SCR Equivalent Circuit
Category: Transistor Logic
Designer: RANA MUHAMMAD ADNAN KHALIL
Notes
This circuit is designed and simulated in Proteus 7.10. This is an equivalent circuit of SCR (thyristor or silicon controlled rectifier). The working of this circuit is given below. 1. To understand this equivalent circuit of SCR, prior knowledge of NPN and PNP transistor is recommended. 2. As we know that npn transistor is turned ON at 0.7V (one logic). 3. The pnp transistor is turned ON at 0V (zero logic). 4. The SCR equivalent circuit is the combination of both PNP and NPN transistor. 5. The pnp transistor is placed at upper side and npn transistor is placed at lower side. 6. The collector of pnp transistor is connected with the base of npn transistor and this point is called "Gate" of SCR. 7. The base of pnp transistor is connected with the collector of npn transistor. 8. Now SCR is ready, the emitter of the pnp transistor is called "Anode" of SCR and emitter of npn transistor is called "Cathode" of SCR. 9. Now this equivalent circuit behave exactly as SCR behavior in circuit. 10. To understand the working of SCR see "SCR Test Circuit" in SCR, TRIAC and DIAC category. 11. To turn ON this SCR, first close the switch S1 and then close switch S2. 11. Once the npn transistor (Q2) is turned ON by giving positive pulse using switch S2 (closed), the Vce of Q2 (npn) will goes towards zero and this point is connected with the base of pnp (Q1) transistor and once the zero signal approaches base of Q1 (pnp), the Q2 will turn on. 12. When Q1 once turned ON, the collector of Q1 (pnp) is connected with base of Q2 (npn). Therefore the voltage and current it reached at base of Q2 (npn), now Q2 base is became independent of switch S2. 13. Now the SCR is permanently ON, because logics for both npn and pnp transistors are true. 14. To turn OFF this SCR you have to cut its main supply by opening the switch S1. Application: 1. This circuit can be used anywhere for replacement of SCR. 2. If you don't have SCR but have combination of npn and pnp transistor, and you need an SCR, SCR can be made using this equivalent circuit. Download these files for performing more experiments.