What Is a TRIAC?
TRIAC stands for “Triode for Alternating Current” and is a type of semiconductor switch with three terminals.
TRIAC has a structure in which two thyristors are connected in parallel so that they are in opposite directions to each other, and the bi-directional current switching operation can be controlled by a single gate.
Due to their ability to carry current in both directions, TRIACs are used as AC switches. AC power can also be easily controlled by shifting the phase of the gate input.
Uses of TRIACs
TIRACs are used in remote control switches for high-power home appliances such as TVs and air conditioners because they can control high-current switching with a small gate signal.
In addition, since the amount of power can be controlled by shifting the phase of the gate input relative to AC, TRIACs are used in various applications, including dimmers for lighting, ballasts to maintain a constant current for fluorescent lights, speed control of motors for fans, air conditioners, washing machines, etc. They are also used for temperature control of refrigerators, speed control of AC trains, and industrial equipment using motors.
Principle of TRIACs
Thyristor, which constitutes TRIACs, has a PNPN four-layer structure, which can be represented by an equivalent circuit consisting of PNP-type and NPN-type bipolar transistors combined and connected to PNP-type gate, and NPN-type anode, PNP-type cathode, and NPN-type gate.
When a gate signal is input and a forward voltage is applied between the anode and cathode, the two transistors enter the ON state. The on-state of both transistors is positively returned to each other’s gate input, resulting in a stable on-state. Once the current begins to flow between the anode and cathode, it continues to flow even when the gate signal is lost.
When a reverse voltage is applied between the anode and cathode, the thyristor enters off-state and the current is interrupted. Thus, when an alternating current is applied between the anode and cathode of the thyristor, it works by supplying power for only half of an AC cycle and blocking the current in the reverse direction.
TRIACs consist of two thyristors that operate in this manner, connected in parallel so that they are in opposite directions to each other.
When a gate current is applied, the thyristor connected in the forward direction turns on, and current flows only as long as the forward voltage is applied to the TRIACs. At the end of the half-cycle of alternating current, the thyristor that was in the ON state becomes reverse biased and goes off state, and no current flows.
Then, when the gate current is input again in the second half cycle of reverse bias, the thyristor on the opposite side turns on this time. In this way, a single gated input controls the switching timing of the current in both directions.
In addition, shifting the phase of the gate current relative to the AC current changes the time that the TRIACs are in the ON state, thereby controlling the amount of power supplied.