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From: firstname.lastname@example.org (N. Thornton)
Subject: Re: TRIAC "wears out" on dimmer circuit
Date: 28 Oct 2002 13:27:16 -0800
NNTP-Posting-Date: 28 Oct 2002 21:27:16 GMT
"Phil Allison" wrote in message news:...
> "N. Thornton" wrote in message
> > Secondly the bulb surge currents. 12x20w = 240 watts at 240v, 1A rms
> > running. With bulbs at a tenth their R at switch on, that will be 10A.
> > With mains at Vpeak in the cycle, that will be 14A. With noise on the
> > mains as well it will be higher still at times.
> > If you pick a 143 triac it should cope with that easily. But that
> > isn't the end of the story.
> ** The single cycle rating is 100 amps and 20 watt bulbs have extremely
> short time constants. The 12 amp triac should be good for 1000 watts worth
> of them.
OK, that sounds good, though that wasn't my experience.
> > Triac voltage: 240v supplies see occasional spikes of way above 240 x
> > 1.4. In fact if you dont have a filter on the input I would go for a
> > 1kV rated part for reliability. Filtering is very desirable. A 400v
> > triac with no filter is a no-no really.
> ** Excess voltage merely triggers a triac on.
When it is off, with cold bulbs. Excess voltage plus cold bulbs equals
high surge current. Have to account for that.
> > Thirdly I assume you want this thing to survive. Dimmers are famous
> > for dying when a bulb blows. When a filament bulb blows it sometimes
> > produces an arc across the filament, and i can effortlessly exceed 60A
> > until the bulb fuse blows. So I would want a triac that can handle
> > more than just the running load.
> ** An arc across the remaining ends will never be anything like 60
Its higher in fact. There was a thread on here about it recently. You
can check out the figures by checking it on google.
> > Moral of the story: avoid capacitor surges, use a bigger triac, and
> > fuse carefully to catch bulb failures.
> ** The cap may be a problem - nothing else is.
My own experience has been that triacs need a lot more headroom to
survive long term. And triacs are known for dying when bulbs fail too,
these are real issues I think.
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