Servicing Fluorescent Lights:
Matt J. Mccullar
Forum Fair Arcade
Arlington, Texas
PURPOSE
As the tallest person on the payroll, I have the
glamorous duty of repairing fluorescent lights not
only in the video game marquees, but in the ceiling as
well. I spent a few hours doing this last week. Fortu-
nately, the only bad parts were bulbs, but after
spitting out dead moths and noticing several dark
marquees on our games, I decided to find out more
about how fluorescent lights work so our marquees
illuminate properly and so the players can see around
the room. Let's see how they work.
FLUORESCENT/INCANDESCENT
A fluorescent bulb is not exactly like an incandescent;
a fluorescent bulb's current flows through a gas
instead of a wire. It does have filaments, though, that
are on opposite ends of a gas-filled tube, as seen in
Figure 1. The tube is filled with a mixture of argon gas
and mercury vapor. This gas will not conduct with
just 120 volts AC. But when the starting switch is
closed, the wire circuit is completed and current
flows. This heats up the filaments in the tube, making
them glow. But the tube won't light yet. When the
starting switch is released, the ballast coil produces
a brief pulse of high voltage. This "kick" is powerful
enough to ionize the gas in the tube. Ionized gas is a
good conductor, so now current flows through the gas
in the tube. Now that current flows through the tube
instead of the switch, the ballast acts as a resistor and
controls the current flow. Once the high voltage has
ignited the gas, a much lower voltage keeps it going.
It's like revving a car up to highway speed, then
cruising for a hundred miles.
LET THERE BE LIGHT
Okay, so current flows through the argon-mercury
gas. Does that produce light?Yes, but not the kind we
can see. If the tube is nothing but clear glass, the
result would be invisible ultra-violet light. So, to
make it visible, the inside of the tube is coated with
phosphors. Phosphors' claim to fame is that they glow
with visible light when they are hit by ultraviolet light.
Tube manufacturers offer several colors of lamps by
mixing different amounts of chemicals in with the
phosphors to get white, yellow, blue or any other
color. The glow of the phosphors is the actual light
from a fluorescent tube.
STARTERS
The previous circuit example is used for desktop
lamps, which require someone to push the starter
button. But if we had that on all our equipment, we'd
spend each day doing nothing but pushing buttons to
get our marquees and overhead lights turned on!
That's whywe have the automatic starter. It ~ooks like
a small tin can. It acts like a switch, but it stays closed
just long enough for the fluorescent tube to fire. It
does this automatically, but if it fails it can gtve you
a real headache.
12.0 \(AG,
BALLAST
FLUORESCENT TUB~
+
FILAMENT
FILAMENT
PHOSPHORS
STARTER
SWITCH
Figure 1
Basic Fluorescent Operation
The glow starter (his real name) contains a capacitor
and a glow switch (see Figure 2). A glow switch is a
glass tube containing two pieces of metal, each with
a contact on the end; and neon gas. The metal pieces
are each bimetallic and bend when heated, just like
what happens inside a thermostat. To understand
why things are put together this way, take a look at
Figure 3. When the circuit is first turned on, 120 Vac~
flows through the ballast, the tube filaments - and
stops when it hits the glow switch, because there is no
continuity. But the 120 Vac is powerful enough to
heat up the neon gas in the glow switch bulb and
make it glow. This heat causes the bunetallic strips to