Testing a fluorescent bulb

kudzu9November 8, 2008

I've got a light box that isn't lighting up when I turn on the switch. It uses an F8T5D bulb. Is it possible to test this bulb with a multimeter? If so, what do I do? Thanks.

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There should be a filament that provides continuity between the pins at each end.

However, there are many other things that change about a fluorescent besides burning out filaments. I don't know (and am too lazy to look up) the type number. Some older ones use a starter assembly that can go bad. That is typically a little can that twists into a socket.

Also, old ballasts sometimes get weak. When new tubes don't fix a light, it is time to suspect the ballast. I just replaced four ballasts that were over 25 years old in a church where new tubes wouldn't start reliably and that fixed them. I suspect a capacitor in the ballasts dried out and failed.

    Bookmark   November 8, 2008 at 11:20PM
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You can't test the tube without putting it in another known working fixture. The filaments could test fine but the tube is no good.

    Bookmark   November 9, 2008 at 9:41AM
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Unfortunately, it's a 12", small diameter fluorescent and I don't have any other fixtures to try it out in. What kinds of diagnostic tests can I do on the light box if I can't test the tube with a multimeter?

    Bookmark   November 9, 2008 at 6:04PM
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The best way to test the fixture is with another fluorescent light tube.

    Bookmark   November 10, 2008 at 8:33AM
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Here is the fluorescent test procedure used by me
1.Replace the tube.
2. If #1 doesn't work replace the ballast or the whole fixture.

    Bookmark   November 10, 2008 at 9:02AM
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Is there a way to test the ballast with a multimeter?

    Bookmark   November 12, 2008 at 8:54PM
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You can test for open windings if you have a diagram showing how the many wires are connected.

However, a more common age problem, particularly when the light "sometimes" works, is the capacitors inside the ballast get weak. That's hard to set up a test for because of interaction with the inductances. If I had to come up with some test, I'd measure resonant frequency, but I never have put in the effort to figure out what was normal.

    Bookmark   November 13, 2008 at 12:08AM
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I have an odd situation.
I have a fluoresent fixture with 6 bulbs, none work when replaced, power is on. It has 3 ballasts 2 are cool to the touch 1 is warm. By touch can you tell which ones are working or do all 3 need to be replaced?
Thank you for any help.

    Bookmark   November 14, 2008 at 8:01AM
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It sounds like none of them are working, if no tubes light up.
Have you confirmed that there is the proper voltage to the ballasts? Did they all go out at the same time?

    Bookmark   November 14, 2008 at 8:13AM
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Replace fluorescent tubes in pairs if they are wired in pairs, to get things working.

Then if you are trying to economize, try the ones you took out one at a time in a known good fixture with a good mate to find out if the suspect bulbs work well. If they don't light reliably and at about the same brightness as the new one then they are not worth saving, as you will be replacing them again soon. Finally, if you have a pair of still-usable ones of the same age, put them together in a fixture.

If a new pair doesn't work, test with a voltmeter for any sign of power at the bulbs. There should be a few volts across the pins at one end of the bulb, and the same voltage across the other end. Then there should be line voltage or more between ends of the tube. If there is a missing voltage, you have a bad ballast.

If there is no voltage anywhere at the tubes, then check whether the power is coming into the ballast.

    Bookmark   November 14, 2008 at 11:07AM
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I have two 6W T5 flourescent bulbs (9") which i want to use. But i dont have any kind of fixtures. Can someone help me in developing a circuit so that i can use the bulbs.

    Bookmark   April 15, 2011 at 2:55PM
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Anybody reading this thread needs to realize...fluorescent bulbs of any type do **NOT** have a filament. They *cannot* be tested for continuity.

This post was edited by cuba_pete on Tue, May 13, 14 at 13:42

    Bookmark   May 13, 2014 at 1:39PM
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Denis Barker

Florescent tubes usually have two filaments, one at either end between the pins. If a continuity test shows an open circuit between the pins your tube is definitely dead. It may also have failed for other reasons so the presence of continuity does not mean the tube must be good.

    Bookmark   March 11, 2015 at 5:35PM
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