The silicon controlled rectifier and the triac are both solid-state electronic components that turn electrical currents on and off. Unlike some switches, which return to a stable "off" state, SCRs and triacs "latch" on or off, and remain that way until certain conditions change. Because of their switching and latching actions, both devices are called thyristors. While they have many similarities, important differences exist between their operation and use.
Silicon Controlled Rectifier
An SCR is a modified diode that conducts electricity in one direction, blocking it from going the other way. The diode is a two-lead device; the leads are called the cathode and the anode. The SCR has a third lead called the gate. Normally, the device does not conduct until it receives a voltage at the gate; then it remains on until the voltage across the cathode and anode drops past a critical point. It typically switches large currents many thousands of times per second.
About the Triac
Like an SCR, the triac has three leads and acts as a current switch but it is more complex than an SCR, because it conducts electricity in two directions. This makes the triac more useful in alternating current circuits than an SCR, because the current direction for AC changes 120 times per second.
Though a triac conducts in both directions, the diode conducts somewhat unequally in each direction. The SCR, when turned on, conducts in only one direction. The asymmetry of the triac's conduction complicates its use. As an AC circuit turns a triac on and off, the resulting waveform's positive and negative cycles become uneven, producing harsh electrical noise and interference.
Electric power control equipment, such as lamp dimmers and energy-saving circuits in appliances, might use either triacs or SCRs, depending on the circuit design. High-power industrial equipment uses SCRs. As a pair of SCRs can emulate a triac, and with fewer symmetry problems, designers prefer these devices in high-voltage and high-current environments. The switching asymmetry in triacs limits their use to lower-power applications.