Rashtrasant Tukadoji Maharaj Nagpur University 2011 B.E Electronics

Section-A

1. (a) Draw and discuss dynamic characteristics of a thyristor during its turn on and turn off processes. Show the variation if voltage across the thyristor and current through it during these dynamic processes. Indicate clearly the different intervals into which turn-on and turn-off times can be subdivided.
(b) discuss the characteristics of Silicon unilateral switch, Triac and GTO.

2. (a) What do you mean by commutation of SCR? elaborate the various methods of forced commutation? discuss class C commutation technique.
(b) discuss the working of PUT relaxation oscillator. What is the advantage of relaxation oscillator over UJT relaxation oscillator?

3. (a) A single phase full wave diode bridge rectifier is supplied with 230V, AC supply. Resistive load of 60O is connected across output of rectifier. compute DC and RMS power consumed by load. compute peak current flowing through load.
(b) explain the effect of source inductance on the performance of a single phase fully controlled converter indicating clearly the connection of different SCR during 1 cycle.

4. (a) discuss the operation of 3 phase bridge diode rectifier supplying lightly inductive load with suitable waveforms. Draw related waveforms.
(b) elaborate the purposes of filters in rectifier circuits? Differentiate ranging from a.c. and d.c. filters.

5. Write short notes on (any two):
(a) di/dt rating of SCR.
(b) Integral cycle control.
(c) Thermal runaway.

Section-B

6. (a) Show that for the midpoint single phase converter, the avg. reduction in load voltage due to commutation is directly proportional to load current. Draw neat output voltage and output current waveform.
(b) A single phase converter employs 2 SCR and centre tapped transformer with turn ratio 2:1. The load is inductive (R=10p and L=1mH) and delay angle is 40°. Determine the avg. load voltage, avg. load current. Supply voltage is 230V at 50Hz. The load current can be presumed to be ripple free.

7. (a) How are the drawbacks of basic series inverter overcome? discuss the working of replaced series inverter.
(b) elaborate the different techniques for voltage and harmonic control in inverter? discuss single pulse width modulation scheme.

8. (a) elaborate the various control techniques used in chopper? discuss.
(b) Draw and discuss the operation of 4 quadrant chopper.

9. (a) discuss the operation of 3-phase bridge inverter in 180° conduction mode. Draw output phase and line voltage waveform.
(b) discuss sinusoidal PWM as used in PWM inverter.

10. Write short notes on (any two):
(a) UPS
(b) Jones chopper
(c) AC voltage regulator.

Earning:   Approval pending.