Inductive Timing Light
Question on switching AC with a Triac?
I have some confusion over how often the signal has to be applied to the Gate when switching AC on and off. It stems from the fact that I read that a Triac will stop conducting when the current goes to Zero, which happens 60 times a second with AC. Or so I beleive 
If I have a small signal applied to the Gate, when the AC crosses Zero, will it then stop conducting and I have to re-apply the signal to the Gate each time to reset it. Or, if I leave the signal on the Gate, will the Triac continue to conduct even though the AC will cross Zero.
I also realise for saftey reasons I should use an Opto-Isolator but I want to leave that out if this discussion in order to understand what is going on at the Gate in order to handle AC crossing Zero 60 times a second. We can also assume I am only switching lights on and off and not controlling brightness. Also no inductive loads.
Any help would be great.
The simple answer is that for low duty cycle switching, you can connect your gate drive transistor to the line side of the triac and just use it as a low-speed switch. (Be careful of the isolation requirements, as you noted in your question.)
If you are using the triac to delay the firing angle, alpha, then you will need to apply the gate signal on BOTH half cycles.
Remember, the triac is basically just two SCRs connected back-to-back, with shared anodes, cathodes, and gates. Thus, if you fire the triac every half-cycle, it will not provide the rectification discussed by the previous answerer.
With only switching lights on and off (i.e. low duty cycle), just switch the gate to the line and don’t worry about the few microseconds of off-time each half-cycle. The inductance of the source will be more than enough to have the current overlap the voltage zero-crossing.
Wells Distrubutor – Less (DIS) Ignition Testing and Troubleshooting
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