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 0o to 90o. In actual practice
firing angle can be varied between 3o to 90o. 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 0o
to 180o. However due to low voltage at 0o and 180o
thyristor cannot be turn-on. Hence practically the range of firing angle is
between 3o and 177o.
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 0o to 180o, gate current is small. Especially in R-C firing circuit, near 180o 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 circuit.