ELECTRICAL ENERGY GENRATION THROU GH SOUND
PROJECT REPORT SUBMITTED BY: INDRESH KUNWAR GAURAV SHAKYA SHIVJE
Acknowledgement We would like to give our sincere thanks to the people who are directly or indirectly attached in completion of our project “ELECTRICAL ENRGY THROUGH SOUND” during the academic year 2009-2010. year 2009-2010.
We express our deep obligation to “Mr. FAHEEMULLAH, H.O.D. of ELECTRICAL ENGINEERING” for his kind initiation, invaluable guidance and keen interest throughout the project work, and preparation of this report. We are also grateful to “Mr. “Mr. SHAHEBAN” Electrical Engineering Department for his worthy help in progressing the work. Last but not least we wish to express our sincere thanks to all our professors, friends and colleagues who helped us directly or indirectly in the completion of this project.
Contents •
1 Constr Con struct uction ion
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2 Pola Polari rity ty
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3 Safe Sa fety ty
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4 Electrical behavior of electrolytics
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5 Capaci Cap acitan tance ce
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6 Vari Varian ants ts
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7 Se Seee also al so
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8 Extern Ext ernal al links lin ks
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References
INTRODUCTION This project is to construct a Sound Battery (generation of electrical energy by sound energy) and to study the current variation
by different sound frequencies frequencies in front of speaker working as microphone under Faraday’s Law. The main idea behind this project is that Now day’s there are so many vehicles running on the road’s with which there is a lot’s of noise pollution on the road’s. Government has made many rules to stop this but still it is same as before. We have try to solve this problem, but we also cannot get the solution for this. So finally we decided that if we cannot stop this noise pollution, we can take use of this noise pollution in our daily life by generating electricity with it. The project appears to be very simple (and it is ), but the important basic electronic operation are very profound. Virtually every electronic circuit in our house uses one or more of the concepts we are about to consider consider.. In brief the circuit operation is as follows. Loud sounds received by the speaker generate AC voltage. The transformer steps these AC AC voltages up to a higher level so that may be more effective in causing a meter reading. The diode (bridge rectifier) changes the AC voltage into DC voltage which is required to drive the meter or LED (light emmiting diode). The meter uses the DC current to move the pointer (needle) of the meter so that we can see the effect of the generated electrical energy.
APPARATUS
The project is basically used to generation of electrical energy by sound energy, this consists of some components which are :-
1.
Speaker
2.
Transformer (step up)
3.
Diode (bridge rectifier)
4.
Electrolytic capacitor
5.
Micro Volt Meter or LED
SPEAKER
A loudspeaker, or speaker, is an electromechanical transducer which transducer which converts an electrical signal into sound. sound. The term loudspeaker is used to refer to both the device itself, and a complete system consisting of one or more loudspeaker drivers (as the individual units are often called) in an enclosure. enclosure.
Sound Basics
To understand what and how speakers work, you first need to understand how sound works. Inside your ear is a very thin piece of skin called the eardrum. When your eardrum vibrates, your brain interprets the vibrations as sound -that's how you hear . Rapid changes in air pressure are the most common thing to vibrate your eardrum. An object produces sound when it vibrates in air (sound can also travel through liquids and solids, but air is the transmission medium when we listen to speakers). When something vibrates, it moves the air particles around it. Those air particles in turn move the air particles around them, carrying the pulse of the vibration through the air as a traveling disturbance. To see how this works, let's look at a simple vibrating object -- a bell. When you ring a bell, the metal vibrates -- flexes in and out -- rapidly. When it flexes out on one side, it pushes out on the surrounding air particles on that side. These air particles then collide with the particles in front of them, which collide with the particles in front of them and so on. When the bell flexes away, it pulls in on these surrounding air particles, creating a drop in pressure that pulls in on more surrounding air particles, which creates another drop in pressure that pulls in particles that are even farther out and so on. This decreasing of pressure is called rarefaction. .
In this way, a vibrating object sends a wave of pressure fluctuation through the atmosphere. When the fluctuation wave reaches your ear, it vibrates the eardrum back and forth. Our brain interprets this motion as sound.
Differentiating Sound
We hear different sounds from different vibrating objects because of variations in: Sound-wave frequency - A higher wave frequency simply means that the air pressure fluctuates faster. We hear this as a higher pitch. When there are fewer fluctuations in a period of time, the pitch is lower. •
Air-pressure level - This is the wave's amplitude, which determines how loud the sound is. Sound waves with greater amplitudes move our ear drums more, and we register this sensation as a higher volume.
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A microphone works something like our ears. It has a diaphragm that is vibrated by sound waves in an area. The signal from a microphone gets encoded on a tape or CD or CD as an electrical signal. When you play this signal back on your stereo, the amplifier sends it to the speaker, which re-interprets it into physical vibrations. Good speakers are optimized to produce extremely accurate fluctuations in air pressure, just like the ones originally picked up by the microphone. In the next section, we'll see how the speaker accomplishes this.
Making Sound
In the last section, we saw that sound travels in waves of air pressure fluctuation, and that we hear sounds differently depending on the frequency and amplitude of these waves. We also learned that microphones translate sound waves into electrical signals, which can be encoded onto CDs, tapes, LPs, etc. Players convert this stored information back into an electric current for use in the stereo system. A speaker speaker is essentially the final translation machine -- the reverse of the microphone. microphone. It takes the electrical signal and translates it back into physical vibrations to create sound waves. When everything is working as it should, the speaker produces nearly the same vibrations that the microphone originally recorded and encoded on a tape, CD, LP, etc.
Making Sound: Magnets
So how does the fluctuation make the speaker coil move back and forth? The electromagnet is positioned in a constant magnetic field created by a permanent magnet. These two magnets -- the electromagnet and the permanent magnet -- interact with each other as any two magnets do. The positive end of the electromagnet is attracted to the negative pole of the permanent magnetic field, and the negative pole of the electromagnet is repelled by the permanent magnet's negative pole. When the electromagnet's polar orientation switches, so does the direction of repulsion and attraction. In this way, the alternating current constantly reverses the magnetic forces between the voice coil and the permanent magnet. This pushes the coil back and forth rapidly, like a piston. When the coil moves, it pushes and pulls on the speaker cone. This vibrates the air in front of the speaker, creating sound waves. The electrical audio signal can also be interpreted as a wave. The frequency and amplitude of this wave, which represents the original sound wave, dictates the rate and distance that the voice coil moves.
This, in turn, determines the frequency and amplitude of the sound waves produced by the diaphragm. Different driver sizes are better suited for certain frequency ranges. For this reason, loudspeaker units typically divide a wide frequency range among multiple drivers. In the next section, we'll find out how speakers divide up the frequency range, and we'll look at the main driver types used in loudspeakers. loudspeakers.
Making Sound: Voice Coil
The voice coil is a basic electromagnet. electromagnet. you know that an electromagnet is a coil of wire, usually wrapped around a piece of magnetic metal, such as iron. Running electrical current through the wire creates a magnetic field around the coil, magnetizing the metal it is wrapped around. The field acts just like the magnetic field around a permanent magnet: It has a polar orientation -- a "north" end and and a "south" end -- and it is attracted to iron objects. But unlike a permanent magnet, in an electromagnet you can alter the orientation of the poles. If you reverse the flow of the current, the north and south ends of the electromagnet switch. This is exactly what a stereo signal does -- it constantly reverses the flow of electricity. If you've ever hooked up a stereo system, then you know that there are two output wires for each speaker -- typically a black one and a red one.
Essentially, the amplifier is amplifier is constantly switching the electrical signal, fluctuating between a positive charge and a negative charge on the red wire and black wire. Since electrons always flow in the same direction between positively charged particles and negatively charged particles, the current going through the speaker moves one way and then reverses and flows the other way. This alternating current causes the polar orientation of the electromagnet to reverse itself many times a second.
Types Of Speakers: A woofer is woofer is a driver capable of reproducing low (bass) frequencies. The usable frequency range varies widely according to design. Whilst some woofers can cover the audio band from bass to 3 kHz, kHz, others only work up to 1 kHz or less. Some woofers are capable of very deep bass performance in the proper enclosure, while others become unusable below 50 or 60 Hz. Hz. A tweeter is tweeter is a driver capable of reproducing the higher end of the audio spectrum, usually from about somewhere around 3-5 kHz up to 20 kHz and beyond. A mid-range speaker , also called a squawker, is designed to cover the middle of the audio spectrum, typically from 200 Hz to about 45 kHz. Midranges are used when the other drivers are incapable of adequately covering the mid audio range.
Transformer
A transformer transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling, coupling , without requiring relative motion between its parts. A transformer comprises two or more coupled windings, windings , and, in most cases, a magnetic core to concentrate magnetic flux. flux . A changing voltage applied to one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings. The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved. Transformers Transformers come in a range of sizes from a thumbnail-sized thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. grids. All operate with the same basic principles and with many similarities in their parts.
Coupling by mutual induction The principles of the transformer are illustrated by consideration of a hypothetical hypothetical ideal transformer. transformer. In this case, the core requires negligible magnemotive force to sustain flux, and all flux linking the primary winding also links the secondary winding. The hypothetical ideal transformer has no resistance in its coils. A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time varying magnetic flux that passes through (links) both
primary and secondary windings. Whenever the amount of current in a coil changes, a voltage is induced in the neighboring coil. The effect, called mutual inductance, inductance , is an example of electromagnetic induction.
If a time-varying voltage is applied to the primary winding of turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through a magnetic circuit. circuit . The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF) in opposition to . In accordance with Faraday's law of induction, induction , the voltage induced across the primary winding is proportional to the rate of change of flux:
and where vP and vS are the voltages across the primary winding and secondary winding,
NP and NS are the numbers of turns in the primary winding and secondary winding, dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary windings. In the hypothetical ideal transformer, the primary and secondary windings are perfectly coupled, or equivalently equivalentl y, . Substituting and solving for the voltages shows that:
where vp and vs are voltages across primary and secondary, Np and Ns are the numbers of turns in the primary and secondary, respectively. Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. The EMF in the secondary winding will cause current to flow in a secondary circuit. The MMF produced by current in the secondary winding opposes the MMF of the primary winding and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, increased current flows in the primary circuit. The resulting increase in MMF due to the primary current offsets the effect of the opposing secondary MMF. In this way, the electrical energy fed into the primary winding is delivered to the secondary winding. In addition, the flux density will always stay the same as long as the primary voltage is steady. For example, suppose a power of power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2. P = EI (power = electromotive force × current) 50 W = 2 V × 25 A in the primary circuit if the load is a resistive load. (See note 1) Now with transformer change:
50 W = 25 V × 2 A in the secondary circuit. Since a direct current by definition does not change, it produces a steady MMF and so steady flux in the core; this quantity does not change and so cannot induce a voltage in the secondary winding. In a practical transformer, transformer, direct current applied to the winding will create only heat.
The universal electromotive force (EMF) equation If the flux in the core is sinusoidal, sinusoidal , the relationship for either winding between its number of turns, voltage, magnetic flux density and core cross-sectional area is given by the universal emf equation (from Faraday's law):
where E is the sinusoidal rms or root or root mean square voltage of the winding, f is the frequency in hertz, hertz, N is the number of turns of wire on the winding, a is the cross-sectional cross-sectional area of the core in square metres B is the peak magnetic flux density in teslas, teslas, Other consistent systems of units can be used with the appropriate conversions in the equation.
Operation at different frequencies
The equation shows that the EMF of a transformer at a given flux density increases with frequency. By operating at higher frequencies, transformers can be physically more compact without reaching saturation, saturation, and a given core is able to transfer more power. However, other properties of the transformer, transformer, such as losses within the core and skin-effect, also increase with frequency. Generally, operation of a transformer at its designed voltage but at a higher frequency than intended will lead to reduced magnetising (no load primary) current. At a frequency lower than the design value, with the rated voltage applied, the magnetising current may increase to an excessive level. Operation of a power transformer at other than its design frequency may require assessment of voltages, losses, and cooling to establish if safe operation is practical. For example, transformers at hydroelectric generating stations may be equipped with overexcitation protection, so-called "volts per hertz" protection relays, relays, to protect the transformer from overvoltage at higher-than-rated frequency which may occur if a generator loses its connected load.
Classifications
Transformers Transformers are adapted to numerous engineering applications applications and may be classified in many ways: By power level (from fraction of a volt-ampere(VA) volt-ampere(VA) to over a thousand MVA), By application (power supply, impedance matching, circuit isolation), By frequency range (power, audio, radio frequency(RF)) By voltage class (a few volts to about 750 kilovolts) By cooling type (air cooled, oil filled, fan cooled, water cooled, etc.) By purpose (distribution, rectifier, arc furnace, amplifier output, etc.). By ratio of the number of turns in the coils
Circuit symbols
Standard symbols Transformer with two windings and iron core. Transformer with three windings. The dots show the relative winding configuration of the windings. Step-down or step-up transformer. The symbol shows which winding has more turns, but does not usually show the exact ratio. Transformer with electrostatic screen, which prevents capacitive coupling between the.
WINDINGS
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Energy Losses
An ideal transformer would have no losses, and would therefore be 100% efficient. In practice, energy is dissipated due both to the resistance of the windings known as copper loss or I2R loss, and to magnetic effects primarily attributable attributable to the core (known as iron loss). loss). Transformers are, in general, highly efficient: large power transformers (over 50 MVA) may attain an efficiency as high as 99.75%. Small transformers, such as a plug-in "power brick" used to power small consumer electronics, may be less than 85% efficient.
Transformer losses
Winding resistance
Current flowing through the windings causes resistive heating of the conductors (I2 R loss). At higher frequencies, frequencies, skin effect and proximity effect create additional winding resistance and losses.
Eddy currents
Induced eddy currents circulate within the core, causing resistive heating. Silicon is added to the steel to help in controlling eddy currents. Adding Adding silicon also has the advantage of stopping ageing of the electrical steel that was a problem years ago.
Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis within the magnetic core. The amount of hysteresis is a function of the particular core material.
Magnetostriction
Magnetic flux in the core causes it to physically expand and contract slightly with the alternating magnetic field (producing a buzzing sound), an effect known as magnetostriction. magnetostriction. This in turn causes losses due to frictional heating in susceptible ferromagnetic cores.
Mechanical losses
In addition to magnetostriction, magnetostriction, the alternating magnetic field causes fluctuating electromagnetic electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, creating a familiar humming or buzzing noise, and consuming a small amount of power.
Stray losses Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of the leakage flux may induce eddy currents within nearby conductive objects, such as the transformer's support structure, and be converted to heat.
Cooling system Large power transformers may be equipped with cooling fans, fans , oil pumps or water-cooled or water-cooled heat exchangers designed to remove the heat caused by copper and iron losses. The power used to operate the cooling system is typically considered part of the losses of the transformer. Losses may be either load-dependent('load-losses') load-dependent('load-losses') or independent independent of it ('no-load loss'). Winding resistance dominates load-losses, whereas hysteresis and eddy currents losses contribute to over 99% of the noload loss.
Construction A) Cores
i) Steel cores
Transformers Transformers for use at power or audio frequencies have cores made of many thin laminations of silicon steel. steel . By concentrating the magnetic flux, more of it is usefully linked by both primary and secondary windings. windings. Since the steel core is conductive, it, too, has
currents induced in it by the changing magnetic flux. Each layer is insulated from the adjacent layer to reduce the energy lost to eddy current heating of the core. The thin laminations are used to r educe the eddy currents, and the insulation is used to keep the laminations from acting as a solid piece of steel. The thinner the laminations, the lower the eddy currents, and the lower the losses. Very thin laminations are generally used on high frequency transformers. The cost goes up when using thinner laminations mainly over the labor in stacking them. A typical laminated core is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". In the EI transformer, the laminations are stacked in what is known as an interleaved fashion. Due to this interleaving a second gap in parallel (in an analogy to electronic circuits) to the gap between E and I is formed between the E-pieces. The E-pieces are pressed together to reduce the gap width to that of the insulation. The gap area is very large, so that the effective gap width is very small (in analogy to a capacitor ). ). For this to work the flux has to gradually flow from one E to the other. That means that on one end all flux is only on every second E. That means saturation occurs at half the flux density. Using a longer E and wedging it with two small Is will increase the overlap and additionally make the grains more parallel to the flux (think of a wooden frame for a window). If an air gap is needed (which is unlikely considering the low remanence available for steel), all the E's are stacked on one side, and all the I's on the other creating a gap. The cut core or C-core is made by winding a silicon steel strip around a rectangular form. After the required thickness is achieved, it is removed from the form and the laminations are bonded together. It is then cut in two forming two C shapes. The faces of the cuts are then ground smooth so they fit very tight with a very small gap to reduce losses. The core is then assembled by placing the two C halves together, and holding them closed by a steel strap. Usually two Ccores are used to shorten the return path for the magnetic flux resulting in a form similar to the EI. More cores would necessitate a triangular cross-section. Like toroidal cores, they have the advantage, that the flux is always in the oriented parallel the grains. Due to the bending of the core, some area is lost for a rectangular winging. A steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remanent magnetism is reduced, usually after a few cycles of the applied
alternating current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long overhead power transmission lines, induced currents due to geomagnetic geomagnetic disturbances during solar storms can cause saturation of the core, and false operation of transformer protection transformer protection devices. devices . Steel cores develop a larger hysteresis loss due to eddy currents as the operating frequency is increased. Ferrite, or thinner steel laminations for the core are typically used for frequencies above 1kHz. The thinner steel laminations serve to reduce the eddy currents. Some types of very thin steel laminations can operate at up to 10 kHz or higher. higher. Ferrite is used in higher frequency applications, extending to the VHF band and beyond. Aircraft traditionally use 400 Hz power systems since the slight increase in thermal losses is more than offset by the reduction in core and winding weight. Military gear includes 400 Hz (and other frequencies) to supply power for radar or radar or servomechanisms. servomechanisms. Distribution transformers can achieve low off-load losses by using cores made with low loss high permeability silicon steel and amorphous (non-crystalline) steel, so-called " metal glasses" glasses " — the high cost of the core material is offset by the lower losses incurred at light load, over the life of the transformer. transformer. In order to maintain good voltage regulation, distribution transformers are designed to have very low leakage inductance . Certain special purpose transformers use long magnetic paths, insert air gaps, or add magnetic shunts (which bypass a portion of magnetic flux that would otherwise link the primary and secondary windings) windings) in order to intentionally intentionally add leakage inductance. The additional leakage inductance limits the secondary winding's short circuit current to a safe, or a controlled, level. This technique is used to stabilize the output current for loads that exhibit negative resistance such as electric arcs, arcs, mercury vapor lamps, lamps , and neon signs, signs , or safely handle loads that may become periodically short-circuited such as electric arc welders. Gaps are also used to keep a transformer from saturating, especially audio transformers that have a DC component added.
ii) Solid cores
Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above mains frequencies and up to a few tens of kilohertz kilohertz.. These materials combine high magnetic permeability with high bulk electrical resistivity. resistivity. At even higher, radio-frequencies radio-frequencies (RF), other types of cores made from non-conductive magnetic ceramic materials, called ferrites, ferrites, are common. Some RF transformers also have moveable cores (sometimes called slugs) which allow adjustment of the coupling coefficient coefficient (and bandwidth) of tuned radio-frequency circuits. Cores are available in a wide variety of shapes, including toroids. Other shapes include so-called E-cores and C-cores.
iii) Air cores High-frequency High-frequency transformers may also use air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.
iv) Toroidal cores
Various transformers. The top right is toroidal. The bottom right is from a 12 VAC wall wart supply. Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel or permalloy or permalloy wound into a coil, from powdered iron, or ferrite or ferrite,, depending on operating frequency. The strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. reluctance. The closed ring shape eliminates air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular crosssections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic electromagnetic interference. interference .
Ferrite toroid cores are used at higher frequencies, typically between between a few tens of kilohertz to a megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. supplies . Toroidal transformers are more efficient than the cheaper laminated EI types of similar power level. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantages are higher cost and limited size. A drawback of toroidal transformer construction is the higher cost of windings. As As a consequence, consequence, toroidal transformers tr ansformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings. When fitting a toroidal transformer, transformer, it is important to avoid making an unintentional short-circuit through the core. This can happen if the steel mounting bolt in the middle of the core is allowed to touch metalwork at both ends, making a loop of conductive material that passes through the hole in the toroid. Such a loop could result in a dangerously large current flowing in the bolt.
B) Windings
The wire of the adjacent turns in a coil, and in the different windings, must be electrically insulated from each other. other. The wire used is generally magnet wire. wire. Magnet wire is a copper wire with a coating of varnish or some other synthetic coating. Transformers Transformers for years have used Formvar wire, Formvar wire, which is a varnished type of magnet wire. The conducting material used for the winding depends upon the application. Small power and signal transformers are wound with solid copper wire, insulated usually with enamel, enamel , and sometimes additional insulation. Larger power transformers may be wound with wire, copper, copper, or aluminium rectangular conductors. Strip conductors are
used for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire to minimize the skin effect losses in the conductors. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is insulated from the other, other, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. This "transposition" "transposition" equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size is. (see reference (1) below) For signal transformers, the windings may be arranged in a way to minimise leakage inductance and stray capacitance to improve highfrequency response. This This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding. Windings on both the primary and secondary of power transformers may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltage ratio. Taps may be connected to an automatic, on-load tap changer type changer type of switchgear of switchgear for voltage for voltage regulation of distribution of distribution circuits. Audio-frequency Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in amplifier in a push-pull type circuit. Modulation transformers in AM transmitters are very similar. Tapped transformers are also used as components of amplifiers, oscillators, and for feedback linearization of amplifier circuits.
C) Insulation of windings
The turns of the windings must be insulated from each other to ensure that the current travels through the entire winding. The potential difference between between adjacent turns is usually small, so that enamel insulation is usually sufficient for small power transformers.
Supplemental sheet or tape insulation is usually employed between winding layers in larger transformers. The transformer may also be immersed in transformer oil that provides further insulation. Although Although the oil is primarily used to cool the transformer, transformer, it also helps to reduce the formation of corona discharge within high voltage transformers. By cooling the windings, the insulation will not break down as easily due to heat. To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely sealed against moisture ingress. Thus the oil serves as both a cooling medium to remove heat from the core and coil, and as part of the insulation system. Certain power transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, vacuum , air spaces within the windings are replaced with epoxy, thereby sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers suitable for damp or dirty environments, but at increased manufacturing cost.
D) Shielding
Where transformers are intended for minimum electrostatic coupling between primary and secondary circuits, an electrostatic shield can be placed between windings to reduce the capacitance between primary and secondary windings. The shield may be a single layer of metal foil, insulated where it overlaps to prevent it acting as a shorted turn, or a single layer winding between primary and secondary. The shield is connected to earth ground. Transformers Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer, transformer, or to prevent the transformer from affecting the operation of nearby devices that may be sensitive to stray fields such as CRTs. CRTs. Small signal transformers do not generate significant amounts of heat. Power transformers rated up to a few kilowatts rely on natural convective air-cooling. Specific provision provision must be made for cooling of
high-power transformers. Transformers handling higher power, or having a high duty cycle can be fan-cooled. Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulphur or sulphur hexafluoride gas. The windings of high-power or high-voltage transformers are immersed in transformer oil — a highly refined mineral oil, oil, that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability and toxic effects of PCB by-products, and its accumulation in the environment, it is no longer permitted in new equipment. Old transformers that still contain PCB should be examined on a weekly basis for leakage. If found to be leaking, it should be changed out, and professionally professionally decontaminated or scrapped in an environmentally safe manner. Today, oday, non-toxic, no n-toxic, stable silicone-based silicone-based oils, or fluorinated or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil. The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) watts) may have cooling fans, oil pumps and even oil to water heat exchangers. exchangers . Oil-filled transformers undergo prolonged drying processes, using vapor-phase heat transfer, electrical self-heating, the application of a vacuum, vacuum , or combinations of these, to ensure that the transformer is completely free of water of water vapor before vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled power transformers may be equipped with Buchholz relays which are safety devices that sense gas build-up inside the transformer (a side effect of an electric arc inside the windings), and thus switches off the transformer. Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses,
but not the core steel loss. These are cooled by liquid nitrogen or helium. helium.
E) Terminals
Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide electrical insulation without letting the transformer leak oil.
F) Enclosure
Small transformers often have no enclosure. Transformers Transformers may have a shield enclosure, as described above. Larger units may be enclosed to prevent contact with live parts, and to contain the cooling medium (oil or pressurized gas).
Transformer types and uses A) Autotransformers An autotransformer has autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end
and the tap. It can be connected to a 230-volt supply to drive 115-volt equipment, or reversed to drive 230-volt equipment from 115 volts. As the same winding is used for input and output, the flux in the core is partially cancelled, and a smaller core can be used. F or voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than a true (two-winding) transformer of the same rating. In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up. The difference difference is usually slight enough to allow reversal where the actual voltage level is not critical. By exposing part of the winding coils and making the secondary connection through a sliding brush, brush, an autotransformer with a nearcontinuously continuously variable turns ratio can be obtained, allowing for very small increments of voltage.
B) Constant voltage transformer (ferro-resonance) By arranging particular magnetic properties of a transformer core, and installing a resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage constant regardless (within some limits) of any varian variance ce in the primar primary y supply supply withou withoutt addit addition ional al circui circuitry try or manual adjustment. CVA transformers run hotter than standard power tran transf sfor orme mers rs,, for for the the reg regulat ulatin ing g acti action on is depe depend nden entt on core core saturation, which reduces efficiency somewhat. The output voltage of a transformer varies some with varying load resistances, even with a constant voltage input. The degree of vari varia ance nce is affe affect cted ed by the the prima rimary ry and and seco second ndar ary y wind windin ing g inductances, among other factors, not the least of which includes winding resistance and the degree of mutual inductance (magnetic coupling) between the primary and secondary windings. For power transformer applications, where the transformer is seen by the load
(ideally) as a constant source of voltage, it is good to have the secondary voltage vary as little as possible for wide variances in load current. The measure of how well a power transformer maintains constant seco second ndar ary y volt voltag age e over over a rang range e of load load curre current nts s is call called ed the the transf transform ormer' er's s volta voltage ge regula regulatio tion. n. It can can be calcul calculate ated d from from the following formula:
Characteristic of CVT
Circuit diagram
C) Polyphase transformers
Physical view
For three-phase For three-phase power , three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Δ, Δ-Y, Δ-Δ and Y-Y. A vector group indicates the configuration of the windings and the phase angle difference difference between them. If a winding is connected to earth (grounded), grounded ), the earth connection point is usually the center point of a Y winding. If the secondary is a Δ winding, the ground may be connected to a center tap on one winding ( high leg delta) delta ) or one phase may be grounded (corner grounded delta). A special purpose polyphase transformer is the zigzag transformer . There are many possible configurations configurations that may involve more or fewer than six windings and various tap connections.
D) Resonant transformers A resonant transformer operates at the resonant frequency of one or more of its coils and (usually) an external capacitor . The resonant coil, usually the secondary, acts as an inductor , and is connected in series with a capacitor . When the primary coil is driven by a periodic source of alternating of alternating current, current , such as a square or Sawtooth or Sawtooth wave at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. breakdown . These devices are used to generate high alternating voltages, and the current available can be much larger than that from electrostatic machines machines such as the Van de Graaff generator or generator or Wimshurst Wimshurst machine. machine . Examples:
Tesla coil Oudin coil (or Oudin resonator; named after its inventor Paul Oudin) Oudin ) D'Arsonval apparatus Ignition coil or induction or induction coil used in the ignition system of a petrol engine
Flyback transformer of transformer of a CRT television set or video monitor. Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the transformer's tr ansformer's secondary is resonated with the cable's capacitance. Other applications of resonant transformers are as coupling between stages of a superheterodyne superheterodyne receiver , where the selectivity of the receiver is provided by the tuned transformers of the intermediatefrequency amplifiers. A voltage-regulating transformer uses a resonant winding and allows part of the core to go into saturation on each half-cycle of the alternating current. This effect stabilizes the output of the regulating transformer, transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an AC power supply. However, due to the hysteresis losses accompanying this type of operation, efficiency is low.
E) Instrument transformers i) current transformer
A current transformer is a type of "instrument transformer" that is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary. This accuracy is directly related to a number of factors including the following: burden, rating factor, load, external electromagnetic fields, temperature and physical CT configuration. configuration. The burden in a CT metering circuit is essentially the amount of impedance (largely resistive) present. Typical burden ratings for CTs are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with
a burden rating of B-0.2 can tolerate up to 0.2Ω of impedance in the metering circuit before its output current is no longer a fixed ratio to the primary current. Items that contribute to the burden of a current measurement circuit are switch blocks meters and intermediate conductors. The most common source of excess burden in a current measurement circuit is the conductor between the meter and the CT. Often times, substation meters are located significant distances from the meter cabinets and the excessive length of small gauge conductor creates a large resistance. Rating factor is a factor by which the nominal full load current of a CT can be multiplied to determine its absolute maximum measurable primary current. Conversely, the minimum primary current a CT can accurately measure is "light load," or 10% of the nominal current. The rating factor of a CT is largely dependent upon ambient temperature. Most CTs have rating factors for 35 degrees Celsius and 55 degrees Celsius. A CT usually demonstrates reduced capacity to maintain accuracy with rising ambient temperature. It is important to be mindful of ambient temperatures and resultant rating factors when CTs are installed inside pad-mounted transformers or poorly ventilated mechanical rooms. Recently, manufacturers have been moving towards lower nominal primary currents with greater rating factors. This is made possible by the development of more efficient ferrites and their corresponding hysteresis curves. This is a distinct advantage over previous CTs because it increases their range of accuracy. For example, a 200:5 CT with a rating factor of 4.0 is most accurate between 20A (light load)and 800A (4.0 times the nominal rating, or "full load," of the CT) of primary current. While previous revisions of CTs were on the order of 500:5 with a rating factor of 1.5 yielding an effective range of 50A to 750A. This is an 11% increase in effective range for two CTs that would be used at similar services. Not to mention, the relative cost of a 500:5 CT is significantly greater than that of a 200:5. Physical CT configuration is another important factor in reliable CT accuracy. While all electrical engineers are quite comfortable with Gauss' Law, there are some issues when attempting to apply theory to the real world. When conductors passing through through a CT are not centered in the circular (or oval) void, slight inaccuracies may occur. occur. It is important to center primary conductors as they pass through CTs to promote the greatest level of CT accuracy. accuracy. Afterall, in an electric metering circuit, the most inaccurate component is the CT.
Current transformers (CTs) are commonly used in metering and protective relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured. Curr Curren entt tran transf sfor orme mers rs are are ofte often n cons constr truc ucte ted d by pass passin ing g a sing single le primary turn (either an insulated cable or an uninsulated bus bar) through a well-insulated toroidal core wrapped with many turns of wire. Current transformers are used extensively for measuring current and monitoring the operation of the power grid. grid. The CT is typically described by its current ratio from primary to secondary. Common secondaries are 1 or 5 amperes. For example, a 4000:5 CT would prov provid ide e an outp output ut curre current nt of 5 ampe ampere res s when when the the prim primar ary y was was passing 4000 amperes. The secondary winding can be single ratio or multi ratio, with five taps being common for multi ratio CTs. Typically, the secondary connection points are labelled as 1s1, 1s2, 2s1, 2s2 and so on. The multi ratio CTs are typically used for current matching in current differential protective relaying applications. Often, multiple CTs CTs will will be instal installed led as a "stack "stack"" for variou various s uses uses (for (for exampl example, e, protection devices and revenue metering m etering may use separate CTs). CTs). For a three-stacked CT application, the secondary winding connection points are typically labelled Xn, Yn, Zn. Care must be taken that the secondary of a current transformer is not disconnected from its load whil while e curr curren entt is flow flowin ing g in the the prima rimary ry,, as this this will will prod produc uce e a dang danger erou ousl sly y high high volt voltag age e acro across ss the the open open seco second ndar ary y and and may may permanently affect the accuracy of the transformer. Specially constructed wideband current transformers are also used (usu (usual ally ly with with an oscilloscope) oscilloscope) to measure waveforms of high frequency or pulsed currents within pulsed power systems. One type of specia specially lly const construc ructe ted d wideb wideband and transf transform ormer er provid provides es a voltag voltage e outpu outputt that that is propo proporti rtiona onall to the measur measured ed curren current. t. Anothe Anotherr type type (called a Rogowski coil) coil ) requires an external integrator in order to provide a voltage output that is proportional to the measured current. Unlike CTs used for power circuitry, wideband CTs are rated in output volts per ampere of primary current.
ii) Voltage Voltage transformers transf ormers
Volta oltage ge trans transfo form rmer ers s (VTs) (VTs) or pote potent ntia iall tran transf sfor orme mers rs (PT (PTs) are are anot anothe herr type type of inst instru rume ment nt tran transf sfor orme merr, used used for for mete meterin ring g and and protec protectio tion n in highhigh-vol volta tage ge circui circuits. ts. They They are design designed ed to presen presentt negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays. relays. The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground. The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays. While VTs were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VTs for most secondary service voltages. For new, or rework, meter packages, VTs are typically only installed in primary voltage (typically 12.5kV) or generation voltage (13.2kV) meter packages.
iii) Pulse transformers
A pulse transformer transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. lines . Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. semiconductors . Special high voltage pulse transformers are also used to generate high power pulses for
radar , particle accelerators, accelerators , or other high energy pulsed power applications. To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors. The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.
iv) RF transformers (transmission line transformers)
For radio For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or bifilar or coaxial coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique. The core material increases the inductance dramatically, thereby raising its Q factor . The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector ( detector ) stage in antique regenerative radio receivers.
v) Baluns
Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or bifilar or coaxial coaxial
cable and are similar to transmission line transformers in construction and operation.
vi) Audio transformers
Audio transformers are usually the factor which limit sound quality; electronic circuits with wide frequency response and low distortion are relatively simple to design. Transformers are also used in DI boxes to convert impedance from high-impedance high-impedance instruments (for ( for example, bass guitars) guitars) to enable them to be connected to a microphone input on the mixing console. console . A particularly particularly critical component is the output transformer of an audio power amplifier . Valve circuits for quality reproduction have long been produced with no other (inter-stage) audio transformers, but an output transformer is needed to couple the relatively high impedance (up to a few hundred ohms depending upon configuration) configuration) of the output valve(s) to the low impedance of a loudspeaker . loudspeaker . (The valves can deliver a low current at a high voltage; the speakers require high current at low voltage.) Solid-state power amplifiers may need no output transformer at all. For good low-frequency response a relatively large iron core is required; high power handling increases the required core size. Good high-frequency response requires carefully designed and implemented windings without excessive leakage inductance or stray or stray capacitance. capacitance. All this makes for an expensive component. Early transistor audio transistor audio power amplifiers often had output transformers, but they were eliminated as designers discovered how to design amplifiers without them.
vii) Speaker transformers
In the same way that transformers are used to create high voltage power transmission circuits that minimize transmission losses, speaker transformers allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal speaker voltages. This application is common in public address (e.g., Tannoy) Tannoy ) applications. Such circuits are commonly referred to as constant voltage or 70 volt speaker circuits although the audio waveform is obviously a constantly changing voltage. voltage. At the audio amplifier, a large audio transformer may be used to stepup the low impedance, low-voltage output of the amplifier to the designed line voltage of the speaker circuit. Then, a smaller transformer at each speaker returns the voltage and impedance to ordinary speaker levels. The speaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in a number of discrete steps. Use of a constant-voltage speaker circuit means that there is no need to worry about the impedance presented to the amplifier output (which would clearly be too low if all of the speakers were arranged in parallel and would be too complex a design problem if the speakers were arranged in series-parallel). The use of higher transmission voltage and impedance means that power lost in the connecting wire is minimized, even with the use of small-gauge conductors (and leads to the term constant voltage as the line voltage doesn't change much as additional speakers are added to the system). In addition, the ability to adjust, locally, the volume of each speaker (without the complexity and power loss of an L pad) pad) is a useful feature. viii) Small Signal transformers
Moving coil phonograph cartridges produce a very small voltage. In order for this to be amplified with a reasonable signal-noise ratio, a transformer is usually used to convert the voltage to the range of the more common moving-magnet cartridges.
ix) 'Interstage' and coupling transformers
A use for interstage transformers is in the case of push-pull amplifiers where an inverted signal is required. Here two secondary windings wired in opposite polarities may be used to drive the output devices.
Uses of transformers
For supplying power from an alternating current power grid to equipment which uses a different voltage.
For regulating the secondary output of a constant voltage (or ferro-resonant), in which a combination of core saturation and the resonance of a tank circuit prevents changes in the primary voltage from appearing on the secondary.
Electric power transmission over long distances.
Large, specially constructed power transformers are used for electric arc furnaces used in steelmaking. steelmaking.
Rotating transformers are designed so that one winding turns while the other remains stationary. A common use was the video head system as used in VHS and Beta video tape players. These can pass power or radio signals from a stationary mounting to a rotating mechanism, or radar antenna radar antenna..
Other rotary transformers are precisely constructed in order to measure distances or angles. Usually they have a single primary and two or more secondaries, and electronic circuits measure the different amplitudes of the currents in the secondaries. See synchro and resolver .
Sliding transformers can pass power or signals from a stationary mounting to a moving part such as a machine tool head.
A transformer-like device is used for position measurement. See linear variable differential transformer . transformer .
Some rotary transformers are used to couple signals between two parts which rotate in relation to each other.
Small transformers are often used internally to couple different stages of radio of radio receivers and audio amplifiers. amplifiers .
Transformers Transformers may be used as external accessories for impedance matching; for example to match a microphone to an amplifier.
Balanced-to-unbalanced conversion. A special type of transformer called a balun is used in radio and audio circuits to convert between balanced linecircuits and unbalanced transmission lines such as antenna downleads.
Flyback transformers are built using ferrite cores. They supply high voltage to the CRTs at the frequency of the horizontal oscillator. In the case of television sets, this is about 15.7kHz. It may be as high as 75 - 120kHz for high-resolution computer computer monitors.
Switching power supply transformers usually operate between 30-1000 kHz. The tiny cores found in wristwatch backlight power supplies produce audible sound (about 1 kHz).
Types of transformers In general, transformers are used for two purposes: signal matching and power supplies.
Power Transformers Power transformers are used to convert from one voltage to another, at significant power levels.
Step-up transformers A "step-up transformer" allows a device that requires a high voltage power supply to operate from a lower voltage source. The transformer takes in the low voltage at a high current and puts out the high voltage at a low current. Examples: •
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You are a Swiss visiting the U.S.A., and want to operate your 220VAC shaver off of the available 110 VAC. The CRT display tube of your computer monitor requires thousands of volts, but must run off of 110 VAC from the wall.
Limitations •
Transformers alone cannot do the following:
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Convert DC to AC or vice versa
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Change the voltage or current of DC
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Change the AC supply frequency. frequency.
Diode •
•
A diode is a tube or semiconductor device, intended to pass current in only one direction. Diode can be used for a wide variety of different purposes. The semiconductor diode is far more common than the tube diode in modern electronic circuits. A diode consists consists of N-type semiconductor semiconductor material, material, usually germanium or silicon, and P-type material. Electrons flow into the N-type material and out of the Pterminal. The schematic symbol for a semiconductor semiconductor diode is shown at B. Positive current flows in the direction of the arrow. Electron movement is contrary to the arrow. The positive terminal of a
diode is called Anode, and the negative terminal is called Cathode, under conditions of forward bias (conduction). Semiconductor diode can be very small, and still handle hundreds or even thousands of volta at several amperes. The older tube type diodes are much bulkier and less efficient than
the semiconductor diodes. Some of the tube type diodes require a separate power supply for the purpose of heatihg a filament. Semiconductor diodes are used for many different purposes in electronics. They can be used as amplifiers, frequency controllers, oscillators, voltage regulators, switches, mixers, and in many other types of circuits.
Diode Bridge A diode bridge or bridge rectifier (occasionally called a Graetz bridge) is an arrangement of four diodes connected in a bridge circuit as shown below, that provides the same polarity of output voltage for any polarity of the input voltage. When used in its most common application, for conversion of alternating current (AC) input into direct current (DC) output, it is known as a bridge rectifier . The bridge recitifier provides full wave rectification from a two wire AC input (saving the cost of a center tapped transformer) but has two diode drops rather than one reducing efficiency over a center tap based design for the same output voltage.
Schematic of a diode bridge •
The essential feature of this arrangement is that for both polarities of the voltage at the bridge input, the polarity of the output is constant.
Basic operation •
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When the input connected at the left corner of the diamond is positive with respect to the one connected at the right hand corner, current flows to the right along the upper colored path to the output, and returns to the input supply via the lower one.
When the right hand corner is positive relative to the left hand corner, current flows along the upper colored path and returns to the supply via the lower colored path.
In each case, the upper right output remains positive with respect to the lower right one. Since this is true whether the input is AC or DC, this circuit not only produces DC power when supplied with AC power: it also can provide what is sometimes called "reverse polarity protection". That is, it permits normal
functioning when batteries are installed backwards or DC inputpower supply wiring "has its wires crossed" (and protects the circuitry it powers against damage that might occur without this circuit in place). •
Prior to availability of integrated electronics, such a bridge rectifier was always constructed from discrete components. Since about 1950, a single four-terminal component containing the four diodes connected in the bridge configuration became a standard commercial component and is now available with various voltage and current ratings.
How Bridge Rectifier works When four diodes are connected, the circuit is called a BRIDGE RECTIFIER. The input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners.
One complete cycle of operation will be discussed to help you understand how this circuit works. We have discussed transformers in previous modules in the NEETS series and will not go into their characteristics at this time. Let us assume the transformer is working
properly and there is a positive potential at point A and a negative potential at point B. The positive potential at point A will forward bias D3 and reverse bias D4. The negative potential potential at point B will forward bias D1 and reverse bias D2. At this time D3 and D1 are forward biased and will allow current flow to pass through them; D4 and D2 are reverse biased and will block current flow. The path for current flow is from point B through D1, up through R L, through D3, through the secondary of the transformer back to point B. This path is indicated by the solid arrows. Waveforms (1) and (2) can be observed across D1 and D3. One-half cycle later the polarity across the secondary of the transformer reverses, forward biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from point A through D4, up through R L, through D2, through the secondary of T1, and back to point A. This path is indicated by the broken arrows. Waveforms (3) and (4) can be observed across D2 and D4. You should have noted that the current flow through R L is always in the same direction. In flowing through R L this current develops a voltage corresponding to that shown in waveform (5). Since current flows through the load (R L) during both half cycles of the applied voltage, this bridge rectifier is a full-wave rectifier.
Electrolytic Capacitor An electrolytic capacitor is a type of capacitor typically capacitor typically with a larger capacitance per unit volume than other types, making them valuable in relatively high-current and low-frequency electrical circuits. circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output, rectifier output, and especially in the absence of rechargeable batteries that can provide similar low-frequency current capacity. They are also widely used as coupling capacitors capacitors in circuits where AC should be conducted but DC should not; the large value of the capacitance allows them to pass very low frequencies. The electrolytic capacitor was invented in 1921 by Julius Edgar Lilienfeld. Lilienfeld. It was largely responsible for the development of mainspowered radio receivers, since it permitted the filtering of the 50-60 hertz power supplied to residences, after it was rectified to power the radio tubes. This was not practical without the small volume and low cost of electrolytic capacitors.
Construction Aluminium electrolytic capacitors are constructed from two conducting aluminium foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular
face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture. Tantalum capacitors are more expensive than aluminum-based capacitors, and generally only usable at low voltage, but they have higher capacitance per unit volume and lower impedance at high frequencies, thus they are popular in miniature applications such as cellular telephones telephones..
Polarity In aluminum electrolytic capacitors, ca pacitors, the layer of insulating aluminum oxide on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that provides high capacitance. The aluminum oxide layer can withstand an electric field strength of the order of 109 volts per metre. The combination of high capacitance and high voltage result in high energy density. Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, denoting the adjacent terminal that should be more negative than the other. This is necessary because a reverse-bias voltage will destroy the center layer of dielectric material via electrochemical reduction (see Redox reactions). Without the dielectric material the capacitor will short circuit, circuit, and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode. Modern capacitors have a safety valve on one circular face to vent the hot gas/liquid, but ruptures can still be dramatic. Electrolytics can withstand a reverse bias for a short period of time, but they will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.
Safety The electrolyte is usually boric acid or sodium or sodium borate in aqueous solution together with various sugars or ethylene glycol which are added to retard evaporation. Care should be taken to avoid ingestion of or eye contact with the electrolyte, and any areas of the body where skin contact has occurred should be washed in good time. It is important to follow safe working practice and to use appropriate protective equipment, notably gloves and sa fety glasses, when working with the electrolyte. Some very old tantalum electrolytics, often called "Wet-slug", contain the more hazardous sulfuric acid, however most of these have corroded away by now. Particular care needs to be taken with electrolytic capacitors capacitors in that the device should not be reverse-biased, i.e. connected to a circuit where the negative terminal voltage is more positive than the positive terminal voltage. Were this to happen the capacitor may self-distruct with spectacular and damaging results, risking significant personal injury.
Electrical behavior of electrolytics A common modeling circuit for an electrolytic capacitor has the following schematic: schematic:
where Rleakage is the leakage resistance, RESR is the equivalent series resistance, resistance , LESL the equivalent series inductance (L being the conventional conventional symbol for inductance). RESR must be as small as possible since it determines the loss power when power when the capacitor is used to smooth voltage. Loss power scales quadratically with the ripple current flowing through and
linearly with RESR. Low ESR capacitors are imperative for high efficiencies in power supplies. It should be pointed out that this is only a simple model and does not include all the effects associated with real electrolytic capacitors. Since the electrolytes evaporate, design life is most often rated in hours at a set temperature. temperature. For example, typically as 2000 hours at 105 degrees Celsius (which is the highest working temperature). Design life doubles for each 10 degrees lower, reaching 15 years at 45 degrees.
Capacitance The capacitance value of any a ny capacitor is a measure of the amount of electric charge stored per unit of potential difference difference between the plates. The basic unit of capacitance is a farad, farad, however this unit is often too large for practical uses, so microfarad and picofarad are more commonly used. Many conditions determine a capacitor's value, such as the thickness of the dielectric and the plate area. area. In the manufacturing m anufacturing process, process, electrolytic capacitors are made to conform to a set of preferred numbers. numbers. By multiplying these base numbers by a power of ten, ten , any practical capacitor value can be achieved, which is suitable for most applications. A standardized set of capacitor base numbers was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers 1.0, 1.5, 2.2, 3.3, 4.7 or 6.8 by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF). Most electrolytic capacitors have a tolerance range of 20 %, meaning that the manufacturer guarantees that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance.
Variants
Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct of direct current else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail as the water within the capacitor evaporates. Electrolytic capacitance values are not as tightly-specified tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a "guaranteed minimum value" and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification specification is acceptable.
An electrolytic capacitors come in several types: Aluminum electrolytic capacitor:
compact but lossy, these are available in the range of <1 µF to 1,000,000 µF with working voltages up to several hundred volts DC. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. Over a long time, the liquid can dry out, causing the capacitor to fail. Bipolar electrolytics contain two capacitors connected in series opposition and are used for coupling AC signals. Tantalum: Tantalum:
compact, low-voltage devices up to about 100 µF, these have a lower energy density and are more accurate than aluminum electrolytics. Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance and little DC leakage, and very low impedance at low frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected backwards or exposed to spikes above their voltage rating. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode ele ctrode is formed of sintered tantalum grains, with the dielectric electrochemically electrochemically formed as a thin layer of oxide of oxide.. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The anode electrode is formed of a chemically deposited semi-conductive semi-conductive layer of manganese dioxide, dioxide , which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) polypyrrole) that reduces internal resistance and eliminates a selfignition failure.
Electrolytic double-layer capacitors Electrolytic double-layer capacitors (EDLCs), also known as supercapacitors or ultracapacitors, have very high capacitance values but low voltage ratings. They use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as the dielectric. As the energy stored is inversely proportional to the thickness of the dielectric, these capacitors have an extremely high energy density. The electrodes are made of activated carbon, carbon, which has a high surface area per unit volume, further increasing the capacitor's energy density. Individual EDLCs can have capacitances of hundreds or even thousands of farads. For example, the Korean company NessCap offers units up to 5000 farads (5 kF) at 2.7 V, useful for electric vehicles and solar energy applications. Smaller units (in the 0.1 F – 10 F range) r ange) are frequently used instead instead of (or in addition to) batteries to supply standby power to memory circuits and clocks. The electrodes for EDLCS could also be made by transition metal oxides, eg. RuO2, IrO2, NiO, etc. Electrodes made by metal oxides store the charges by two mechanism: double layer effect, the same
with active carbon, and pseudocapacitance , which can store more energy than double layer effects. Aerogel capacitors
Aerogel capacitors, using carbon aerogel to attain immense electrode surface area, can attain huge values, up to thousands of farads. EDLCs can be used as replacements for batteries in applications where a high discharge current is required, e.g. in electric vehicles. vehicles . They can also be recharged hundreds of thousands of times, unlike conventional conventional batteries which last for only a few hundred or thousand recharge cycles. However, capacitor voltage drops faster than battery voltage during discharge, so a DC to DC converter may be used to maintain voltage and to make more of the energy stored in the capacitor usable.
Volt Meter A voltmeter is an instrument used for measuring the potential difference between two points in an electric circuit. The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter . One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance. The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
Potentiometer
A voltmeter may also be realized using a potentiometer , which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the p otentiometer. Then the wiper position is adjusted to change the potentiometer's effective resistance until a balance is obtained and no current is detected. At this time, record the potentiometer's wiper position. For example, if our potentiometer were a length of very long wire and our wiper were some sort of metal wand in contact with that wire, record the length of wire between the wiper and the end of the wiper that is in our circuit. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.
One may also measure voltage using a potentiometer potentiometer in the nullbalance method. method . The potentiometer's resistance is changed at the wiper until the null detector shows detector shows zero voltage between the two circuits. There are many implementations implementations for null detectors, including nanovolt-sensitive integrated circuits, circuits , simple audio circuits that click to indicate voltage difference, and transformed ammeters, ammeters, as discussed at the top of this article. For more on circuit transformations, note Thevenin's theorem and Norton's theorem. theorem .
VU meter A VU meter is often included in analog audio equipment to display a signal level in volume units. units . The response of a VU meter (black line) compared to instantaneous input level (grey area) of a drum beat. beat. Level is in dB and time is in seconds
It is intentionally a "slow" measurement, averaging out peaks and troughs of short duration to reflect the perceived loudness of the material. It was originally developed in 1939 by the combined effort of Bell Labs and broadcasters CBS and NBC, NBC, for measuring and standardizing the levels of telephone lines. The instrument used to measure VU is called the volume indicator (VI) instrument. Most users ignore this and call it a VU meter. 0 VU was defined:
The reading of the volume indicator sha ll be 0 vu when it is connected connected to a 600-ohm 600- ohm resistance in which is flowing one milliwatt of sine-wave of sine-wave power at 1000 cycles per second The typical VU scale is from −20 to +3. The rise and fall times of the meter are both 300 milliseconds, meaning that if a constant sine wave of amplitude 0 VU is applied suddenly, the meter will take 300 milliseconds to reach the 0 on the scale. It behaves as a full-wave averaging instrument, and is not optimal for measuring peak levels. The behaviour of VU meters is defined in ANSI C16.5-1942, British Standard BS 6840, and IEC 60268-17.
How to use a VU meter Tape and cassette decks typically used physical meters similar to needles on a compass. The needles would be "pegged" when they hit the physical pegs which stopped the maximum motion of the needle. The first high fidelity deck, the Advent used only one meter, which could be switched to average both channels, or either channel, but this was never adopted by any other design. When LEDs were developed, they were often used to indicate peak levels, and later arrays of them replaced mechanical meters, and later LCD and fluorescent displays which are not subject to Newton's laws of motion to limit reaction time. Computer recording software often emulates an array of LEDs. The Nagra V, the latest digital version of a professional reel-to-reel recorder dating from the 1960s still uses analog meters.
Peak levels may be displayed in addition to the current level. As a rule, recording levels should be set so that they do not exceed the red area beyond 0 VU or only rarely. If the level is set too low, noise levels will be high. If set too high, the sound quality and frequency response is typically poorer at high recording levels, and clipping effects can be especially severe for a digital recording system. Systems tailored for voice often incorporate automatic level control, usually not defeatable in inexpensive recorders. VCR's only rarely included VU meters when they provided a manual level control, which is typically required for recording live m usic rather than compressed television or radio broadcasts.
