How Degaussing Coils Work
RANDY FROMM
Plus Some Alternative Methods For Degaussing Picture Tubes
Copyright 2/11/91 By Randy Fromm
I perform quite a few interesting demonstrations and experiments at the
Arcade School. One of my favorites is to show the relationship between
magnetism and electricity by holding a powerful magnet against the
picture tube of a videogame monitor. This causes the picture tube to
display a beautiful rainbow pattern that follows the invisible magnetic
field that surrounds the magnet. But when I pull the magnet away from
the screen, the colors remain skewed in large areas of the picture; The
picture tube has become magnetized and the display looks lousy!
Let's take a look at the picture tube and see how a little magnetism can
really mess-up the picture in a color monitor. Take a close look at the
screen on the front of a color picture tube. There are thin vertical
stripes of "phosphor" coating the inside of the glass. There are three
different types of phosphor, laid down in alternating stripes. Each
glows with a different color when struck by a stream of high-energy
electrons. These energetic electron "rays" come from three "electron
guns" that live in the skinny neck of the picture tube (also known as the
cathode ray tube or CRT).
When properly aligned, the electrons from each of the three electron guns
will strike only a single color phosphor. Electrons from the so-called
"red gun" will strike only the red colored phosphor, the green gun hits
green phosphor and blue phosphor gets zapped by the blue gun. This
alignment process is known as the "purity and convergence procedure."
Six slightly magnetic rings, centered over the electron gun assembly in
the neck of the CRT, are used to bend the streams of electrons so that
each strikes the correct color phosphor.
If the picture tube becomes magnetized in any way, the streams of
electrons may hit the wrong color phosphor. It doesn't take much of a
magnetic field to cause a noticeable change in color. Just the magnetic
field of the Earth is more than enough to do it. This is especially
apparent in a monitor that is mounted "face-up" such as in a cocktail
table or rifle video. Turn the game on and then rotate it. As you spin
the game, you probably will see the colors shift as the picture tube
intercepts the magnetic field of the Earth.
To prevent the Earth (or anything else) from permanently magnetizing the
CRT, all color monitors use a "degaussing
coil." A degaussing coil is a demagnetizing coil that surrounds the
outside of the picture tube. The degaussing coil is powered by 120 volts
AC. The rapidly changing field quickly demagnetizes the CRT by
scrambling any molecules that might be aligned through magnetic
attraction.
But the degaussing coil can't remain continuously energized while the
monitor is in normal operation. Disregarding the fact that it will burn-
up if left continuously energized, the magnetic field produced by the
coil will produce some radical distortion in the picture. Most monitors
use an "automatic degaussing circuit" to control the coil. The
degaussing coil is only activated when the monitor is first turned on,
and then only briefly at that. To reactivate the coil, the monitor must
be allowed to cool for ten to twenty minutes.
Let's look at a typical circuit and see how it works. The key this
circuit is a component called a "thermistor"; a component that changes
its resistance with a change in temperature. In a degaussing circuit,
the thermistor will increase in resistance as its temperature rises.
This type of thermistor is also known as a posistor. As the temperature
of the thermistor drops, so does its resistance.
The basic degaussing circuit is simple. It contains just two
components: the thermistor and the coil itself. They are connected in
series and to the 120 volts AC input of the monitor. When power is
applied to the monitor, the degaussing circuit is powered as well. At
this time, the thermistor is cold and has a low resistance. The
alternating current flows through the thermistor and the degaussing coil,
creating the AC magnetic field and demagnetizing the CRT.
But as soon as current begins to flow through the thermistor, it begins
to heat. As it heats, its resistance increases rapidly until just a tiny
trickle of current is allowed to pass; not enough to cause a noticeable
effect on the screen but just enough to keep the thermistor nice and warm.
This prevents the degaussing coil from activating again until the
monitor is turned off for a while and the thermistor has had a chance to
cool.
A slightly more sophisticated version of the automatic degaussing coil
circuit uses a 15 ohm resistor and a pair of diodes in addition to the
thermistor and the coil itself. The addition of the resistor assures that
the thermistor will heat rapidly (whether or not the degaussing coil is
plugged in, by the way) and cut off the current to the coil.
The diodes are used to choke off any residual flow of AC to the coil when
the thermistor is at its maximum resistance. Notice the unusual
connection of these two diodes. They are connected in parallel, with the
anode of one diode connected to the cathode of the other and vice-versa.
This arrangement allows AC to pass through the diodes and to the
degaussing coil when the thermistor is cold. When the heated thermistor
cuts off most of the current, the small amount of residual current that's
left is cut off from the coil by the voltage drop of the diodes. This
assures that there will be absolutely no interference from the degaussing
coil; it is completely shut down.
Make Your Own Degaussing Coil
If you operate games in a convenience store or other 24 hour locations,
your automatic degaussing circuit will never have a chance to operate
(remember, it needs to cool for about twenty minutes to operate properly.
) You can use your own degaussing coil to demagnetize the CRT manually.
Degaussing coils can be purchased from just about any good electronics
retail outlet or from industry parts supply houses like Wico.
You also can make your own degaussing coil from parts you have salvaged
from a junk color monitor. The obvious method is simply to remove the
degaussing coil from the old picture tube and connect it to a line cord
with a momentary switch in the line. The use of a momentary switch is
critical as the coil may burn up if left energized for extended periods
of time.
Another, novel approach is to use the yoke of the junk monitor as a
degaussing coil. Specifically, the vertical deflection coil of the yoke
is connected to the line cord and switch. If you want to try it yourself,
make certain that you connect to the vertical deflection coil only, not
to the horizontal deflection coil. Use an ohmeter to identify the two
coils. The vertical deflection coil will have a much higher resistance
than the horizontal deflection coil. Typically, the vertical deflection
coil will have around 30 ohms of resistance while the horizontal will
have only 2 or 3 ohms. If you connect the 120 volts AC to the wrong coil,
you'll probably blow the circuit breaker, with the possible accompaniment
of smoke and pyrotechnics.
Another workable degaussing coil substitute is a bulk tape eraser
(normally used to erase magnetic tapes or disks.) You can even use a
soldering gun to degauss a CRT. It must be the transformer type such as
a Weller 8200 or similar.
Whether it's storebought, salvaged, jury-rigged or homemade, the method of
using your coil to degauss a picture tube is the same. The CRT can be
degaussed with the monitor on or off. I like to have it on so I can see
the results right away. Besides, it's neat to see how the powerful
magnetic field of the degaussing coil distorts the picture. Start at the
center of the CRT. Hold the momentary switch closed while moving the
degaussing coil in a spiral across the screen. When the spiral extends
well beyond the edge of the picture tube, move it away from the CRT, turn
the coil at a 90 degree angle and release the momentary switch to turn
off the coil. Turning the coil at the end is simply a precautionary
measure to assure that the magnetic field of the degaussing coil doesn't
have any effect on the CRT at the moment the coil is shut off. In
practice, I don't find it to be necessary. Likewise I have found that
degaussing the front of the picture tube is generally all that is
necessary; "The Book" says you also should degauss the sides and top of
the CRT and chassis.
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RANDY FROMM YMN...@prodigy.com