There was an amateur radio event recently in Oklahoma
City that I attended which had a large flea market in which all
sorts of things were out there for sale. One man had boxes of
mostly new parts that one could buy for a few Dollars and they
were decent parts so I bought one of the boxes.
As I was sorting through it later to put all the parts
in drawers and bottles, I ran across about a half-dozen neon
lamps and wondered how many younger amateurs these days know
what these things can do.
They are kind of an echo of days mostly gone by, but
they are interesting little devices.
I will describe them as looking kind of like small clear
Christmas lights. The bulbs are about the size of a Braille
cell, as one fellow I used to know, described it. They are
pointed on one end where the glass was sealed and have two bare
wires coming out the base where they can be soldered in to a
circuit.
They've been around for maybe a hundred years and used
to be used in anything that needed a little orange-colored light
to tell you if it was on.
Basically, they went where we presently see LED's. A
coffee pot, toaster or other appliance made during most of the
20TH century may have had one to signal that it was on or
heating.
They were popular because they drew only 1 twenty-fifth
of a Watt and, if properly current-limited, they lasted the life
of the device.
Another common use was in circuit testers to warn
electricians that a line was live or verify that power was being
applied.
They are almost the color of school buses, life jackets
and anything else that is supposed to be bright orange and get
your attention. Even some night lights used a few neon bulbs to
faintly light someone's way.
If that's all they did, there wouldn't be much point in
writing this, but they are a class of electronic devices called
gas discharge tubes.
Those are light bulb-like devices that have almost a
total vacuum in them except for a little bit of neon, argon or
some other rare gas.
If you read then for continuity with an ohm meter, they
don't show any continuity at all. They look just like an open
circuit.
If you put enough voltage across them, however, they
ionize suddenly when you reach a critical voltage called the
breakdown voltage.
The gas begins to conduct electricity because electrons
are being knocked out of their orbits in the atoms of neon or
whatever gas is there. Each electron then tries to fall back in
to orbit and it is this going in and out of orbit that releases
light energy to make the bulb glow.
The breakdown effect makes a lot of weird circuits
possible which is what makes these bulbs interesting to play
around with.
If you gradually lower the voltage across the bulb, it
will eventually de-ionize and go out so what a person sees is
the light gets dimmer as you drop the voltage but then comes a
point where it goes dark just like someone turned it off.
The breakdown voltage is higher than the voltage needed
to keep it drawing current so it is possible to put a voltage on
a neon bulb that is a little too low for it to light, but almost
high enough. If you do anything to increase the voltage, it
suddenly comes on and stays that way.
The other interesting thing is that the voltage across a
lit neon bulb stays about the same even if you try to feed in
more voltage. The current goes up fast and if there is no
limiting resistor, the bulb will quickly fail. This effect has
made them useful for voltage regulators.
I am not going to bore everybody with much more of this,
but things from elevator buttons in the sixties or so, older
electric organs and tons of other pieces of industrial and
consumer gear from the fifties and sixties used those little
bulbs for more than just small orange lights.
Martin
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