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Triac Circuit Help

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Active Member
Hi all,

I have a triac circuit here:

It is used to control fluorescent lights. I'm trying to figure out if R4 and C1 are needed?
I'm told that they were added in because leakage current would cause a particular ballast to flicker every 5 minutes when it was supposed to be off. (The ballast would charge up over the 5 minnute period and would think the voltage was applied, so it would try to turn on). I guess this extra RC circuit is supposed to prevent that?

Can someone help me out here?



Well-Known Member
R4 & C1 together form a snubber network. It's purpose is to limit the DV/DT presented to the triac. If this parameter is not controlled properly, it can cause commutation where the triac very quickly switches state every half cycle. This action causes the triac to heat very quickly and can destroy it. Also half cycling into an inductive load, like a transformer, or in your case a lighting ballast, effectively looks like feeding it with D.C rather than the A.C it was designed for, again with the detrimental effect of rapid heating and possibly damaging the windings.

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