Government College University, Faisalabad Department of Telecommunication Engineering,
Applied Physics Lab Manual
109717267.doc 2
Prepared & Edited by: Dr. Ijaz Ahmad Khan (Assistant Professor) Engr. Maria Hanif (Lab Engineer)
Verified by: Engr. Kasif Nisar Paracha (Lecturer)
Approved by: Engr. Muhammad Afzal Sipra (TI, M) Associate Professor and Chairman Telecommunication Engineering Department
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TABLE OF LAB EXPERIMENTS Sr. No.
1. 2.
Experiment
Page No.
To analyze simple circuits using Ohm’s Law. 6
To determine the frequency of AC supply.
10
3.
To use potentiometer as a voltage divider.
13
4.
To demonstrate current flow in rectifier circuits by using LED’s. To study the characteristics of RLC series (acceptor) circuit.
16
5. 6. 7.
8. 9. 10.
11. 12
To study the characteristics of RLC parallel (rejecter) circuit. To observe voltage distribution in capacitive circuits and explore series and parallel combinations of capacitors. To study differentiator & integrator circuits.
21 25 29 33
To use LDR in circuit design.
40
To determine the value of an unknown small resistance using a Carey Foster’s bridge.
44
To determine the height of inaccessible object. Semester Project
51
PREFACE 109717267.doc 4
48
The purpose of the science of Physics is to explain natural phenomena. It is the laboratory where new discoveries are being made. This is where the physicist is making observations for the purpose of identifying the patterns which may later be fit to mathematical equations. Theories are constructed to describe patterns observed and are tested by further experimentation. The laboratory of each and every subject taught in the degree of Bachelors in Electrical Engineering is of very much importance in every University. Fully equipped laboratories meeting the industrial demands under the supervision of qualified, talented and practically motivated lab assistants and lab engineers is also a basic criterion of the Pakistan Engineering Council. This Manual has been formulated considering all these above mentioned points. This manual is according to the equipment supplied by the RIMS, USA and meets the requirements of all the course of Applied Physics as per the curriculum of G. C. University Faisalabad.
With Regards Engr. Maria Hanif
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General Lab Instructions Each student group consists of a maximum of 2-4 students. Each group member is
responsible in submitting lab report upon completion of each experiment on their practical Note book. Students are to wear proper attire i.e. shoe or sandal instead of slipper. Excessive jewelleries are not advisable as they might cause electrical shock. A permanent record in ink of observations as well as results should be maintained by each student and enclosed with the report. The recorded data and observations from the lab manual need to be approved and signed by the lab instructor upon completion of each experiment. Before beginning connecting up, it is essential to check that all sources of supply at the bench are switched off. Start connecting up the experiment circuit by wiring up the main circuit path, then adds the parallel branches as indicated in the circuit diagram. After the circuit has been connected correctly, remove all unused leads from the experiment area, set the voltage supplies at the minimum value, and check the meters are set for the intended mode of operation. The students may ask the lab instructor to check the correctness of their circuit before switching on. When the experiment has been satisfactory completed and the results approved by the instructor, the students may disconnect the circuit and return the components and instruments to the locker tidily. Chairs are to be slid in properly.
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Experiment No. 1 OHM’S LAW OBJECTIVE • In this lab you will learn how to analyze simple circuits using Ohm’s Law
EQUIPMENT: • Digital Multimeter (DMM) •
Resistors:
•
E-PAL (Training board)
THEORY: Ohm's law states that the current through a conductor is directly proportional to the potential difference across the ends of the conductor. Mathematically V = IR
where I is the current (amperes), V is the potential difference (volts) and R is the resistance of the conductor (ohms). If, R is constant than there is a linear relation between V and I.
Fig.1: Ohm’s law
The law was named after the German physicist Georg Ohm. The slope of a V(I) graph is R. This relationship can be checked by measuring the current through and voltage across each resistor. When measuring current be sure to place the ammeter in series with the components in question. Current flows through a circuit. When measuring voltage be sure to place the voltmeter in parallel with the resistance. Voltage drops across a resistor. No doubt you noticed in the DC Circuits lab that voltages recorded across the series components very nearly summed to the power supply voltage. And it's almost certain that you discovered that 109717267.doc 7
voltage measured across each component in the parallel configuration was quite close to the power supply setting. Therefore, your astute observations should help you with this experiment as well.
CIRCUIT DIAGRAM:
Fig.2 Resisters in series
Fig.3 Resister in parallel combination
Procedure: 1. Connect the circuit according to circuit diagram of Figure 2.
2. Let R1 = 180 Ohms, R2 = 220 Ohms, R3 = 330 Ohms 3.
Set the power supply to 12 volts.
4.
Measure the current (in milliamps) through and voltage drop (in volts) across each resistor.
5. Connect the circuit according to circuit diagram of Figure 3.
6. Let R1 = 180 Ohms, R2 = 220 Ohms, R3 = 330 Ohms. 7. Set the power supply to 3 volts. 8. Measure the current (in milliamps) through and voltage drop (in volts) across each resistor. 109717267.doc 8
9. Also measure the current between the + side of the battery or power supply and R1.
CALCULATIONS: For circuit # 1 (series): 1. Calculate the total resistance. 2. Calculate the theoretical current (in Amps) value using the total resistance value and the
value of the input voltage. 3. Calculate the theoretical value of the voltage drop across each resistor by using the
theoretical current value. 4. Calculate the percent error for the current (use the average of your measured values (in
Amps) as your experimental value). 5. Calculate the percent error for the voltage drops across each resistor.
For Circuit # 2 (parallel): 1. Calculate the total resistance. 2. Calculate the theoretical value of the total current (in Amps) by using the total resistance
and the value of the input voltage. 3. Calculate the theoretical value of the current (in Amps) through each resistor. You may
assume that the voltage across each resistor is the same as the input voltage. 4. Calculate the percent error for the total current. 5. Calculate the percent error for the values of the currents through each resistor.
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RESULTS & CALCULATIONS: For circuit # 1: RT (calculated) = ___________, I(calculated) using calculated value of RT and input voltage V= ___________ VR1 (calculated) = ___________, VR2(calculated) = ___________ VR3 (calculated) = ___________
For circuit # 2: RT (calculated) = ___________, I(calculated) using calculated value of RT and input voltage V= ___________ IR1 (calculated) = ___________, IR2(calculated) = ___________ IR3 (calculated) = ___________
DISCUSSION AND CONCLUSION: 1) ------------------------------------------------------------------------------------------2) ------------------------------------------------------------------------------------------3) --------------------------------------------------------------------------------------------
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Experiment No. 2 MELDE’S EXPERIMENT OBJECTIVE: •
In this lab you will learn how to determine the frequency of AC supply.
EQUIPMENT: •
A stand with clamp and pulley
•
A light weight pan
•
A weight box
•
Balance
•
A battery with eliminator and connecting wires
Fig.1 Melde's experiment
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THEORY: “ An experiment to study transverse vibrations in a long, horizontal thread when one end of the thread is attached to a point of a vibrator , while the other passes over a pulley and has weights suspended from it to control the tension in the thread. “ Melde's experiment is a scientific experiment carried out by the German physicist Franz Melde on the standing waves produced in a tense cable originally set oscillating by a tuning fork, later improved with connection to an electric vibrator. This experiment attempted to demonstrate that mechanical waves undergo interference phenomena. In the experiment, mechanical waves traveled in opposite directions from immobile points, called nodes. These waves were called standing waves by Melde since the position of the nodes and loops (points where the cord vibrated) stayed static.
Transverse wave motion: A transverse wave motion is that wave motion, in which individual particles of the medium execute simple harmonic motion about their mean position in a direction perpendicular to the direction of propagation of wave motion.
Fig.2 Transverse wave
Tension: In physics, tension is (magnitude of) the pulling force exerted by a string, cable, chain, or similar solid object on another object.
PROCEDURE: 1) Find the weight of pan and mass of thread 1cm in length. 2) Set the apparatus according to figure. 3) Excide the steel strip by passing A.C. current through the coil of electromagnet and put a 109717267.doc 12
small weight in the scale pan. The thread will vibrates under the forced vibration of steel strip. 4) Take the readings according to table and calculate the frequency by using this formula:
n=1/2l(√T/m) 5) Increase the weights in pan and calculate the frequency again.
Fig.3 Block diagram of melde’s experiment
RESULTS & CALCULATIONS: Table 1
No. of Obs.
Mass of weigh t in pan gms
1 2 3 4
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No. of loops p
Distan ce betwe en extrem e nodes L cms.
Lengt h of each loop l=L/p
Mass of the pan+ wts. Added to it W
Tension in dynes T=Wx9 81
n=1/2l√(T/ m)
cms.
gms
dynes
vib./sec.
DISCUSSION AND CONCLUSION: 1. ------------------------------------------------------------------------------------------2. -------------------------------------------------------------------------------------------
3. --------------------------------------------------------------------------------------------
Experiment No.3 TO STUDY POTENTIAL DIVIDER CIRCUIT OBJECTIVE: •
In this lab you will learn how to use potentiometer as a voltage divider.
EQUIPMENT / COMPONENTS REQUIRED: • • • •
DMM TRAINER BOARD Resistor Variable resistor
LAB SAFETY CONCERNS: • • • •
Make sure all circuit connections are correct, and no shorted wires exist. Adjust the power supply to the proper voltage before connecting it to the circuit Adjust signal generator to the proper level before connecting it to the circuit When using electrolytic capacitors, the arrows must point to the negative side of current flow.
THEORY: A potentiometer is a manually adjustable electrical resistor that uses three terminals. In many electrical devices, potentiometers are what establish the levels of output. For example, in a loudspeaker, a potentiometer is used to adjust the volume. In a television set, computer monitor or light dimmer, it can be used to control the brightness of the screen or light bulb.
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Fig.1 Potentiometer
Fig.2 Circuit diagram of potentiometer
Working: Potentiometers, sometimes called pots, are relatively simple devices. One terminal of the potentiometer is connected to a power source, and another is hooked up to a ground — a point with no voltage or resistance and which serves as a neutral reference point. The third terminal slides across a strip of resistive material. This resistive strip generally has a low resistance at one end, and its resistance gradually increases to a maximum resistance at the other end. The third terminal serves as the connection between the power source and ground, and it usually is operated by the user through the use of a knob or lever. The user can adjust the position of the third terminal along the resistive strip to manually increase or decrease resistance. The amount of resistance determines how much current flows through a circuit. When used to regulate current, the potentiometer is limited by the maximum resistivity of the strip.
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Fig.3 Circuitry of variable resistor
PROCEDURE: Construct the circuit given in figure below
Fig.4Circuit diagram
CALCULATIONS 1) Calculate & Measure the value of Vout. 2) What happens if 9k is replaced by 1k? 3) What happens if 9k is replaced with 2k? 4) What happens if 1k is replaced with zero? 5) .Replace +12V with ground. Measure Vout and answer all of the above questions.
CONCLUSION: 1. --------------------------------------------------------------------------------------------------2. --------------------------------------------------------------------------------------------------109717267.doc 16
3. ---------------------------------------------------------------------------------------------------
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Experiment No. 4 RECTIFIER CIRCUIT OBJECTIVE: To demonstrate current flow in Rectifier Circuits by using LED’s
EQUIPMENT / COMPONENTS REQUIRED: • • • •
DMM Trainer Board Resistor Diode
THEORY: Half-Wave Rectifiers An easy way to convert ac to pulsating dc is, to simply allow half of the ac cycle to pass, while blocking current to prevent it from flowing during the other half cycle. The figure below shows the resulting output. Such circuits are known as half-wave rectifiers because they only work on half of the incoming ac wave.
Fig.1 Half Wave Rectifier
Fig.2 Output of Half wave rectifier
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Full-Wave Rectifiers The more common approach is to manipulate the incoming ac wave so that both halves are used to cause output current to flow in the same direction. The resulting waveform is shown below. Because these circuits operate on the entire incoming ac wave, they are known as full-wave rectifiers.
Fig.3 Full-Wave Rectifier
Fig.4 output of Full wave rectifier
PROCEDURE: For half wave rectification • • •
Construct the circuit given in figure below. Observe the output using oscilloscope. Make the following measurements and take the snap short of output signal. .
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Fig.5 Circuit Diagram
CALCULATIONS Table 1 Sr. no.
GRAPH
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Input
Output
(Vrms)
(V)
1
20
2
30
For full-wave rectification •
Construct the circuit given in figure below.
Fig.6 Circuit Diagram • •
Observe the out put using oscilloscope. Make the following measurements and take the snap short of out put signal. Table 2 Sr. no.
GRAPH
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In put
Out put
Vrms
V
1
20
2
30
CONCLUSION: 1. -----------------------------------------------------------------------------------------------------------2. -----------------------------------------------------------------------------------------------------------3. ------------------------------------------------------------------------------------------------------------
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Experiment No. 5 ACCEPTOR CIRCUITS OBJECTIVE: •
To study the characteristics of RLC series (acceptor) circuit
EQUIPMENT / COMPONENTS REQUIRED: •
Audio oscillator
•
Resistors (100-1000 ohm)
•
Capacitors (0.1µf)
•
Inductance (500mH)
•
DMM
THEORY: A given combination of R, L and C in series allows the current to flow in certain frequency ranges only. For this reason it is known as an acceptor circuit i.e., it accepts some specific frequencies. A series-resonant circuit that has a low impedance at the frequency to which it is tuned and a higher impedance at all other frequencies At resonance frequency, XC=XL and XL-XC = 0. Therefore, the only electrical characteristic left in the circuit to oppose current is the internal resistance of the two components and resistance used. Hence, at resonance frequency, Z = R.
Fig.1 Acceptor Circuit
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RESONANCE: For a certain frequency of the sinusoidal voltage applied to RLC series circuit the current flowing in the circuit has a maximum value. This phenomenon is called resonance.
RESPONSE CURVE: When RLC series circuit is excited by a sinusoidal voltage, the current changes with frequency of applied voltage. A graph between current n frequencies is known as the response curve.
Fig.2 Response curve
QUALITY FECTOR: It is the ratio between the resonance frequency and the bandwidth, i.e. Quality factor=Q= fr/∆f
PROCEDURE: •
Construct the circuit given in figure below.
Fig.3 Circuit diagram •
Set the oscillator to resonance frequency.
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•
Switch on the oscillator and observe the current flowing in the circuit with the help of multi-meter.
•
Observe the current by increasing and decreasing the frequency and note the corresponding current.
•
Record two more sets of observations for different values of R.
GRAPH:
CALCULATIONS:
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fr=………hertz
R ohm s
No. of Obs. freq. (Hz) Current (mA) freq. (Hz) Current (mA) freq. (Hz) Current (mA)
R1=
R2=
R3=
1
2
3
4
5
6
7
Results from graph: •
Lower limit frequency= f1 =………….hz
•
Upper limit frequency= f2 =………….hz
•
Bandwidth
•
Quality factor =Q=fr/∆f=………
= f2 – f1=∆f=……hz
DISCUSSION AND CONCLUSION: 1) ------------------------------------------------------------------------------------------2) ------------------------------------------------------------------------------------------3) -------------------------------------------------------------------------------------------
Experiment No. 6 REJECTER CIRCUITS 109717267.doc 26
8
9
OBJECTIVE: •
To study the characteristics of RLC parallel (rejecter) circuit
EQUIPMENT / COMPONENTS REQUIRED: •
Audio oscillator
•
Resistors (100-1000 ohm)
•
Capacitors (0.1µf)
•
Inductance (500mH)
•
DMM
THEORY: A given combination of R, L and C in parallel dis-allows the current to flow in certain frequency ranges only. For this reason it is known as a rejecter circuit i.e., it rejects some specific frequencies. A parallel-resonant circuit that has a high impedance at the frequency to which it is tuned and a low impedance at all other frequencies At resonance the impedance of the capacitor becomes equal to that of inductor (XC=XL) hence impedance is maximum so no current flows. At low frequency the capacitive reactance is high so all current flows through inductor and when the frequency is high all current flows through capacitor because at that point reactance of the capacitor is low. So we obtained a V shape graph. It’s also used in radio stations where we have to reject current for certain frequency range.
Zeq = (1/ZL) + (1/ZC) + (1/ZR)
Fig.1 Circuit diagram 109717267.doc 27
RESONANCE: For a certain frequency of the sinusoidal voltage applied to RLC parallel circuit the current flowing in the circuit has a minimum value. This phenomenon is called resonance.
RESPONSE CURVE: When RLC parallel circuit is excited by a sinusoidal voltage, the current changes with frequency of applied voltage. A graph between current n frequency is known as the response curve.
Fig.2 Response Curve
QUALITY FACTOR: It is the ratio between the resonance frequency and the bandwidth, i.e. Quality factor=Q= fr/∆f
PROCEDURE: •
Construct the circuit given in figure below.
•
Set the oscillator to resonance frequency.
•
Switch on the oscillator and observe the current flowing in the circuit with the help
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of multimeter. •
Observe the current by increasing and decreasing the frequency and note the corresponding current.
•
Record two more sets of observations for different values of R.
Fig.3 Circuit diagram
GRAPH:
CALCULATIONS:
fr=………hertz Table 1 R ohm
No. of Obs.
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1
2
3
4
5
6
7
8
9
s freq. (Hz) Current (mA) freq. (Hz) Current (mA) freq. (Hz) Current (mA)
R1=
R2=
R3=
Results from graph •
Lower limit frequency= f1 =………….hz
•
Upper limit frequency= f2 =………….hz
•
Bandwidth
•
Quality factor =Q=fr/∆f=………
= f2 – f1=∆f=……hz
DISCUSSION AND CONCLUSION: 1) ------------------------------------------------------------------------------------------2) -------------------------------------------------------------------------------------------
3) --------------------------------------------------------------------------------------------
Experiment No. 7 STUDY OF CAPACITORS OBJECTIVE: •
In this experiment we will determine how voltages are distributed in capacitor circuits and explore series and parallel combinations of capacitors.
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EQUIPMENT: • • •
DMM Trainer board Capacitors of different values
THEORY: The capacitance is a measure of a device’s ability to store charge. Capacitors are passive electronic devices which have fixed values of capacitance and negligible resistance. The capacitance, C, is the charge stored in the device, Q, divided by the voltage difference across the device, ∆V:
C = Q/∆V. (1) The SI unit of capacitance is the farad, 1 F = 1 C/V, In general, the capacitance can be calculated knowing the geometry of the device. For most practical devices, the capacitor consists of capacitor plates which are thin sheets of metal separated by a dielectric, insulating material. For this reason, the schematic symbol of a capacitor is has two vertical lines a small distance apart (representing the capacitor plates) connected to two lines representing the connecting wires (or leads).
There are two ways to connect capacitors in an electronic circuit - series or parallel connection.
SERIES: In a series connection the components are connected at a single point, end to end as shown below:
Fig.1 Capacitor in series
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For a series connection, the charge on each capacitor will be the same and the voltage drops will add. We can find the equivalent capacitance, Ceq, from
Q · 1/Ceq = ∆V = ∆V1 + ∆V2 = Q/C1 + Q/C2 = Q [1/C1 + 1/C2] (3) So
1/Ceq = 1/C1 + 1/C2 (4) PARALLEL: In the parallel connection, the components are connected together at both ends as shown below:
Fig.2 Capacitor in parallel
For a parallel connection, the voltage drops will be the same, but the charges will add. Then the equivalent capacitance can be calculated by adding the charges:
Ceq∆V = Q = Q1 + Q2 = C1∆V + C2∆V = [C1 + C2] ∆V
(5)
So Ceq =C1 + C2
(6)
NOTE: Here we will use AC, the voltage is actually ∆V = I/ωC, where I is the current and ωis the angular frequency. We don’t actually measure I or ω here, and the analysis is the same .
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PROCEDURE: From the Diagram below, perform the following steps as mentioned under the diagram.
CIRCUIT DIAGRAM:
Fig.3 Circuit diagram
1. Turn on the power supply and set the AC voltage to 10 V. Measure the accurate voltage with the multimeter and record it below: Vac = ___________________V 2. Connect two 0.1 µF capacitors in series. Measure V2 (across C2) and record it below. V2 (measured) = ____________ V 3. Compute the expected value of V2 using Vac, the values of C1 and C2 with equations 3 and 4. V2 (expected) = ____________ V % difference = |measured - expected| / measured x 100 % =__________ 4. Connect a third 0.1uF capacitor in parallel with C2. Compute their equivalent capacitance. Ceq = _________ µF. Measure and compute the voltage across the equivalent capacitance. Veq (measured) = ___________ V, Veq (expected) = _________V, % difference = __________ 5. Now remove the third capacitor and replace it with a 0.01uF capacitor. Compute their equivalent capacitance. Ceq = _________ µF.
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Measure and compute the voltage across the equivalent capacitance. Veq (measured) = ___________ V, Veq (expected) = _________V, % difference = __________ 6. Now connect the 0.1uF and the 0.01uF capacitor in series. Compute the equivalent capacitance. Ceq = _________µF. Measure and compute the voltage across the equivalent capacitance. Veq (measured) = ___________ V, Veq (expected) = _________V, % difference = __________ 7. This method can be used to find an unknown capacitance. Replace C1 with the unknown value capacitor and determine its capacitance by measuring V2 and using equations 3 and 4. V1 = ___________ V, C1 = _________ uF.
DISCUSSION AND CONCLUSION: 1) ------------------------------------------------------------------------------------------2) ------------------------------------------------------------------------------------------3) --------------------------------------------------------------------------------------------
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Experiment No. 8 STUDY OF RC WAVEFORMS OBJECTIVE: •
To study differentiator & integrator circuits
EQUIPMENT: • DMM • TRAINER BOARD • Resistor Oscilloscope
THEORY: Square Wave Signal: If we apply a Square Wave voltage to the RC circuit whose frequency matches that exactly of the 5RC time constant of the circuit, then the voltage waveform across the capacitor would look something like this:
Fig.1 5RC Input Waveform:
The voltage drop across the capacitor alternates between charging up to Vc and discharging down to zero according to the input voltage. Here in this example, the frequency (and therefore the time period, ƒ=1/T) of the input square wave voltage exactly matches that of the RC time constant, ƒ=1/RC, and the capacitor is allowed to fully charge and fully discharge on every cycle resulting in a perfectly matched RC waveform. If the time period of the input waveform is made longer (lower frequency, ƒ<1/RC) for example a time period equivalent to say "8RC", the capacitor would then stay fully charged longer and also 109717267.doc 35
stay fully discharged longer. As shown in fig.2. If however we reduced the time period of the input waveform (higher frequency, ƒ>1/RC), to "4RC" the capacitor would not have sufficient time to either fully charge or discharge and the resultant voltage drop across the capacitor, Vc will be less than its maximum input voltage as shown below in fig.3.
Fig.2 An 8RC Input Waveform:
Fig.3 A 4RC Input Waveform
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Frequency Responses The Integrator The Integrator is a type of Low Pass Filter circuit that converts a square wave input signal into a triangular waveform output. As seen above, if the RC time constant is long compared to the time period of the input RC waveform the resultant output will be triangular in shape and the higher the input frequency the lower will be the output amplitude compared to that of the input. Consider the output across the capacitor at high frequency i.e.
This means that the capacitor has insufficient time to charge up and so its voltage is very small. Thus the input voltage approximately equals the voltage across the resistor.
Fig.4 Circuit diagram
By ohm’s law
So
Thus output is the integral of input voltage.
The Differentiator The Differentiator is a High Pass Filter type circuit that converts a square wave input signal into high frequency spikes at its output. If the RC time constant is short compared to the time period of the input waveform the capacitor will become fully charged quickly before the next change in the cycle. When the capacitor is fully charged the output voltage across the resistor is zero. The arrival of the falling edge of the input waveform causes the capacitor to reverse charge giving a negative
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output spike, then as the square wave input changes during each cycle the output spike changes from a positive value to a negative value.
Fig.5 Circuit diagram
Consider the output across the resistor at low frequency i.e.,
This means that the capacitor has time to charge up until its voltage is almost equal to the source's voltage.
Thus the output voltage is the derivative of input voltage.
Cut-off Frequency In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced (attenuated or reflected) rather than passing through.
fc=1/2πRC Where RC is the time constant of the circuit previously defined and can be replaced by tau, T. This is another example of how the Time Domain and the Frequency Domain concepts are related.
PROCEDURE: 1. Draw the circuits of Differentiator & Integrator and verify the information provided above
in the discussion. 2. Do your waveforms match those that are provided in the figures above, Explain any differences.
GRAPH OF THE OUTPUT OF INTEGRATOR CIRCUIT 109717267.doc 38
GRAPH OF THE OUTPUT OF DIFFRENTIATOR CIRCUIT 109717267.doc 39
DISCUSSION AND CONCLUSION 1) ------------------------------------------------------------------------------------------109717267.doc 40
2) ------------------------------------------------------------------------------------------3) --------------------------------------------------------------------------------------------
LIGHT DEPENDENT RESISTOR (LDR) LIGHT DEPENDENT RESISTOR (LDR)
Experiment No. 9 LIGHT
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LIGHT DEPENDENT RESISTOR (LDR)
OBJECTIVE: •
In this lab you will learn how to use LDR in Circuit design
EQUIPMENT / COMPONENTS REQUIRED: • • • •
DMM TRAINER BOARD Resistor LDR
THEORY: A photo resistor or light dependent resistor is a component that is sensitive to light. When light falls upon it then the resistance changes. Values of the resistance of the LDR may change over many orders of magnitude the value of the resistance falling as the level of light increases. It is not uncommon for the values of resistance of an LDR or photo resistor to be several mega ohms in darkness and then to fall to a few hundred ohms in bright light. With such a wide variation in resistance, LDRs are easy to use and there are many LDR circuits available. LDRs are made from semiconductor materials to enable them to have their light sensitive properties. Many materials can be used, but one popular material for these photo resistors is cadmium sulphide (CdS).
Fig.1 LDR
How an LDR works It is relatively easy to understand the basics of how an LDR works without delving into complicated explanations. It is first necessary to understand that an electrical current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high
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resistance have very few free electrons, and therefore it is hard to make them move and hence a current to flow. An LDR or photo resistor is made any semiconductor material with a high resistance. It has a high resistance because there are very few electrons that are free and able to move - the vast majority of the electrons are locked into the crystal lattice and unable to move. Therefore in this state there is a high LDR resistance. As light falls on the semiconductor, the light photons are absorbed by the semiconductor lattice and some of their energy is transferred to the electrons. This gives some of them sufficient energy to break free from the crystal lattice so that they can then conduct electricity. This results in a lowering of the resistance of the semiconductor and hence the overall LDR resistance. The process is progressive, and as more light shines on the LDR semiconductor, so more electrons are released to conduct electricity and the resistance falls further.
PROCEDURE: 1. Construct the circuit according to fig1. 2. Start with, using a 100 resistor as the test resistor. 3. Make measurements of Vout first with the LDR in the light and then with the LDR in
the shade. Write these results in the table below: Table1 No. of Obs.
Fixed resistor value
Vout in the light
1 2 3 4 5
Voltage change= Vout in shade – Vout in light
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Vout in the shade
Voltage change
CIRCUIT DIAGRAM No. 1:
Fig.1 Circuit diagram
CONCLUSION: 1. With this circuit, is Vout HIGH or LOW in the light?
2. Which test resistor gives the biggest voltage change between light and shade? 3. Which resistor would you use to make your light sensor most sensitive to changes in illumination? 4. Repeat the procedure by changing the position of LDR and fixed resistor.
5. Now take Vout across the LDR
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CIRCUIT DIAGRAM 2:
Fig.2 Circuit diagram Table 2 No. of Obs.
Fixed resistor value
Vout in the light
Vout in the shade
Voltage change
1 2 3 4 5
CONCLUSION: 1. With the second circuit, is Vout HIGH or LOW in the light?
2. Which test resistor gives the biggest voltage change between light and shade?
3. Which resistor would you use to make your light sensor most sensitive to changes in illumination?
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Experiment No. 10 CAREY FOSTER’S BRIDGE OBJECTIVE: To determine the value of an unknown small resistance using a Carey Foster’s bridge
EQUIPMENT: • • • • • • • •
An unknown resistance A known resistance(variable) A DMM A dry cell A plug key A meter bridge with jockey A sorting plate A copper strip
THEORY: The Carey Foster bridge is an electrical circuit that can be used to measure very small resistances. It works on the same principle as Wheatstone’s bridge, which consists of four resistances, P, Q, R and S that are connected to each other as shown in the circuit diagram below. In this circuit, G is a galvanometer, E is a lead accumulator, and K1 and K are the galvanometer key and the battery key respectively. If the values of the resistances are adjusted so that no current flows through the galvanometer, then if any three of the resistances P, Q, R and S are known, the fourth unknown resistance can be determined by using the relationship
P/ Q= R/S
Fig.1 Wheatstone’s bridge 109717267.doc 46
PROCEDURE: The connections are made as shown in fig.2 such that potential divider acts as two equal resistances P & Q. Now connect the copper strip in right gap of the bridge and a known resistances (R) in the left gap. The null point is determined and it’s distance l′1 from the bridge is measured. Interchange the position of R and copper strip and note the distance l′ 2 of the new null point from the left end. Take at least three readings for different values of known resistances. Now the given wire whose resistance is to be determined (say Y) is placed in the right gap in place of copper strip and known resistance in left gap (X).Find the null point and the distance l1 and similarly l2 when X & Y are interchanged. Take at least three readings for different values of Y.
Fig.2 Block diagram Wheatstone’s bridge
FORMULAS USED For unknown resistance Y: Y=X-ρ(l2-l1)Ω Where X is the resistance introduced in the resistance box l1 is the length of the balance point in the bridge wire where resistance box is in the left gap l2 is the length of the balance point in the bridge wire where resistance box is in the right gap
The resistance per unit length (ρ )of the bridge wire is determined by: ρ= R/( l′2-l′1) ohm/cm Where R is the known resistance in the resistance box.
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CALCULATIONS: Table 1For determination of ρ Distance of the null point Sr. No.
R ohm
When X in left gap l′1 (cm)
When X in left gap l′2(cm)
Shift in balance point l′2-l′1 cm
ρ= R/( l′2-l′1)
ohm/cm
1 2 3 4 5
Table 2 For determination of X
Distance of the null point Sr. No.
X ohm
When Y in left gap l1 (cm)
When Yin left gap l2(cm)
Shift in balance point l2-l1 cm
Y=X- ( l2-l1)
ohm/cm
1 2 3 4 5
CONCLUSION: 1) What is the principle of Wheatstone bridge? ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------2) What is the principle of Carey Foster’s bridge bridge? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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3) When is the C.F. bridge most sensitive? -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Experiment No. 11 109717267.doc 49
SEXTANT OBJECTIVE •
To determine the height of inaccessible object.
EQUIPMENT: •
Sextant
•
Rigid stand with clamp
•
Measuring tape
•
Plump line spirit level
•
Colored chalks
THEORY A sextant is an instrument used to indirectly measure distances. A measuring tape or meter stick is incapable of measuring the distance to a star. Sextants have been used for centuries to make this sort of indirect measurement. In this activity sextants will be made and used to measure the height of a very tall object, such as a flagpole or a building.
Fig.1 Sextant
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PROCEDURE 1. Choose an observation point from which you can clearly see both the top and the bottom of
the object you wish to measure. Determine the exact distance between the observation point and the base of the object. 2. Set the sextant to zero and look at the object through the eyepiece, adjusting your view until it is in the center of the frame. 3. Adjust the sextant arm to split the screen in two halves. Continue moving the arm until the top half of the object on one side of the image is aligned with the bottom half of the object on the other side of the image. 4. Use a scientific calculator to find the height of the object by multiplying its distance from the observation point by the tan of the angle that you measured. For example, if you were 150 feet from the base of the object, and the recorded angle was 75 degrees, the height of the object would be 150 x tan 75 = 560 feet.
OBSERVATIONS & CALCULATIONS • •
Vernier constant of the sextant =………… Initial reading of the vernier against SS for the two mirrors parallel =………… • Zero correction =………….. • Height of the mark Q above the ground =……........
Table 1 Sextant at point R
Sr. No. 1 2 3
Initial reading A
Final reading B
Angle θ1=B-A
Sextant at point S
Initial reading C
Final reading D
Angle θ2=D-C
Horizontal distance between the points R & S (d)
DISCUSSION AND CONCLUSION 1) ------------------------------------------------------------------------------------------109717267.doc 51
Vertical distance between the top and reference mark Q h=d/ (cotθ2 -cotθ1)
2) ------------------------------------------------------------------------------------------3) --------------------------------------------------------------------------------------------
(SEMESTER PROJECT) ELECTRIC MOTOR
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OBJECTIVE: •
The objective of this project is to build a simple electric motor in order to explore the inter-relationship of electricity & magnetism.
THEORY: An electric motor is an electromechanical device that converts electrical energy into mechanical energy. Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo; some electric motors can also be used as generators, for example, a traction motor on a vehicle may perform both tasks. Electric motors and generators are commonly referred to as electric machines.
USES: Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives.
OPERATING PRINCIPLE: Nearly all electric motors are based around magnetism (exceptions include piezoelectric motors and ultrasonic motors). In these motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft. One, or both, of these fields must be made to change with the rotation of the motor. This is done by switching the poles on and off at the right time, or varying the strength of the pole.
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