Thyristor circuits basics

Summary

Phase control and zero-voltage switching are the basic power control methods used in the SCR and triac circuits. Phase control is achieved by resistive or resistive—capacitive networks or by triggering pulses derived from such devices as diacs, unijunction transistors, two-transistor trigger circuits, and neon bulbs.

In order to turn off the thyristor, it is necessary to reduce the main current below the holding-current level. In an ac circuit, the current passes through zero every half cycle so the turn off of the thyristor also is assured every half cycle. The commutation of a thyristor in a DC circuit requires, however, the use of additional circuitry.

To avoid either false triggering or damage to a thyristor due to transients, some simple protective networks are usually recommended.

Practical applications of SCRs and/or triacs are shown in the circuits of light dimmers, universal motor controllers, heat controllers, and DC-to-AC inverters.