Fluorescent tubes with electronic ballast

[Roger Mills]

I have a bathroom mirror with two 18" 15W fluorescent tubes inside, similar to

A couple of days ago, one of the tubes didn't come on. I took the front off and swapped the tubes over in order to try to ascertain whether the problem was with the tube itself, or with whatever was powering it.

The effect of swapping the tubes was that *neither* of them now works - and they continued not to work when I swapped them back!

The tubes are driven by some sort of electronic ballast which has mains in at the top and 8 outputs at the bottom - 2 for each end of each tube.

Do any of you have any experience of these things? If a tube fails, can that kill its side of the ballast? Could swapping the tubes have killed the other side, too?

Is there any way that I can test a tube by using a multi-meter? I've tried measuring DC resistance across the pins. On both tubes, there appears to be virtually no resistance between the two pins at the one end and infinite resistance between the pins at the other end, but I'm not sure what - if anything - that tells me!

Would the best plan be to replace the ballast and both tubes? I can't find an identical ballast but this one is similar and would probably fit:

This is 2 x 18w rather than 2 x 15w, but I could get this plus 2 x 18 watt tubes for about £20 - which is a lot cheaper than replacing the whole thing for £70.

Any comments?

[Dave Plowman (News)]

Tubes with heaters would normally be something like 15 ohms between pins. And the same at both ends.

[tabbypurr]

yes

no

no

both tubes are dead

Probably just dead tubes, but always the unlikely chance a ballast may have killed a tube. I'd fit 2 new tubes, that will probably fix it.

NT

[Andrew Gabriel]

They each have a low voltage filament connected across them, so should be quite low resistance. Open circuit means the tube is dead. It also means the ballast isn't very smart - it should shutdown the tubes before it burns out the filaments. Burned out filaments means the ballast carried on trying to run the tubes *after* they were already dead, and that can make the tube ends very hot. It has been known to crack or melt the glass and overheat things near the tube ends (such as the lampholders).

Ballast might well not be dead. I would try with two new tubes.

18W tubes are 2' long, which probably won't fit.

[ARW]

Buy LED tubes and forget the ballast.

[Roger Mills]

Well spotted! I had misread the description of the 18w tubes.

I can't find a 2 x 15w ballast anywhere. Would one designed for 2 x 18w be ok with 15w tubes?

[Roger Mills]

Interesting suggestion! Can I get LED tubes which are direct replacements for 'ordinary' tubes - same length and diameter, and pins on the ends which fit the existing holders?

If so, how would these need to the wired? Currently there are 4 pairs of wires which connect to the output end of the ballast. Would each pair of wires have to be directly connected to the incoming mains?

[harry]

There are several failure modes. There is a filiment between the two pins at each end ie, two in total. Thes e can go open circuit. Unusual with electronic ballast.

The tubes are filled with sub-atmospheric argon with a smidgen of mercury. If air leaks in they are kaput.

Check out the wiring and tube contacts, you might have disturbed something. The tube contacts are very fragile these days. Also prone to corrosion esp. in kitchens &c. Also connection on ballast.

[harry]

You might find it cheaper to buy a complete fitting than a new ballast. That's what happened last time I had similar problem.

[harry]

I looked at the lumens per wat, electronic fluorescent/LED There was not much difference. Price, LED twice the cost. It was in the kitchen so not much used. So I went fluorescent. Things may have changed, LED has come down a lot in price recently.

[ARW]

and the link has a wiring diagram on it!

I do not believe that you need any help to work out how to bypass the ballast and wire that circuit.

[Roger Mills]

Fair comment!

Only problem is that nobody seems to make T8 LED tubes less than 2' long, and mine are 18"! Have you seen any as short as this?

[tabbypurr]

LED tape can be whatever length you want it

NT

[pamela]

Interesting but that lamp is only 900 lumens. Not really as bright as a fluorescent.

[Dave Plowman (News)]

Yes - but it's the latest thing. LED. So must be better. ;-)

[ARW]

Stop complaining and start bragging about extra few inches.

[bert]

In article , Roger Mills writes

\isn't a B&Q one by any chance. I had similar problem and ballast for

2x18w works OK. Can't remember/find where I bought it. Original was Chinese.

[Andrew Gabriel]

It's a bit of a cross of technologies. 15W 18" T8 tubes are an old halophosphase tube size, as opposed to modern triphosphor T8 tubes, although any you buy today will be triphosphor. They were the first size of fluorescent tube available in the US, but were never particularly common in the UK. They operate at 300mA which doesn't well match any other tube size, so using a different ballast may not work well.

However, it might not be the ballast.

[alan_m]

18" was/is common for lighting 2ft fish tanks.

[Roger Mills]

Mine came from Argos - probably the one I cited, or very similar if not. B&Q appear to sell one which is the same size, and looks the same - but uses 8w T5 tubes instead of 15w T8's.