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Testing Fluorescent Tubes for Cathode Emission

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 WB5AGZ  Stillwater, OK 
Information Technology Division Network Operations Group
Reply to
Martin McCormick
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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 . . .

Reply to
electricitym

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

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Important: Anyth> Many fluorescent light fixtures use two tubes in series per

Reply to
Sam Goldwasser

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.

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Martin McCormick WB5AGZ  Stillwater, OK 
Information Technology Division Network Operations Group
Reply to
Martin McCormick

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.

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Reply to
sofie

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.

Reply to
James Sweet

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.

Reply to
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 
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Reply to
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.

Reply to
James Sweet

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