Power electronics digital notes b. Tech III year

Resistance Firing Circuit 
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The circuit below shows the resistance triggering of SCR where it is employed to drive the load 
from the input AC supply. Resistance and diode combination circuit acts as a gate control circuitry 
to switch the SCR in the desired condition. 

As the positive voltage applied, the SCR is forward biased and doesn’t conduct until its gate 
current is more than minimum gate current of the SCR. 

When the gate current is applied by varying the resistance R2 such that the gate current should be 
more than the minimum value of gate current, the SCR is turned ON. And hence the load current 
starts flowing through the SCR. 

The SCR remains ON until the anode current is equal to the holding current of the SCR. And it 
will switch OFF when the voltage applied is zero. So the load current is zero as the SCR acts as 
open switch. 

The diode protects the gate drive circuit from reverse gate voltage during the negative half cycle 
of the input. And Resistance R1 limits the current flowing through the gate terminal and its value 
is such that the gate current should not exceed the maximum gate current. 

It is the simplest and economical type of triggering but limited for few applications due to its 
disadvantages. 

In this, the triggering angle is limited to 90 degrees only. Because the applied voltage is maximum 
at 90 degrees so the gate current has to reach minimum gate current value somewhere between 
zero to 90 degrees. 

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Figure: 1. 23. R Firing circuit for SCR and corresponding waveforms 
Resistance 
–
 Capacitacne (RC) Firing Circuit 

The limitation of resistance firing circuit can be overcome by the RC triggering circuit which 
provides the firing angle control from 0 to 180 degrees. By changing the phase and amplitude of 
the gate current, a large variation of firing angle is obtained using this circuit. 

Below figure shows the RC triggering circuit consisting of two diodes with an RC network 
connected to turn the SCR. 

By varying the variable resistance, triggering or firing angle is controlled in a full positive half 
cycle of the input signal. 

During the negative half cycle of the input signal, capacitor charges with lower plate positive 
through diode D2 up to the maximum supply voltage Vmax. This voltage remains at -Vmax across 
the capacitor till supply voltage attains zero crossing. 

During the positive half cycle of the input, the SCR becomes forward biased and the capacitor 
starts charging through variable resistance to the triggering voltage value of the SCR. 

When the capacitor charging voltage is equal to the gate trigger voltage, SCR is turned ON and the 
capacitor holds a small voltage. Therefore the capacitor voltage is helpful for triggering the SCR 
even after 90 degrees of the input waveform. 

In this, diode D1 prevents the negative voltage between the gate and cathode during the negative 
half cycle of the input through diode D2. 

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