08-12-2012, 02:23 PM
DC CHOPPER CIRCUITS
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Procedure :
Study of triggering circuit
1. Plug-in the mains power chord on to a 230V A.C supply. Switch on S1, S2
Can be kept off.
2. Connect probe one across common and pin 2 of the first 555 IC with ground
lead clipped to common. Observe waveforms at various points (A,B,C,D)with
the other probe. Sketch them on a graph paper, with the display in the first trace as the reference. Vary the pulse width and frequency controls and
observe the variation in the above waveforms.
3. Connect one probe on the input(primary) of a pulse transformer and the other
on its output, the ground of the second probe being on the cathode point(k).
Switch on S2.Plot the output voltages of the points below waveforms noted earlier. (Note : Two GND can be connected here as the two sides are isolated). Switch off S1 and S2.
Class D chopper
1. Connect of the circuit as in fig. set current limit of power supply low, and the voltage to 15 volt. Switch S2 on panel off. Switch - on power supply and S1. (S1 on panel).
2. Connect the C. R. O probe across C. and check whether it has charged to about the supply the supply voltage with the dot as positive. Set pulse – width and frequency to minimum. Switch –on S2. The chopper goes into operation with a humming noise.
3. Observe on the C. R. O. the charge on the capacitor oscillates. Observe voltage waveform across Th1 , Th2 , C, L and the load. Observe also the current waveforms through Th1 ,C and load using the 0.1 standard resistance. Plot all of them on a graph paper in time phase relation with the reference. Note the turn off time offered by the circuit to Th1. Change pulse width and frequency, one-at-a-time and note variations in the waveforms. Note the input and output meter reading also.
4. With the C. R. O. displaying the Th1 turn-off, increase loading of the chopper till failure occurs. Repeat, now with the probe connected across C. Note the minimum turn-off time for which commutation does not fail and the rate of charging of the capacitor at the instant.
5. Connect an inductance in series with the rheostat and observe voltage waveform across C and the load with a F.W.D across the R-L load repeat the observations. (Note : In case of commutation failure, switch-off power supply and S2 switch-on power supply and S2, in that order). If the fault persists, look for the fault and switch on as above. Tabulate for Resistive and Inductive loads.