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From: "Bevan Weiss"
Subject: Re: TRIAC "wears out" on dimmer circuit
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X-Original-Trace: 26 Oct 2002 19:09:13 +1300, news.orcon.net.nz
Date: Sat, 26 Oct 2002 19:09:16 +1300
NNTP-Posting-Date: Sat, 26 Oct 2002 19:09:13 NZDT
I would have gone more for...
If the TRIAC is being trigger capacitively via the optocoupler, do you have
a pulldown on the TRIAC to ensure that the voltage on the TRIAC side of the
capacitor doesn't float high??
I haven't really worked with TRAIC's (assuming they are very high input
impedance devices such as MOSFETs)... so please excuse me if I'm totally off
"Robert Baer" wrote in message
> John Harris wrote:
> > Hello,
> > I built a circuit to fade a string of twelve 20W tungsten filament
> > light bulbs connected in parallel (for outdoor use). This translates
> > to a peak current of less than 2A at 240Vac (after the initial
> > switch-on surge). I used a TIC236D triac, which is rated for 12A at
> > 400V. The circuit worked perfectly for a few days (around 60 hours of
> > operation), but then a fault developed, whereby the string would only
> > fade from fully on to around half brightness, instead of fully off.
> > When the opto-transistor that I used to trigger the triac (via a
> > capacitor) was disconnected from the driving signal, the lights
> > remained at half brightness instead of remaining off. When the circuit
> > was switched on from cold, it would work fine for a few minutes and
> > then the problem would re-appear, even though everything was well
> > heatsinked and the triac remained barely warm to the touch anyway. I
> > replaced the offending triac and the fault disappeared for another few
> > days before returning again. The string is around 10 metres long and
> > no bulbs have blown. There is heavy interference on the AM radio band.
> > I have a 240V 0.47uF capacitor connected across the triac output, but
> > no inductor in series. I tried using inductors before and after the
> > triac stage with no noticeable reduction in EMI. There is obviously a
> > design flaw on my part and any suggestions would be appreciated.
> > John.
> Tear apart a standard 600W light dimmer, and see what is used for the
> inductor; in fact steal that inductor and use it in the same place in
> your circuit.
> BTW, the low RFI versions use a larger and different inductor.
> Also please note the surge or turn-on current is much larger than you
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