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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: TRIAC "wears out" on dimmer circuit
Date: Fri, 25 Oct 2002 00:41:39 GMT
NNTP-Posting-Date: Thu, 24 Oct 2002 17:41:39 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
John Harris wrote:
> 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.
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
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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