I'm hoping someone on here can give me a simple explanation of what a TRIAC does.

To give this request some context, I'm an embedded systems software engineer, so I interface with hardware, but I have no formal training in it. I was looking over a schematic for work and I saw a bunch of TRIACs being placed, but I am not familiar with this. I read the Wiki, but it's a bit much for me.

Can some one give me a basic example of what a TRIAC does and why you'd need them?

EX. I know a resistor, resists the flow of electricity
I know that a capacitor stores electric charge
I know diode's allow the flow of current in only one direction.

So like these basic concepts, can someone explain a TRIAC to me? Thanks.


3 Answers 3


A triac is used to control an AC load, just like a transistor can be used to control a DC load. It is drawn like a couple of diodes in each direction which are then triggered to turn on. They will turn off when the current drops below a certain threshold. So if you just want it on or off you leave the signal on or off and at the next zero-crossing of the AC it will be in that state.

You can also get fancy and turn it on only sometimes, which is where the microprocessor programming comes in. Once you have a zero cross detector you can then turn it on in a "gentle" manner as well as keeping trac of percentages on/off if you are trying to reduce the power or control a speed by leaving it off for some cycles.


A triac is basically an electronic switch for alternate current. What it does is to close a contact between two terminals when a current is applied to its "Gate". It will continue allowing current to flow until it drops below a certain threshold.

It can be seen as an alternative to the relay.

See also this question

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    \$\begingroup\$ Note that when you use it as an alternative to a relay it does not provide insulation. \$\endgroup\$ Sep 9, 2013 at 8:35

A triac is a structure which can be seen as two transistors intertwined. The transistors act as switches. When the triac is fired (pulse on the gate) one transistor is switched on, which in turn switches the second transistor on. Then the second transistor keeps the first one switched on, even after the pulse was stopped. That way a short pulse switches on the triac, and both transistors keep each other on until the load current is removed.


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