What is a DIAC? Operation Applications Circuits A DIAC is a bi-directional semiconductor switch that can be turned on in both forward and reverse polarities above a certain voltage: it is often used to provide defined switching for a triac. A DIAC is a full-wave or bi-directional semiconductor switch that can be turned on in both forward and reverse polarities. The name DIAC comes from the words  DI ode  AC  switch. The DIAC is an electronics component that is widely used to assist even triggering of a TRIAC when used in AC switches and as a result they are often found in light dimmers such as those used in domestic lighting. These electronic components are also widely used in starter circuits for fluorescent lamps. Although the term is not often seen, DIACs may also be called symmetrical trigger diodes - a term resulting from the symmetry of their characteristic curve. DIACs come in a variety of formats. As discrete components they may be contained in small leaded packag...