A TRIAC is an electrical component that has two leads used to connect an alternating current (AC) and a third lead used to trigger the device. Unlike some other devices, such as transistors and diodes, a TRIAC can conduct current in either direction between its two conductive leads. The trigger portion of the device, called its gate, turns the device on or off to varying degrees. Using the gate in conjunction with the phase of an alternating current, a TRIAC can be set to allow only a portion of an AC signal to pass through it and often is used in devices such as dimmer switches and electrical motor speed controls.
The word TRIAC, created by merging triode with AC, was originally a trade name used by General Electric for its version of a silicon-based, gate-controlled, full-wave AC switch. Since its original release, however, the word has become the general name for all such devices. In proper terms, the devices are referred to as bidirectional or bilateral triode thyristors. Occasionally, the device is simply called a thyristor, which is convenient but not entirely accurate, as the device is essentially a configuration of two thyristors.
A thyristor is a specialized semiconductor device typically made of four layers of silicon fused together. The four individual layers of silicon are treated so that they possess the alternating electrical charges of positive-negative-positive-negative, or PNPN. Each end of the layers serves as a connector to access the thyristor. The positive end is the device’s anode and the negative end its cathode. A gate connection is also made to the positively charged layer sandwiched between the two negatively charged layers.
Under static conditions, the alternating layers of charge resist allowing an electrical current to flow through the thyristor. There is, however, a limit to the amount of voltage the device can resist. If the voltage applied to the device exceeds that limit, the device will succumb to an effect called an avalanche and begin to conduct the electrical current.
In order to control the thyristor, a negative voltage is applied to its gate. This alters the charge in the positive layer to a more negative inclination, which can trigger an avalanche. By varying the voltage at the gate, the avalanche point of the thyristor can be varied, allowing the device to conduct electrical current only at or above a predetermined voltage.
AC signals continuously alternate from full positive voltage toward zero voltage, then toward full negative voltage, back toward zero voltage, and then back toward full positive voltage again. This means that an AC signal is constantly changing its voltage level. As a result, by varying the gate voltage of a thyristor, the percentage of the AC voltage that can pass through the device can be varied and controlled.
Thyristors, however, can only conduct an electrical current in one direction, which will block half of the AC voltage in the same way a diode would. In order to use the full AC voltage, a TRIAC is constructed of two thyristors. By connecting the anode of one thyristor to the cathode of the other on one end, and the remaining cathode and anode on the other end, the two devices can conduct a single AC voltage in both directions. The two gates, also interconnected, allow one control signal at the gate to control an AC signal passing through the TRIAC. In this way, a TRIAC can provide any desired portion of an AC voltage to a device, such as a motor, and by varying the gate voltage, vary the speed of the motor.