12/24V DC TO 240V AC INVERTER As supplied this kit is designed to convert 12V DC to 240V AC at 50Hz. With a few component changes the circuit can be ...
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. . L A T A F E B O T E V O R P D L U O C T U P T U O S R E T R E V N I E H T H T I W T C A T N O C Y D O B Y N A
BRIEF CIRCUIT DESCRIPTION Automatic switch on section of the circuit................Resistor
R1 provides current to forward biassed
diodes D1-D4 and produces a DC voltage of approximately 1.2V at N2. If any external load is present across the 240V output terminals this voltage is applied via the transformer secondary winding and the external load to the anode of D17, thus Q1 is turned on via R2, the collector of Q1 is pulled low, Q2 is turned on and it provides current to the 8.2V zener ZD1. Q3 is a series regulator that produces a regulated output voltage of approximately 8V. In summary, if a load is connected to the inverter, the 8V regulated rail will appear and the inverter will start up. If no load is present, the 8V regulated rail would drop to 0V and the only current drawn by the whole circuit is drawn by R1, approximately 200uA. Inverter section of the circuit.................IC1
(555) is configured as a square wave oscillator, its frequency
is adjustable by VR1. Emitter follower transistor Q4 and Q5 are used to drive a beat indicator led, LED1. This led flashes at the difference frequency between the output of IC1 and the applied input to the Test terminal....More on this in the construction and adjustment notes. The output from IC1 is also applied to two seperate time delay networks made up by D18/R13/C8 and D19/R14/C9/VR2. These networks delay the switch on of the Mosfets by approximately 3.3mS. This results in a drive waveform that produces a modified square wave output: 350V Peak, 1/3 off, 1/3 on, 1/3 on etc. The resultant waveform produced by the inverter has the same RMS and peak voltage (240V) as a standard sine wave mains voltage. One of the delay networks is made adjustable so that the two delay times can be made equal....More on this in the construction and adjustment notes. Two seperate inverter gate networks are used to drive their corresponding Mosfets in a push-pull action. Two seperate spike protection networks are connected across the Drains and the Sources of the paralleled Mosfets. CONSTRUCTION AND ADJUSTMENT NOTES
The Bipolar (BP) capacitor C2 is labelled as being 100uF on the PCB. A 2.2uF or a 6.8uF bipolar capacitor will be supplied for C2. Mosfets Q5-Q8 require some heat sinking, a piece of sheet aluminium with a total area of about 40 sq. cm should be sufficient for each pair of paralleled Mosfets in applications up to 300W. Note that the drains of the Mosfets (centre pins) are also connected to their mounting tabs. Q5 and Q6 could be placed on a seperate heatsink that is isolated from the rest of the circuitry. Same applies for Q7 and Q8. If the Mosfets are mounted on the same chassis they will all need to be insulated from the chassis. Normal guage mains type insulated wire can be used for wiring the transformer secondary to the output socket and N1 and N2 on the PCB. This wiring the wiring handles relatively low mains output current: Eg. 1A @ 240V with a 240W load. Heavy guage tinned copper wire can be used to interconnect the Mosfets as per the diagram. Alternatively you could strip and tin the red or black wire. Tinned wire from one of these should be OK for up to 300W operation. The transformer should have a centre tapped primary of approximately 8-0-8V and a secondary voltage of 240V. Its power rating determines the total maximum inverter output power capability. The test terminal has a very high input impedance. If one finger is held at this point whilst the other hand is used to grip a insulated lead which is connected to a mains supply LED1 would flash at the difference frequency. There is sufficient capacitance between the hand and an insulated mains lead to apply a standard 50Hz supply to the test terminal. Adjust VR1 until the led stops flashing, at this point the two frequencies are the same (50Hz). LED2 will light whenever there is an 8V rail and therefore indicates that the inverter is running. If points A-B were connected to each other the inverter would run. A switch which would serve as an ON-OFF could be placed across these two points. CONVERSION FOR 24V OPERATION
.......Mosfets Q5-Q8 should have a higher breakdown voltage, 100V
OK: Eg. IRF540's. The transformer required would be 16-0-16V primary and a 240V secondary. Q3 (25V) should ideally have a higher breakdown voltage, BD139 (60V) OK.
