Question

Write short notes about firing circuits, then design a firing circuit to produce the following
angles 30, 90 and 120 degrees.

Answer 1

Anode Cathode Gate Thyristor is a controlled switch i.e. unlike diode, it doesn't get turn on as soon as anode voltage becomes greater than cathode voltage. For thyristor to start conducting, it needs two conditions simultaneously. 1. Voltage at anode greater than cathode 2. Gate signal (current injection in gate). ig 0 v10 0 Vo VO lo 0 a V +a Above waveform illustrate firing angle concept. You van see, anode voltage is positive at t>0 but both io and Vo are zero as thyristor is not conducting in absense of gate singal. CS Scanned with CamScanner

Now after some time t (or angle=2*pi*t/T, where T is time period of input sine wave), gate is fired with current pulse. As soon as gate is fired, both conditions for turning on of thyristor are fulfilled and it gets turned on and current starts flowing in circuit. Now angle at which thyristor gate is fired is called firing angle=2*pi*t/T. To turn on thyristor only a small pulse is required at gate but now to turn it off you need to make its current less than Holding current value.

0-120-3 fining angle = and لاہور == b 0-902 o's Arad T 30XT II rad 185 2K 3 9:30 - ant T ant T t = I art I= I 3 T it - I 12 TL TIL #= Tlz TIA T CSscanned with CamScanner