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Fluorescent light capacitor function and failure mode
(John J. Lee) wrote in message ...
I'm trying to fix a fluorescent kitchen light. It was blowing the
downstairs lighting circuit fuse every time the wall switch was turned
on, even when the fluorescent tube was removed.
The breakage seems to have been caused by me leaving something cooking
on the kitchen grill for ages (never again!), causing the room to
become very hot (condensation on the window, so maybe some in the
light, too), and maybe some grease / carbon to settle on surfaces in
the room (though I can't actually see or feel any). I'm pretty sure
the light was switched on when I left it. There was no fire,
thankfully.
The light has a standard old-fashioned circuit with just a ballast
(just an inductor?) and a "starter" (bimetallic strip thingy).
There's also a big electrolytic capacitor, connected directly across
the mains. So, with the fluorescent tube removed, the circuit should
just be the capacitor connected across the mains.
If I disconnect the capacitor from one side of the connecting block
that it's wired to, switching it on at the wall no longer blows the
fuse (and there's no short across the mains terminals with the
capacitor connected). So it's the capacitor that's at fault, not some
grease or something shorting things out.
First, is the capacitor just there to smooth the voltage, to reduce
flicker? Can I disconnect it and switch it all on again, to see if
anything else is broken?
Second, when I measure the (DC) resistance of the capacitor with a
multimeter, I get 500 kOhms or so. Anybody care to guess what
happened to it to cause it to start breaking down at 250V? Was it my
fault, or was it just a coincidence that it started blowing fuses just
as I left the grill on?
TIA
John
I've never seen a fixture with a large electrolytic across the power
line. Are you from the UK since you use the word "mains" for the
power line? Any capacitor is probably just to filter out
high-frequency noise from the lamp arc, 50 or 60 Hz and many, many
harmonics. The capacitor is probably breaking down when exposed to
the full line voltage, but not under the low voltage from the ohmmeter
section of the multimeter. Almost any capacitor rated at twice the
line voltage will reduce the "noise going back onto the power line.
It is most evident when listening to AM radio. If you can use the
radios in your house with the light on, without the capacitor, then it
probably is ok to continue to use it without interfering with your
neighbors radio reception. But if you are in an apartment house, you
may cause interference to your neighbors radio due to the ac power
lines radiating the noise.
H. R. (Bob) Hofmann
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