Marquees and Ceilings
bend and make their contacts touch together. When
these contacts touch, the neon gas stops glowing
because current is flowing through the glow switch,
the fluorescent tube, and the ballast. At this instant,
it is exactly like having someone push a start button.
The filaments in the fluorescent tube glow and pre-
pare it to fire. With the neon extinguished, the heat
source in the glow switch is removed, and the bime-
tallic strips cool off enough to break the contacts and
bend back to their original separate position. When
the glow switch contacts break, the ballast trans-
former provides its inductive "kick," and the fluores-
cent tube turns on.
- - - - - - --
-
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GLOW
..-SWITCH
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Figure2
Starter: Inside And Out
The starter as a whole is connected in parallel with
the tube, but when the tube is lit there is not enough
voltage to relight the neon gas in the glow switch. So
the contacts stay apart.
Remember the capacitor? It absorbs the arc formed
when the contacts open, so it makes them last longer.
It also reduces radio interference caused by the arc.
If the metal contacts in the starter weld together, or
the capacitor shorts out, current would constantly
flow through the tube filaments, quickly burning
them out. This ruins both the starter and the bulb, so
you'll have to replace them both. Something else to
worry about.
INTERESTING ASIDES
~- An interesting thing about fluorescent bulbs is the
• gas in them will not fire at low temperatures. Once the
bulb is started, however, it will stay lit regardless of
how cold it is. That's why they come on fine early in
the day when the heating system is on, but if the tem-
perature drops in the afternoon to around 50 degrees
F, they won't work if someone shuts them off and tries
to restart them again. A good rule of thumb is if it's too
cold for you in shirt-sleeves, it's too cold for fluores-
cent bulbs.
A fluorescent bulb will glow all by itself, with nothing
connected to it, when hit with strong radio-frequency
(RF) energy. There have been several cases of people
who live near powerful transmitters reporting that
they can't shut off their lights! Aerospace companies
sometimes test certain radio dishes by holding fluo-
rescent tubes in the beam paths. High-voltage power
lines can light them on the ground.
WHAT CAN GO WRONG AND HOW TO FIX IT
The most common failure is a bad bulb, and substi-
tution is the only cure for that. After removing a
suspect bulb, lift one end of it and then the other, and
listen for rattling sounds. Does it sound like some-
thing is rolling around inside the tube? If so, one of
the filaments has broken loose. A tube will not light
if a filament is open, so check for continuity with a
meter on both ends. Sometimes, though, both fila-
ments may be good and the tube will still be bad; the
vacuum seal may be punctured, or the bulb has just
reached the end of its life.
If a tube is black on both ends, the tube is darn close
to dying on you. Each time the lamp is started, the
filaments glow and burn off some of the phosphors
close to them. Over time there won't be very many
phosphors left and this lets you know that the bulb's
age is showing.
READY SET START!
If the bulb won't light at all and bulb subing doesn't
work, you may have a bad starter or a bad ballast
transformer. The starter has two ridged legs under it,
and a light counter-clockwise twist will remove it from
its socket. If a new starter doesn't do the trick, pray
for a loose bulb socket or a blown fuse. The ballast
transformer is just a coil of wire, so check for conti-
nuity; if it has a burning smell and/ or still doesn't
light the tube, replace the ballast. It could have a
shorted turn in it, and not many of us have a coil
tester. You can quickly check for proper voltage by
removing the starter and sticking the meter probes
into its socket. If you don't get 120 Vac you know
something's wrong.
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