Understanding DIACs
In this video I explain the basics of the DIAC - what it does, how it works, how it can be used, and I show how it is used in a circuit with a TRIAC.
The actual word DIAC stands for DIode for Alternating Current and this gives a clue about what these electronic components do in an electronic circuit, often assisting in a switch circuit
A DIAC is a two terminal device with its circuit symbol looking a bit like two parallel diodes in opposite orientations closely placed and merged together The electrodes are designated MT, standing for main terminal that's MT1 and MT2 - they may also be called Anode 1 and Anode 2 on some occasions.
There are two types of structure used for DIACs, the three layer structure and a five layer version. There are some differences between the two, but essentially they perform the same function in a circuit.
In terms of its operation the DIAC is not like an ordinary semiconductor diode that allows current through in only one direction, but instead the DIAC allows current through after a certain breakdown voltage is reached - often it is called a DIAC switch for this reason. This is known as the breakover voltage - typically this is between about 25 and 48 volts. The DB3 is a popular DIAC - the DB3 DIAC has a breakover voltage of around 32 volts.
The most common use for DIACs is with TRIACs. TRIACS are widely used for AC mains control - one of the very popular applications is within light dimmers. However TRIACS are notorious because their switching characteristics are different in different halves of the cycle and that's not good.
To overcome this, a DIAC can be placed in the TRIAC circuit in the gate circuit. As DIACs have a nice defined breakover voltage in both directions, it means that when the DIAC reaches its breakover voltage, the TRIAC will be fired and it will switch.
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