In power electronics, phase control circuits are widely used to regulate the average power delivered to a load by controlling the conduction period of a semiconductor device, most commonly a Silicon Controlled Rectifier (SCR) or Thyristor. The firing angle, often denoted as $\alpha$, is the delay from the beginning of the AC half-cycle to the point at which the SCR is triggered into conduction.

Resistive Trigger Circuit Explained

A pure resistive trigger circuit, also known as an R-trigger circuit, is the simplest type of gate triggering circuit for an SCR. In this circuit, the gate voltage ($V_g$) for the SCR is derived from the AC supply voltage through a variable resistance ($R$) and often a fixed resistance or a diode to protect the gate. The main characteristic of a purely resistive circuit is that all voltages and currents within it are in phase with the applied AC source voltage.

• The instantaneous gate current ($I_g$) is directly proportional to the instantaneous gate voltage ($V_g$), which in turn is in phase with the input AC voltage.
• For the SCR to turn ON, its gate current ($I_g$) must reach a certain minimum value, known as the gate trigger current ($I_{gt}$).

Firing Angle Limitation in R-Trigger Circuits

The fundamental limitation of a pure resistive trigger circuit stems from the phase relationship between the supply voltage and the gate voltage. Let's understand why the maximum firing angle is limited to 90°:

• The AC supply voltage (and consequently the voltage across the resistive trigger circuit) starts from zero at 0°, rises to its peak value at 90°, and then falls back to zero at 180° during a positive half-cycle.
• Since the gate current in an R-trigger circuit is in phase with the supply voltage, the maximum possible gate current is available only at the peak of the AC voltage, i.e., at 90°.
• If the required gate trigger current ($I_{gt}$) is not reached by the time the instantaneous supply voltage reaches its peak at 90°, it means that the maximum available gate current at 90° is less than $I_{gt}$.
• After 90°, the instantaneous supply voltage begins to decrease, which means the instantaneous gate current also decreases from its peak value.
• Therefore, if the SCR has not triggered by 90°, it will not trigger at all during that positive half-cycle because the gate current will only continue to fall below the value it had at 90°, making it impossible to reach $I_{gt}$ if it wasn't reached earlier.
• This inherent behavior of a purely resistive trigger network restricts the adjustable range of the firing angle. The SCR can only be triggered somewhere between 0° and 90° of the positive half-cycle.

Conclusion on Firing Angle Range

Due to the direct phase relationship between the gate voltage and the input AC supply voltage, a pure resistive trigger circuit can only provide sufficient gate current for the SCR to turn on within the first 90° of the positive half-cycle. This means the adjustable firing angle for such a circuit is limited from 0° to a maximum of 90°.