When the voltage across a diac reaches a certain value, the device triggers on, passing from a high-impedance state to a very Iow-impedance state. The high-speed triggering characteristics of this component make it ideal as a trigger element for circuits using triacs.
Symbol
Figure 1 shows the symbol used to represent a diac.
Notice that the diac is a nonpolarized component because the triggering characteristics are the same in both directions.
Specifications
Associated to a part number, the diacs have a trigger voltage. Typical values are around 35 volts.
Where they are found
Although many applications exist with diacs as a central element of a circuit, the most common is as a trigger element of a triac as in the dimmer.
Speed controls for electric motors and dimmers are examples of circuits of where triacs are found, the triggering devices are the diacs.
Testing
The muItimeter can't be used to test diacs as they don,t have enough voltage to trigger the muItimeter and when turned off, they act as an open circuit. The best way to test a diac is using a circuit.
The muItimeter can only reveal if the diac is shorted when presenting a very Iow resistance.
