Q20Basic Electrical Engineering

What is an SCR? A Silicon Controlled Rectifier (SCR), widely known simply as a Thyristor, is a heavy-duty, robust, four-layer (P-N-P-N), three-junction semiconductor device. It features three terminals: Anode (A), Cathode (K), and Gate (G). Unlike a standard diode, it acts as a highly controllable unidirectional switch. It can handle massive industrial power loads (thousands of volts and amps) but requires a small trigger current pulse at the Gate terminal to initiate conduction. Once triggered into conduction, the gate loses control, and it remains 'ON' until the main current drops below a holding threshold.

V-I Characteristics & Operating Modes: The static Voltage-Current (V-I) characteristic curve is heavily dependent on the polarity of the applied voltage and the presence of gate current. It exhibits three distinct operating regions:

• 1. Forward Blocking Mode (Off-State): Anode is positive with respect to Cathode, but no Gate current is applied. Junctions J1 and J3 are forward-biased, but inner junction J2 is heavily reverse-biased. The SCR acts as an open switch blocking high forward voltages. Only a tiny forward leakage current flows.
• 2. Forward Conduction Mode (On-State): If the forward voltage is pushed past the Breakover Voltage ($V_{BO}$), or more safely, if a positive Gate current ($I_g$) pulse is injected, junction J2 undergoes avalanche breakdown. The SCR switches perfectly 'ON', acting as a short circuit. Voltage drops to a minimal 1-2V, and massive forward current flows limited only by the external load.
• 3. Reverse Blocking Mode (Off-State): Anode is made negative with respect to Cathode. Junctions J1 and J3 are reverse-biased, acting like two diodes in series blocking the current. Only a tiny reverse leakage current flows until the Reverse Breakdown Voltage ($V_{BR}$) is reached, which would destroy the device.