TRIAC

 Triode for Alternating Current or TRIAC is the proprietary eponym, for an electronic device that could conduct current in either direction while it is turned on, and is generally called as bilateral triode thyristor or bidirectional triode thyristor.

The TRIAC is basically a DIAC with a gate terminal. The gate terminal controls the turn-on conditions of this bilateral device. The gate current can control the action of the device in either direction. This is similar to that of the SCR. However, the characteristics of the TRIAC are somewhat different from those of the DIAC.

By placing the TRIAC in a circuit it is possible lo indicate how it works. In this arrangement the switch is used to select various conditions for the TRIAC. The load can be either a light bulb or an ac motor. When the switch is in position one there is no gale con­nection. The TRIAC docs not conduct. The motor docs not run. There is no trigger voltage applied lo the gale. In position two a diode is placed in the circuit and with is polarity so arranged to allow a trigger voltage applied lo the gale on the positive pulse of the ac applied to the circuit. The TRIAC conducts, but only one-half of the ac sine wave.

This means that only about one-hair to the normal current is applied to the motor. This is the same arrangement as with an SCR. An ac motor may have a problem with this type of pulsating dc voltage. When the switch is moved to position three, the full ac sine-wave voltage is applied to the gate with, of course, a reduc­tion in value caused by the resistor R. Now that both halves of the ac sine wave are applied to the gate, the TRIAC conducts full lime and the full value of ac is applied to the ac motor. The motor then runs at lull speed. R can be made a variable type and its value would then con­trol the amount of ac current that passes through the TRIAC and to the motor.

Questionnaire:

• What is TRIAC? Explain.