Q.Explain the construction and working of SCR half wave rectifier with RC triggering circuit.

The SCR half-wave rectifier with RC triggering places the load in series with the SCR across a transformer secondary, and an RC network fires the gate at an adjustable firing angle so that conduction occurs only from $\alpha$ to $\pi$ in each positive half cycle. …

[!TLDR]

RC-triggered SCR half-wave rectifier: the capacitor voltage lags the input, fires the SCR at $\alpha$, and load voltage appears from $\alpha$ to $\pi$ with $V_{dc}=\frac{V_m}{2\pi}[1+\cos\alpha]$.

Construction: The ac supply feeds a transformer whose secondary has terminals A (top) and B (bottom). The load resistor $R_L$ is connected in series with the SCR (anode A, cathode K, gate G) across the secondary. The gate is driven by an RC triggering circuit, which is the most common form of thyristor power control. It consists of a series resistor $R_1$, a variable resistor $VR_1$, a capacitor C, a diode $D_1$ in series with the gate, and a diode $D_2$ that prevents a negative gate-cathode voltage. The gate signal is taken from the junction of $D_1$ and the RC network.

Working: During the positive half cycle terminal A is positive with respect to B. The capacitor charges through $R_1$ and $VR_1$, so the capacitor voltage lags the input by an angle set by $R_1$, $VR_1$ and C. When the high-end capacitor voltage rises to the trigger level, it supplies gate current through $D_1$ and the SCR fires. The instant of firing is the firing angle $\alpha$; from $\alpha$ to $\pi$ the supply voltage appears across the load. During the negative half cycle the SCR is reverse biased and does not conduct, so the load voltage is zero. Because conduction happens in only one half of each cycle, this is a half-wave rectifier. …