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What is Diac?

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I have heard two different stories when it comes to these 5 layer devices. Some would say it's about the current, but I found one that deals with voltage. I like to deal with voltage, as the current is just the requirement which will maintain the voltage. A diac is basically a switch that is activated when the voltage across it exceeds a threshold value. Once it starts to conduct, the voltage across it drops to a minimum.

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Hi Suraj,
If you use an ordinary transistor instead of a a Diac in an AC circuit it would not drive the Triac properly and would likely overheat.

A Diac has equal threshold voltages for both polarities of the AC mains voltage. When the threshold voltage is reached across a Diac, it conducts heavily and therefore supplies a fairly high current (limited by a series resistor) to the Triac's gate.

An ordinary transistor with its base not used would breakdown like a Zener diode at about 50V to 80V with its normal polarity, and turn-on like a very high-voltage Diac. With the dimmer control set very dim or very bright, the Triac might not even turn-on during these half-cycles and operate like a half-wave rectifier.
When the voltage is reversed across the transistor, its C-B junction becomes forward-biased in series with its E-B junction that becomes reverse-biased. A silicon transistor's reverse-biased E-B junction breaksdown like a zener diode at about 6V, so will provide gate current to the Triac for this half-wave of the mains. With 6V across the E-B junction and the full Triac's gate current through it, the junction must dissipate a power that it was never designed to do.

I bet that your light dimmer does not dim smoothly without a Diac.

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