Characteristics: Triac - nptel

1. Explain the effect of increasing the magnitude of the gate current and junction
temperature on (i) forward and reverse break down voltages, (ii) forward and reverse
leakage currents.
15 V
•
•
R
Th
i B
N 1 N 2
2. The thyristor Th is triggered using the pulse transformer shown in figure. The pulse
transformer operates at 10 KHZ with a duty cycle of 40%. The thyristor has maximum
average gate power dissipation limit of 0.5 watts and a maximum allow able gate voltage
limit of 10 volts. Assuming ideal pulse transformer, find out the turns ratio N 1 /N 2 and the
value of R.
Fuse
i 1
V i
220 V
50 HZ
i f
3. A thyristor full bridge converter is used to drive a dc motor as shown in the figure. The
thyristors are fired at a firing angle ∝ = 0° when motor runs at rated speed. The motor has
on armature resistance of 0.2 Ω and negligible armature inductance. Find out the peak
surge current rating of the thyristors such that they are not damaged due to sudden loss of
field excitation to the motor. The protective fuse in series with the motor is designed to
disconnect the motor within 1 2
cycle of fault. Find out the rating of the
2 ∫ i dt
thyristors.
4. Why is it necessary to maximize the peripheral contact area of the gate and the cathode
regions? A thyristor used to control the voltage applied to a load resistance from a 220v,
Version 2 EE IIT, Kharagpur 33