It is a two terminal devices.The diac semiconductor is a full-wave or bidirectional thyristor. It is triggered from a blocking-to-conduction state for either polarity of applied voltage whenever the amplitude of applied voltage exceeds the breakover voltage rating of the diac.It Can conduct in either direction Three layer low voltage, low current semiconductor switch .Commonly used to trigger SCRs and TRIACs
- The DIAC, or "diode for alternating current", is a diode that conducts current only after its breakover voltage, VBO, has been reached momentarily.
- When this occurs, the diode enters the region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode.
- The diode remains "in conduction" until the current through it drops below a value characteristic for the device, called the holding current, IH.
- Below this value, the diode switches back to its high-resistance (non-conducting) state.
- This behavior is bidirectional, meaning typically the same for both directions of current.
- Most DIACs have a three-layer structure with breakover voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.
- DIACs have no gate electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs. Some TRIACs, like Quadrac contain a built-in DIAC in series with the TRIAC's "gate" terminal for this purpose.
- DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2.