Triac protection

[Malcolm]

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.

 

[Tweetldee]

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!!!
--
Tweetldee
Tweetldee at att dot net (Just subsitute the appropriate characters in the
address)

Time is what keeps everything from happening all at once.

 

[Sam Goldwasser]

I've got the same problem with a cheap little 'touch lamp' dimmer. I've
replaced it twice now...almost cheaper to buy an entire new lamp.

Nah, just pick up 1000 triacs on eBay and install a rugged socket.

It may be hard to guarantee reliable protection since it's quite possible
for the lamp to become a momentary short circuit when it blows out. You
could always install some sort of active current limiting but that would
use some power. A fast acting fuse might work at least some of the time
but would cost as much as a triac.

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[G. Skiffington]

Howdy Malcolm.....You might try something as simple as a NE-2
bulb/resistor setup across your light socket and/or one of the miniature
sealed line-voltage pilot lamps across the socket.....these could stay
hidden or even be shown as a help in diagnosing that the light bulb is
just blown.

 

[Sam Goldwasser]

Howdy Malcolm.....You might try something as simple as a NE-2
bulb/resistor setup across your light socket and/or one of the miniature
sealed line-voltage pilot lamps across the socket.....these could stay
hidden or even be shown as a help in diagnosing that the light bulb is
just blown.

And this will protect the triac how?

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the Feedback Form at repairfaq.org. Thanks.

 

[Michael Floyd]

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