Contents
If the value of R G is set too high with respect to the supply voltage, the thyristor may not trigger. Once the circuit has been turned-“ON”, it self latches and stays “ON” even when the push button is released providing the load current is more than the thyristors latching current.
Which is the most common form of thyristor power control?
Phase control is the most common form of thyristor AC power control and a basic AC phase-control circuit can be constructed as shown above. Here the thyristors Gate voltage is derived from the RC charging circuit via the trigger diode, D1.
How is the thyristor connected to the DC supply?
The thyristor is forward biased and is triggered into conduction by briefly closing the normally-open “ON” push button, S 1 which connects the Gate terminal to the DC supply via the Gate resistor, R G thus allowing current to flow into the Gate.
How does an alternating AC supply affect a thyristor?
When connected to an alternating current AC supply, the thyristor behaves differently from the previous DC connected circuit. This is because AC power reverses polarity periodically and therefore any thyristor used in an AC circuit will automatically be reverse-biased causing it to turn-“OFF” during one-half of each cycle.
What does it mean to turn off a thyristor?
In thyristor technology the switching OFF of a thyristor is conventionally termed commutation. In a.c. circuits, when the current through a thyristor passes through its natural zero, a reverse voltage appears automatically and turns OFF the thyristor. This is a natural commutation. No external circuit is now required to turn OFF the thyristor.
Why does the thyristor keep the gate current on?
The gate current is kept on, at lower value, for some times after the thyristor turned on in order to avoid unwanted turn-off of the device; this is known as the “back-porch” current. A shaped gate current waveform of this type is shown in Fig. 3.36.
Can a mechanical switch be used with a thyristor?
If this is the case we could just replace the thyristor with a large mechanical switch. One way to overcome this problem and reduce the need for a larger more robust “OFF” switch is to connect the switch in parallel with the thyristor as shown.