What is the primary characteristic of a TRIAC?

A TRIAC, which stands for Triode for Alternating Current, is primarily characterized by its ability to conduct in both directions during an AC cycle. This unique feature allows a TRIAC to control and switch AC loads effectively, making it a popular choice for applications such as light dimmers and motor speed controls. When a TRIAC is triggered, it allows current to flow through it regardless of the polarity of the voltage across its terminals, enabling it to conduct during both the positive and negative halves of the AC waveform.

In contrast, the other options do not accurately describe the functioning of a TRIAC. It does not conduct only after being switched on; rather, it can be turned on by an external signal. While it does interact with other components, it does not always need a DIAC to operate, as it can be directly triggered by gate signals. Additionally, a TRIAC is specifically designed for AC applications, rather than being limited to DC circuits. Thus, the defining feature is its ability to conduct during both halves of the AC cycle.