On Wed, 13 Aug 2003 21:50:56 GMT, "Tweetldee"
wrote:

"Malcolm" wrote in message
...
I have a dimmer switch controlling a house light. Sometimes when the light
bulb
blows, the triac in the dimmer is taken out too and becomes a short
circuit.
I've replaced the triac several times over the years, but I was wondering
if it
would be possible to put a quench across the triac to stop it happening
again.

It's a conventional enough circuit for a 240V mains - a 500K pot with 1M
across
it feeding 2 stages of 5K6 / 47nF "filter" to produce the trigger for the
diac
/ triac.Only other component is an inductor - about 40 turns on a short
ferrite
rod.

Any suggestions for the quench components?
Fantrace.

Things can only get better.

As I have read in several places, the cause of the triac failure in these
circuits is because of the manner in which the light bulb fails. When the
bulb fails, the filament melts and the resulting arc inside the bulb causes
extreme transients that exceed the triac's maximum Dv/Dt ratings.
Therefore, the triac fails.
One way to suppress the transients would be to put a snubber across the
triac, and see if that helps. Also, you might consider adding a fast
blowing fuse in series with the triac and load. Be sure that you figure the
fuse rating so that the bulb's turn-on surge won't blow it, but will quickly
blow with a higher than normal load. The fuse wouldn't offer as much
protection as a snubber, but won't hurt.

Cheers!!!
Compared to the triac, a fuse is SUPER SLOW, and wouldn't do a thing
as far as protecting the triac is concerned.

Regards,

Michael Floyd