Snubber needed for triac driving heating element?

A triac is inherently zero current (or reverse current)switching. To prevent dv/dt for reactive loads, a simple RC snubber should normally suffice.

RL

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Triacs *self protect*when over voltaged by turning fully on.

Supply spikes simply trigger them on and they will commutate off at the next zero crossing.

I think SCRs do the same.

.... Phil

I have seen numerous domestic light dimmers fail in the fully-on condition. I suspect the failures could have resulted from huge currents due to lightning, though arcing inside tungsten filament light bulbs as they burn out is also a possibility. As you would be well aware, after the silicon melts, it tends to conduct, regardless of the original function of the device. If you have a different explanation for these failures I would be interested to hear it. I never bothered to dissect the failed units.

I agree, if the triac is not damaged. If somehow it gets melted, it will quite likely stay on. I don't know the exact mechanism by which they get melted, but I believe that it does happen, at least in consumer-grade light dimmers.

? But

tea

rt

heat-update-user-error

o remotely preheat your oven" Really? WTF?

t,

ff.

stay on. It turns off at the next zero crossing. No? It would have to be a scramble in the controlling circuit to make it come on for more than an instant which is what the Verge article is about. That can happen even wit h a relay.

Yes, I have seen it happen a couple of times with touch-controlled dimmers on table lamps when the lamp failed and there was an internal arc in the bulb. The triac became a short circuit (and the 32A circuit breaker on the ring final circuit tripped).

John

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Uses can overload them with a few 100W bulbs.

But they do not fail short due to supply spikes.

They trigger on due to limited dv/dt capacity if the spike is fast or sheer voltage.

..... Phil

With a heavy, noninductive load and very high dV/dt, they can certainly fail short-circuit. The conducting area moves out from the contact, so the die area is effectively much smaller at short times.

Of course mains wiring isn't great at transmitting very sharp spikes, so it's probably safe enough in real life.

Cheers

Phil Hobbs

Phil Hobbs wrote drivel again:

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Critical di/dt failure is rare.

.... Phil