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From: "Fritz Schlunder"
Subject: Re: TRIAC "wears out" on dimmer circuit
Date: Thu, 24 Oct 2002 16:47:43 -0700
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"John Harris" wrote in message
> 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.
Well, considering that the lightbulbs are at half brightness with no gate
signal, that would suggest one of several possibilities:
1. Mains voltage exceeds blocking voltage of triac. Though the mains are
only 240Vac, perhaps they are running high, or else there is something on
the mains that is spewing noise that causes it to exceed 400V which trips
the triac into conduction periodically (say at the peak values of the
waveform thus producing half brightness).
2. dv/dt of mains is too high... This wouldn't be typical for good old
50/60Hz mains, but perhaps there is some noise on the mains that has a high
dv/dt component which is tripping your triac. If this is the case, perhaps
a snubber accross the triac could remedy the problem.
3. Something is triggering the gate without your permission.
4. Triac is bad.
Possible remedies/courses of action for above possible causes of problem:
1. Check voltage by using oscilloscope (make sure the use an isolation
transformer on your o-scope first, or you will prolly break something).
Replace traic with higher voltage device.
2. Check voltage using oscilloscope. Install a snubber accross triac.
3. Double check circuitry, and try placing a resistor between the gate and
main terminal 1 to prevent spurious gate triggering. This isn't a sensitive
gate triac, but maybe that would help anyways. Try various values.
4. Replace triac.
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