Q3
(a) Draw a circuit diagram illustrating how a single thyristor ('silicon controlled rectifier') may be used to provide a variable voltage d.c. output from a single phase a.c. supply.
(b) Explain how the firing angle of the thyristor is varied.
(c) Sketch waveforms for the output voltage when the firing angle is:
(i) 60°;
(ii) 120°.
Reference Answer
### (a) Circuit Diagram: Single Thyristor Controlled Rectifier A single thyristor (Silicon Controlled Rectifier or SCR) can be used to create a variable DC output from a single-phase AC supply using a half-wave controlled rectifier circuit. The key components are: 1. **AC Supply (Vs)**: A single-phase sinusoidal voltage source. 2. **Thyristor (SCR)**: A three-terminal semiconductor device (Anode, Cathode, Gate) that acts as a switch. It allows current to flow from anode to cathode only when it is forward-biased (anode voltage > cathode voltage) AND a positive trigger pulse is applied to its gate terminal.
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