Firing Circuits:

Gate triggering is the most commonly used turn-on method employed to switch on the thyristors. Triggering circuits is also called firing circuits. There are various firing circuits available. R-Firing circuits is simple but suffer from limited firing circuits. Firing angle is limited between 0^o to 90^o.

In actual practice firing angle can be varied between 3^o to 90^o. Limitation of the firing angle range of R-Firing circuit is eliminated by introducing a capacitor and a diode. Thus R-C firing circuits can increase the firing angle limitation range. Theoretically firing angle can be varied from 0^o to 180^o. However due to low voltage at 0^o and 180^o thyristor cannot be turn-on. Hence practically the range of firing angle is between 3^o and 177^o.

Both R and R-C firing circuits suffer from following disadvantage:

• They can be employed in power circuits having only one thyristor
• They are capable of open loop control only
• Due to lower voltages near 0^o to 180^o, gate current is small. Especially in R-C firing circuit, near 180^o gate current is minimum due to maximum value of R. This will increase the turn on time, especially for R-L load, leading to higher turn on loss
• Higher frequency gate signal is desirable for reliable turn on. Both the circuits are not capable of providing the same
• There is no electrical isolation between control circuit and power circuit

However the circuits are simple and cheap. R-C firing circuits is widely used in low power thyristor controllers, such as solid state ac regulators for speed control of fans and blowers. R-C firing circuits can also acts as snubber circuits