DIYbanter - View Single Post - Neon light exploding and falling from fixture

Jeff Wisnia

wrote:
On Sun, 12 Aug 2007 14:01:30 -0700, "Lesley"
wrote:

"RBM" rbm2(remove wrote in message
...

Neon fixtures use very high voltage. A short circuit in the fixture could
cause this. What is this being used for?

As Noozer pointed out, I meant to say fluorescent. The fluorescent lights
are used in the carport. They are under cover and I could not see any water
or anything getting in there. They are exposed to hot and cold temperatures,
mind you, because there is no door on the carport.

Lesley

I thought you meant florescent. You cant change the tubes on a neon
(without special equipment anyhow).

Anyhow, when I was a young boy I was in a store with my father when a
florescent bulb exploded. Both of us were covered with glass and
florescent dust. The store wanted us to go to a hospital but my
father said we were ok. I will never forget that. As a youngster it
was quite frightening.

I'd suspect the ballast is bad. I'd replace the ballast if I were
you. (or the whole fixture). But all you really need to change is the
ballast. The rest of the fixture is just a metal shell and some
wires, plus the lamp "sockets". Unless the bulb fell out of the
socket, there is nothing else that could cause this except the
ballast. You said it exploded in the fixture, so that eliminates the
falling out of a bad socket.

I am glad this does not happen often considering how many places have
florescent lights. I think those old "transformer" type ballasts were
more likely to fail than the modern electronic ones. This dont happen
to be one with the old twist in starters, is it?

Jim

It would be helpful if Don Klipstein jumped in and educated us about
what sort of ballast failure causes a flourescent bulb to "explode".

My understanding of ballast operation is that aside from heating the
filaments for starting their main purpose is to act as a "choke" which
provides an inductive impedance to drop line voltage down to the voltage
level which is correct for the particular bulbs they are rated for.

A fixed resistor could provide the same voltage drop, but would waste
too much power as heat. Back around 1949 my uncle Mo, who was well
versed in electricity and the electronics of that era made me a line
voltage operated "black light" from a small UV flourescent bulb and a
big wirewound resistor, all housed in one of those wooden Velveta cheese
boxes about 10 inches long. (Rememvber them?)

So, maybe if the "choke" in the ballast which provides that inductive
reactance in series with the bulb were to short out, the bulb would
receive full line voltage and the resulting arc would be so intense that
it could overheat the gas in the tube causing its pressure to increase
enough to burst the bulb. Or, maybe it would just run so hot that the
heat would precipitate a mechanical failure.

Just my .02, but I've got a feeling that if the OP didn't know the
difference between neon tube and a flourescent bulb then he/she may not
be able to judge the length of the bulbs "by eye" either. So let's not
get too worked up trying to figure out what's really installed there and
what went wrong.

Me, I'm down to telling the OP to get a qualified pro out there to check
things out, I wouldn't dare suggest that the OP tries working on it
him/herself.

Jeff

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