The term 'diac',  which stands for 'diode for alternating current', refers to a three-layer two-terminal device that can conduct current in two directions. However, a diac only starts to conduct current when the voltage across it momentarily exceeds a certain threshold known as its 'breakdown voltage'.

Once triggered by a momentary voltage higher than its breakdown voltage, the resistance of the diac decreases abruptly.  This results in a sharp increase in current flowing through the diac and a corresponding decrease in the voltage across it.  This conducting state remains as long as the current flowing through the diac is higher than a current threshold known as the diac's 'holding current.'

Once the current through a conducting diac falls below the holding current, the diac switches back to its high-resistance or non-conducting state.

Prior to being triggered into conduction, a non-conducting diac exhibits negative resistance.  This means that increasing the voltage across a non-conducting diac will cause the current flowing through it to decrease, as long as the breakdown voltage is not reached.

Figure 1.  Photo of a diac (left) and the symbol for a diac (right)

Most diacs exhibit a breakdown voltage of around 30 V.  Unlike other thyristors such as the SCR or the triac, the diac has no gate electrode with which it can be triggered.  A diac's primary application is for triggering another device.

Diacs are also known as symmetrical trigger diodes because of the symmetry exhibited by their V-I characteristic curves. Because of this symmetry, the two terminals of the diac are not called 'anode' and 'cathode', and are instead referred to as MT1 and MT2.

See Also:   Triac;  Active Components;  More Articles