Triode - definition

Triacs is a semiconductor Instrumentation designed to control the flow of alternating current (AC), acting as a bidirectional controlled switch. It is an extension of the functionality of the thyristor, allowing current to be conducted in both directions with a suitable gate pulse. Thanks to this property, the triac can be used for power control in AC circuits without the need for Bridge rectifiers or additional switching elements.

The design of the triac is based on a five-layer structure consisting of alternating p-type and n-type regions, forming two symmetrical circuits resembling two coupled Thyristors. The element has three electrodes: the main electrodes designated MT1 (Main Terminal 1) and MT2 (Main Terminal 2), through which the load current flows, and a gate electrode (G - Gate) to which the control pulse is applied. Conduction starts when the voltage between MT1 and MT2 reaches a threshold value and a corresponding pulse is delivered to the gate. Once in the conduction state, the triac remains open until the load current drops below the holding value.

Triacs are widely used in power control systems, lighting dimmers, single-phase motor speed controllers, heater controllers and Solid State Relays. Their ability to switch with alternating voltages in both directions allows for simplified actuator circuit design, particularly in domestic and industrial applications. When designing circuits with triacs, it is important to take into account parameters such as current and voltage rating, turn-on current, gate sensitivity and resistance to disturbances and transients in the mains supply. To ensure proper operation and protection of the triac, additional components such as varistors, diacs or snubber circuits are often used.

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