PipeDown wrote:


Dimmers have been known to be burned out when a bulb burns out. The instant
the filament on the bulb burns, there is a spike in voltage (or was that
current) you might see as that last bright flash as it blows out. This
spike can and does damage the SCR (transistor looking 3 pin device in the
dimmer control) if its current rating is borderline (as it may be in cheaper
switches). Better dimmers have a device rated for a much higher inrush
current but it cost a tiny bit more.



You're right, and reverting to my usual pedagogue mode I'll add that the
effect is called a "tungsten arc" and it can create a near short circuit
sized current spike which can easily blow out the small sized triacs
used in solid state lamp dimmers. It happens maybe once in every 5 to 10
bulb burnouts.

I've seen cases where a 75 watt bulb blows with a bright white flash
when you flip the wall switch on, and when you replace the bulb you find
that you have to go and reset the panel breaker for that circuit before
you can light the new bulb.

What happens (As explained to me.) is that when the bulb filament
breaks, if the broken ends don't snap away from each other fast enough,
a small arc between them begins vaporizing the tungsten filament
material, and filling the bulb with a conductive metal vapor through
which current continues to flow. The effect "runs away" and the current
increases as more of the filament end lengths melt away. I've heard that
some "better" brands of lamp bulbs are made with a deliberately thin
section of lead wire inside their bases to act as a fuse to limit the
severity of that current surge.

I used to occasionally blow out the solid state "touch dimmers" in our
bedside lamps when their 150 watt bulbs blew. I did the numbers on it
and installed fuse holders with 3AG 2 amp quick blow fuses under the
lamp bases. I've had to replace a few blown fuses since then when bulbs
burned out, but never again a dimmer.

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
"Life is like a sewer -- what you get out of it depends on what you put
into it."