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
I read in this news group that bulbs blowing can be a reason that a
dimmer fails. Is this true?
It is true and happens frequently. Also fixtures with candelabra sockets
often develope minor short circuits in the socket, just a momentary pop, not
enough to trip the breaker, but enough to take out a dimmer
of the kitchen in my cookie cutter. It has one of those pug-ugly chandelier
things that uses those damn candle-base bulbs. (as does the other big
ceiling fixture, non-dimmed, also a piece of junk.) Dimmer still works as a
switch, so haven't bothered to replace it yet. Been looking off and on for
replacement ceiling fixtures for the kitchen, but none of them in the stores
seem to have normal bulb sockets. And the current one has one neat feature-
a downward-facing flood bulb in the center, to light up the table enough to
read newspapers at.
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.
Thanks, I realized too late that I forgot to mention that.
Not to be pedagogical, but I sized the fuses for my lamps by looking up
the specs of the triacs in my lamps' touch dimmers, the ones which were
punching through, and found on their maximum I^2*t rating. (eye squared
Then I looked on Buss' website and found that the blowing energy (also
expressed as I^2*t) of their 3AG 2A fast blow fuses was somewhat less
than that of the triac, so I figured it should work.
And work it has....I use two 75 watt bulbs in a "Y" adaptor in each of
those table lamps cause the two are cheaper two purchase than one 150
watter in most places, plus when one bulb dies I still can get light to
see my way around from the other one, assuming it didn't expire with one
of those tungsten arc blasts and take out the fuse. I think I probably
have three or four "normal" bulb burnouts for every one which takes a
fuse along with it.
And some people accuse me of being a geek who "gilds turds"....Wonder
why that is....<G>
Two 75 watt bulbs produces a lot less light and are a lot less efficient
than one 150 watt bulb. You will pay more for the electricity:
75 watt GE "standard" 1190 lumens
150 watt GE "standard" 2850 lumens
That's 20% more light for the same power or 20% more efficient.
Also, you would probably find the quality of light more
desirable from the 150 watt bulb (higher color temperature).
Over the estimated 750 hour lifetime, you will use 112.5 KwH.
If you are fortunate enough to pay only 10 cents per KwH,
thats $22.50 of electricity.
Also note, that when you use a dimmer, although you use less
power, the bulb gets much less efficient.
Well, I followed through and bought some 150 watt bulbs yesterday. While
I was at the store looking at the lumen ratings of frosted incandescents
I noticed that the 150 watt output of a "three way" 50-100-150 watt bulb
was also considerably lower than that of the plain 150 watter.
I replaced the "Y" adaptor and its two 75 watt bulbs in just one of our
two living room table lamps with a 150 watt bulb.
the results were pretty dramatic, the lamp with the 150 watt bulb was
noticably brighter, so I put a 150 watter in the other one too.
Now I need someone to understand why the single 150 watt incandescent
puts out 20 percent more lumens than the pair of 75 watt bulbs. (Are you
reading Don Klipstein?) I'm guessing it might have something to do with
more thermal energy being tossed away through four filament supports
than through two, or something like that.
I'll be kicking myself for a while over not thinking about the lumen
output when I decided to install those "Y" adaptors with pairs of 75
As my mother-in law used to say, "Jeff, for a smart guy, every once in a
while you do stupid pretty good."
I can give you a partial answer. The filaments in higher wattage
bulbs are designed to run at a higher temperature. At the higher
temperatures, more of the radiated energy is in the visible light
portion of the spectrum (see http://en.wikipedia.org/wiki/Black_body )
(however still only around 3-5%). Why don't we just run all
bulbs hotter? That dramatically decreases the lifetime of the
filament. Why can we get away with running higher wattage
filaments hooter than lower wattage ones? That's the part that I
P.S. Halogen is a technology for increasing the lifetime of the
filament, allowing you to run it hotter.
Most people are unaware of that, including many energy conservation
advocacy organization that encourage people to use lower wattage
bulbs. Also, it is getting harder to find light output
figures on many types of bulbs.
Maybe another reason is because the bases of both bulbs are the same
diameter but the 75 watt bulb is physically smaller than the 150 watt
one. That puts its filament closer to its the base which makes the base
obscures a larger solid angle in the total sphere of radiation, thus
blocking a greater percentage of the visible light?
I am sincerely genuinely impressed. The application is specific to the
triac and type of Buss fuse used, but has a good chance of working in
Nitpicking - also check the fuse voltage ratings, and fuseholder ratings.
There may be more room to install a fuse at the light fixture.
Right, but in the dimmer, there might be room for a glass fuse without
a fuseholder. Either one with pigtails (wires soldered to the ends)
or small endcaps that are barely bigger than the metal ends of the
glass fuses. I don't know what Jeff actually used.
There may be enough room for it, but not after you've taped it
so that they don't short against the dimmer box. "Inline" fuse
holders (like the automotive variety) are plastic and don't
expose any metal bits. But they are kinda bulky, as are dimmers.
Wouldn't lay odds you could install one in anything but the deepest
Age and Treachery will Triumph over Youth and Skill
Well, as long as you sort of asked....
All four of the table lamps with touch dimmers in our home are Asian
styles with "full metal jacket" brass bodies. I installed the touch
dimmers in all of them myself, inside the lamp bases.
When the time came to add those fuses I Installed panel mount 3AG fuse
holders through holes I drilled in the side of the lamps bases, near
where the lamp cords exited.
Things being what they are, every few years we get a new cleaning person
and it sometime takes a little time for them to get used to those touch
switches. So, after the first few housecleanings (we're absent when
those happen) I'l maybe find the original manual switch on a lamp's bulb
socket turned to the OFF position, and once a lamp wouldn't turn on when
I touched it because the lady thought the fuseholder cap was a switch
and twisted it so that it sprang out enough to open the circuit. <G>
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