In an RC half wave triggering circuit, the firing angle can be ideally varied between 0 to 180 degrees. Let's understand how this is possible by discussing the working principle and characteristics of the circuit.

Working Principle of RC Half Wave Triggering Circuit:
The RC half wave triggering circuit is an electronic circuit used to control the firing angle of a thyristor. It consists of a resistor (R) and a capacitor (C) connected in series with a thyristor (SCR) and a load. The gate terminal of the thyristor is connected to the junction between the resistor and the capacitor.

When the AC supply voltage is applied across the circuit, the capacitor charges through the resistor. The charging time constant (RC) determines the rate at which the capacitor charges. Once the capacitor voltage reaches the threshold voltage level of the thyristor, it triggers the thyristor into conduction.

Characteristics of RC Half Wave Triggering Circuit:
1. Firing Angle: The firing angle is the delay between the instant when the supply voltage reaches its positive peak and the instant when the thyristor is triggered. In an RC triggering circuit, the firing angle can be varied by adjusting the values of the resistor and the capacitor. By increasing the value of either component, the charging time constant increases, resulting in a longer delay and a larger firing angle. Conversely, decreasing the values of the resistor and the capacitor decreases the charging time constant, leading to a shorter delay and a smaller firing angle.

2. Firing Range: The firing range of the RC triggering circuit is determined by the time required to charge the capacitor to the threshold voltage level of the thyristor. As the charging time constant (RC) increases, the firing range also increases. Similarly, as the charging time constant decreases, the firing range decreases. In an ideal scenario, the firing angle can be varied between 0 to 180 degrees by adjusting the values of the resistor and the capacitor appropriately.

Conclusion:
In summary, the firing angle of an RC half wave triggering circuit can be ideally varied between 0 to 180 degrees by adjusting the values of the resistor and the capacitor. By increasing or decreasing the charging time constant, the delay in triggering the thyristor can be controlled, thereby adjusting the firing angle. This ability to vary the firing angle makes the RC triggering circuit suitable for applications where precise control of power is required, such as in motor speed control or dimmer circuits.