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From: Kevin McMurtrie
Subject: Re: TRIAC "wears out" on dimmer circuit
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Fri, 25 Oct 2002 00:22:18 GMT
NNTP-Posting-Date: Thu, 24 Oct 2002 17:22:18 PDT
In article ,
firstname.lastname@example.org (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.
A few things to consider:
Do not use a capacitor on the output! It's causing current surges. Use
a series inductor. Unlike transistors, inductors are good for SCR
devices because the SCR won't switch off until the coil's current flow
is depleted. A lossy or easily saturated inductor works best because it
will be less likely to ring when the power is turned on.
Filament bulbs have low cold resistances. Add that current to the
capacitor's current and you're probably causing lots of thermal stress
in the semiconductor. You may get better life from the bulbs and your
TRIAC if you adjust your circuit to not dim the bulbs below a faint red
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