Testing Fluorescent Tubes for Cathode Emission

[Martin McCormick]

Many fluorescent light fixtures use two tubes in series per
ballast. If one tube stops working, both are dark. I have a bunch of
4-foot 40-Watt tubes ranging in condition from probably good to most
likely bad. It occurred to me that it should be possible to test the
cathode emission on each tube by applying the 3-volt heater voltage to
the cathodes at each end and then determining what voltage it takes to
cause the gas to ionize. If one feeds the cathodes by either a
by filer transformer or two separate filament-type transformers, then
there should be no common connection between the ends of the tube.
One could use a current-limited AC source or even a current-limited DC
source to apply voltage between the ends of the tube. A good tube
should break down at around 175 to 200 volts while a bad tube will
need a higher voltage due to reduced cathode emission.

One should be able to vary the high voltage source so that the
voltage can be increased until breakdown occurs.

If the current limiting is substantial such as allowing only a
milliamp or two at 200 volts, then the tube would probably glow weakly
but the idea here is to determine breakdown voltage.

Does anybody see any reason why this shouldn't be a valid test method?

Thanks.

 

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Martin McCormick:
.... gosh.... simple... I just install the suspect fluorescent tube
into an operating, good two tube fixture. If the tube works in place
of a good one that I just removed then I keep it, otherwise it is
trash. After "testing" I usually only pair up used tubes with used
tubes and new tubes with new tubes.
electricitym
..
..
..

 

[Sam Goldwasser]

First check for continuity of the filaments. Often, the open and that's
a simple test!

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[Martin McCormick]

First check for continuity of the filaments. Often, the open and that's
a simple test!

Yup. Open filaments are a show stopper all right.

These tubes all have good filaments but are old enough that
some, probably most of the cathodes, are depleted from use.

 

[sofie]

Martin:
If they are depleted from use then you don't need to any more of a test than
trying each one in with a good tube in a working fixture. If they light up
then keep them around.
As electricitym indicate in his reply post you should probably pair them up
with other used tubes. I usually replace them both at a time with two new
tubes in my double tube fixtures so I always end up with an odd amount of
good, used tubes that can be put into an easy access garage or shop two tube
fixture with just one bad tube.

 

[James Sweet]

Many fluorescent light fixtures use two tubes in series per
ballast. If one tube stops working, both are dark. I have a bunch of
4-foot 40-Watt tubes ranging in condition from probably good to most
likely bad. It occurred to me that it should be possible to test the
cathode emission on each tube by applying the 3-volt heater voltage to
the cathodes at each end and then determining what voltage it takes to
cause the gas to ionize. If one feeds the cathodes by either a
by filer transformer or two separate filament-type transformers, then
there should be no common connection between the ends of the tube.
One could use a current-limited AC source or even a current-limited DC
source to apply voltage between the ends of the tube. A good tube
should break down at around 175 to 200 volts while a bad tube will
need a higher voltage due to reduced cathode emission.

One should be able to vary the high voltage source so that the
voltage can be increased until breakdown occurs.

If the current limiting is substantial such as allowing only a
milliamp or two at 200 volts, then the tube would probably glow weakly
but the idea here is to determine breakdown voltage.

Does anybody see any reason why this shouldn't be a valid test method?

Thanks.

The phosphors normally wear out first, when the lumen output drops below
about 70% of new the tube is shot. Just pop each tube into a fixture with a
fairly new tube and toss out any that don't come up to reasonably close to
full brightness after a couple minutes.

 

[James Sweet]

Yup. Open filaments are a show stopper all right.

Not always, I have some electronic ballasts that can fire up a tube with an
open cathode just fine. I used one in the ozonator in my hot tub to squeeze
more life out of a $60(!) bulb that had both cathodes blown open when the
original choke shorted.

 

[mike]

Many fluorescent light fixtures use two tubes in series per
ballast. If one tube stops working, both are dark. I have a bunch of
4-foot 40-Watt tubes ranging in condition from probably good to most
likely bad. It occurred to me that it should be possible to test the
cathode emission on each tube by applying the 3-volt heater voltage to
the cathodes at each end and then determining what voltage it takes to
cause the gas to ionize. If one feeds the cathodes by either a
by filer transformer or two separate filament-type transformers, then
there should be no common connection between the ends of the tube.
One could use a current-limited AC source or even a current-limited DC
source to apply voltage between the ends of the tube. A good tube
should break down at around 175 to 200 volts while a bad tube will
need a higher voltage due to reduced cathode emission.

One should be able to vary the high voltage source so that the
voltage can be increased until breakdown occurs.

If the current limiting is substantial such as allowing only a
milliamp or two at 200 volts, then the tube would probably glow weakly
but the idea here is to determine breakdown voltage.

Does anybody see any reason why this shouldn't be a valid test method?

Thanks.

Ok, but back to the original question.
What about testing of tubes?
I've tried to test laptop backlight bulbs with a transistor curve tracer
without much success.
A 5" tube from a battery-powered fluorescent lamp breaks down at about
500VDC. But there seems to be little correlation between breakdown
and goodness of the bulb. Methinks different mechanisms are in play.
mike

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[James Sweet]

Ok, but back to the original question.
What about testing of tubes?
I've tried to test laptop backlight bulbs with a transistor curve tracer
without much success.
A 5" tube from a battery-powered fluorescent lamp breaks down at about
500VDC. But there seems to be little correlation between breakdown
and goodness of the bulb. Methinks different mechanisms are in play.
mike

You're comparing apples to oranges. A laptop backlight tube is a cold
cathode fluorescent, more like a neon tube. The breakdown voltage is related
to gas fill and pressure, but the "goodness" of the tube is dependent on the
emissive coating on the electrodes and phosphor condition as well. The best,
and only reliable way to tell the condition of a fluorescent tube is to fire
it up on a known good ballast rated to operate that tube. You can tell more
from the brightness of the tube than anything else since they depreciate
gradually with use and will usually get down below 50% before they fail to
light at all.