hobbes wrote:
Hi,

I have a Lutron dimmer that dims a 100w (4 x 25w) light fixture in a
bathroom. I noticed that one of the 25w bulbs was out and that the
dimmer does not dim any more. My question is can a bulb burning out
cause a sufficiently high current transient / short to burn out a
dimmer?

I read in this news group that bulbs blowing can be a reason that a
dimmer fails. Is this true?

best, Mike.


Yes, what occasionally happens when a bulb burns out is that a phenomena
called a "tungsten arc" occurs. The filament break develops an arc
between the broken ends which melts those ends and vaporizes some
tungsten. The arc continues, melting more tungsten and shortening the
remaining filament ends and the current increases as more of the
filaments melt and the density of the vaporized tungsten increases.

It all happens faster than you can say Jill Robinson, usually
accompanied by a bright white flash just as you "turn on the lights".

The current will sometimes surge high enough to blow a panel fuse or
breaker, or as in your case, to fry the triac in a dimmer.

Good quality light bulbs used to have special thin wire "fuses" as part
of their construction which were just thick enough to let the bulbs be
turned on and run but would "blow out" if the bulb developed a tungsten
arc. I don't know if that's still the case.

When you are replacing the dimmer, go to Rat Shack and pick yourself up
an inline 3AG fuse holder and a few 2 amp 3AG fuses. Wire the fuseholder
in series with the hot feed to the dimmer (it should fit inside the
dimmer box) and you'll be pretty well be protected against a future
burnout. You may have to replace a burned out fuse, but the dimmer
should be OK.

That's just what I did with the four table lamps in our home which are
fitted with "touch dimmers". After the second dimmer blew I added 2 amp
fuses to all of them. I've probably replaced six fuses in the last five
years, but all the dimmers are still alive and well.

HTH,

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10^12 furlongs per fortnight.