This document discusses different types of thyristors including silicon-controlled rectifiers (SCRs), triacs, and diacs. SCRs allow current to flow in one direction when triggered by a positive gate signal and can be used to control both AC and DC circuits. Triacs are bidirectional and can control current flowing in either direction when triggered by a positive or negative gate signal. Diacs are required for triggering triacs due to their nonsymmetrical triggering characteristics. The document provides schematics and diagrams of the different thyristor components.

1. Thyristors

2. Silicon-Controlled Rectifiers
Figure 24-1. Simplified SCR.

3. Silicon-Controlled Rectifiers (cont’d.)
Figure 24-4. Schematic symbol for an SCR.

4. Silicon-Controlled Rectifiers (cont’d.)
Figure 24-5. Common SCR packages.

5. Silicon-Controlled Rectifiers (cont’d.)
Figure 24-7. Removing power in a DC circuit.

6. TRIACs
Figure 24-9. Equivalent TRIAC.

7. TRIACs (cont’d.)
Figure 24-10. Simplified TRIAC.

8. TRIACs (cont’d.)
Figure 24-11. Schematic symbol for a TRIAC.

9. TRIACs (cont’d.)
Figure 24-12. Common TRIAC packages.

10. TRIACs (cont’d.)
Figure 24-13. TRIAC AC switch circuit.

11. TRIACs (cont’d.)
Figure 24-14. TRIAC AC control circuit.

12. Bidirectional Trigger Diodes
Figure 24-15. Simplified DIAC.

13. Bidirectional Trigger Diodes
(cont’d.)
Figure 24-17. Schematic symbol for a DIAC.

14. Bidirectional Trigger Diodes
(cont’d.)
Figure 24-18. Variable full-wave phase-control circuit.

15. Testing Thyristors
Commercial test equipment
Ohmmeter

16. Summary
Thyristors include SCRs, TRIACs, and DIACs
An SCR controls current in one direction by a positive gate
signal
SCRs can be used to control current in both AC and DC
circuits
TRIACs are bidirectional triode thyristors

17. Summary (cont’d.)
TRIACs control current in either direction by either a
positive or negative gate signal
Because TRIACs have nonsymmetrical triggering
characteristics, they require the use of a DIAC
DIACs are bidirectional trigger diodes
Thyristors can be tested using commercial transistor testers
or ohmmeters