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 circuit.

R-triggering and RC-triggering circuit notes : Click here to download

 

Turning off SCR [Commutation]

Commutation

• To turn ON a SCR, a low voltage, short duration pulse is applied to the gate (typically 4V, 100µs).
• Once the SCR is turned-on, the gate loses control and the SCR will only turn off when the load current falls virtually to zero, or the SCR is reverse biased.
• The SCR will turn off naturally with a.c. supplies as the voltage reverses (which is called as Natural Commutation), but no such reversal occurs with d.c. supplies and it is necessary to force a voltage reversal if turn-off is to occur. This process is called Forced Commutation.
• The process of turning OFF SCR is defined as "Commutation".
• In all commutation techniques, a reverse voltage is applied across the SCR during the turn OFF process.
• By turning OFF a SCR we bring it from forward conducting to the forward blocking mode.

      The conditions to be satisfied in order to turn OFF an SCR are:

• IA < IH ( Anode current must be less than holding current).
• A reverse voltage is applied to SCR for sufficient time enabling it to recover its  blocking state.

      There are two methods by which a SCR can be turned OFF.
                I. Natural Commutation
               II. Forced Commutation

Natural Commutation:

• In AC circuit, the current always passes through zero for every half cycle.
• As the current passes through natural zero, a reverse Voltage will simultaneously appear across the device. This will turn OFF the device immediately.
• This process is called as natural commutation, since no external circuit is required for this purpose. It is also known as line commutation.
• AC voltage controllers or phase voltage controllers are the example for the natural commutation.

Forced Commutation:

• To turn OFF a SCR, the forward anode current should be brought to zero for sufficient time to allow the removal of charged carriers.
• In case of DC circuits, the forward current should be forced to zero by means of some external circuits. This process is called as forced commutation.

1. Class A or Self commutation
2. Class B or Resonant commutation
3. Class C or Complementary commutation
4. Class D or Impulse or auxillary commutation
5. Class E or External pulse commutation