Triac Efficiency

Yes, you're right. But the capacitor goes through that diode of the RCD and then the resistor would be the discharge path.

You don't want to discharge the capacitor's energy immediately across the contacts as they close, otherwise the contacts might weld closed.

Same reason you wouldn't want to discharge a large HV capacitor directly across a FET or it would blow up the FET... Or, certainly would not be good for it.

boB

You've lost me. I thought you were talking about a DC circuit. The resistor would need to be in series with the cap to prevent a surge current through the contact, but then that limits the protection from the capacitor.

With a FET, you can control the current through it. It doesn't need to be a surge.

Yes, DC.

I suppose a FET could have been made to work as well but a FET would also need to have a drive circuit that limits how long it is on and sufficient over-current protection so it doesn't blow up. But since the on time would have been very short, a heat sink could be either minimal or not even used.

I like simple but effective fixes like an electrolytic capacitor that does all this pretty much by itself. BTW, this particular circuit was for multiple relays and PV circuits and was in the mid 1990s where really good low RdsOn FETs were not so plentiful and low cost. We used the best FETs at the time in our inverters back at Trace Engineering.

And you are absolutely correct about not wanting a series resistor with the capacitor if at all possible. The capacitor was really an elegant solution. I should take a picture of this device. I will do that sooner than later if I run across the PV GFP board which I still have one of.

boB

BTW, guys, it's not the diminishing voltage across the triac that stops its conduction, it is the diminishing current through it. Imagine an inductive load. It will start to turn off when the current goes goes below the holding threshold. If the rate of rise of voltage (due to the inductance) across it is low enough, it will completely turn off.

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When the supply voltage drops to under 1volt, a triac will turn off. It can be re triggered when the voltage rises sufficiently again.

FYI: For a 240V 50Hz supply, this takes about 20 microseconds.

..... Phil

Of course it will turn off because the current drops to zero at 1V.

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....... Phil

Of course. Ohm's law only applies to materials that it applies to. Circular reasoning, true. But it's realistic. Ohm's laws predicts a linear relationship between current and voltage, which is not true for many devices, in particular, semiconductors.

You should know that.

I finally put back my dropbox app back. Here is a picture of that old PV GFP unit I did back around 1995. The stand-up electrolytics are the snubber caps. Not huge, but not small either.