Principle of Operation of SCR

THYRISTOR Turn ON method:-

When Anode is positive with respect to Cathode then thyristor can be turned on by any one of the following method called triggering.

  1) Forward voltage triggering.

  2) Gate triggering.

  3) dv/dt triggering.

  4) Thermal triggering.

  5) Light triggering.

SCR (Silicon Controlled Rectifier):-

SCR stands for Silicon Controlled Rectifier. It is a four layer (P-N-P-N), three junction (J1, J2, J3) & three terminal {A (Anode), K (Cathode), G (Gate)} semiconductor switching device. It is unidirectional device that blocks the current flow from Cathode to Anode. It also blocks the current flow from Anode to Cathode until it is triggered into conduction by a proper Gate signal between Gate & Cathode terminals. SCR have very low resistance in forward conduction & very high resistance in reverse direction. The Voltage rating of SCR is 10KV & RMS Current rating is 3000A with corresponding Power handling capacity is 30MW.

Fig.-1 Schematic Diagram & Symbol of SCR

Two Transistor model of SCR:-

The principle of operation of SCR can be understand by Two Transistor model of SCR. By bisecting the two middle layer in two separate half we can convert the SCR in equivalent two transistor model. In Fig.-2 (J1-J2) considered as PNP & (J2-J3) considered as NPN transistor separately.

Fig.-2 Two Transistor model of SCR