What does a diac do in electronic circuits?

A diac is a semiconductor device that functions primarily as a switching device in electronic circuits. Its main characteristic is that it allows current to flow only after a specific breakdown voltage, known as the trigger voltage or breakover voltage, is reached.

When the voltage across a diac rises and hits this threshold, the diac becomes conductive, allowing current to flow through it. This behavior is crucial in applications such as phase control in light dimmers and motor speed control, where the diac provides a means to switch on the current at a precise voltage level.

The other options do not accurately describe the specific role of a diac. While diacs may influence current flow, they do not measure voltage or limit current directly, nor do they function as simple on/off switches that close at any voltage. Instead, their operation relies heavily on reaching that particular breakdown voltage, making the correct understanding of their function vital in electronic circuit design.