Bathroom illuminated mirror problem

[Roger Mills]

I have an illuminated bathroom mirror which has two unsilvered vertical strips with 15w T8 fluorescent tubes behind them. The tubes are powered via a single 2-way electronic ballast.

The lights have suddenly developed a fault. When turned on, they light normally but then go out after a few (4 or 5) seconds. If I then turn them off and wait a minute or so, I can turn them on again - and the cycle repeats. There's no way they'll stay on more than a few seconds.

Anyone come across this sort of thing? AIUI, the ballast is supposed to deliver a high start-up current and then cut it to a lower level for running. So could it be cutting the current to zero rather than to the proper value?

Is replacing the ballast likely to fix the problem, or could the tubes be at fault?

The ballast is a Kengo KEB-215. I can't find one of these anywhere, or even one of another make with the same spec. The nearest I can find is a Tridonic 22185216 - which is 2 x 18w rather than 2 x 15w. It's also slightly longer, so I'd have to bodge the mounting brackets. Assuming I can make it physically fit, is the difference in electrical spec likely to be a problem?

[ARW]

Go LED?

[Brian Gaff]

No it might not look very nice in that sort of application you will see the brightness hot spots. I would say its the ballast myself. I had a similar issue on a rostrum mounted cctv some years ago and that was the ballast. as for wattage. Hard to say if it would make any difference. It did not on my version, but its been a lot of years since then and things do change. These work by not using the filaments and I often found that if the room was cold they would not start and stay started until they warmed up. Brian

[tabbypurr]

I'd assume it's the ballast. Good time to go to LEDs. Under-run them & they last very well.

Or you could look for bad caps or a fractured connection in the ballast.

NT

[Dave Plowman (News)]

You'd need a lot of LEDs to give as uniform a looking light as a tube, if that matters.

[Robin]

[tabbypurr]

a neat pattern is usually fine.

[Andrew Gabriel]

If it's a good electronic ballast, it will be detecting unbalanced current flow in the tubes, and shutting down in that case. This would be because one of the tubes has run out filament emission coating, and starts operating as a rectifier. If allowed to continue operating, that end of the tube will overheat, and would likely damage the lampholder and anything else near the tube end.

If this is the cause, you need new tubes. Often you will see at least one tube end is blackened too (that's the sputtered off emission coating).

[Roger Mills]

Many thanks for the all the comments.

I think it was probably a bad connection on one of the tubes. I took both tubes out and turned them the other way up - and it started working ok.

For future reference, can you get LED tubes which fit the same sockets? If so, do you junk the ballast and just connect them directly to the mains?

With regards to checking the ballast for failed caps, etc. - the whole thing seems to be pretty much encapsulated, with a few wires emerging from both ends. I doubt whether it could be opened up non-destructively.

[tabbypurr]

both methods are used

NT

[ARW]

You can get them. I have fitted quite a few.

The ballast is bypassed but there are a number of ways to wire them up depending on the LED tube that you buy!

[Brian Gaff]

No you cannot get into those ballasts. Its a nasty trick so you have to replace them. Now I'm intrigued. Are the tubes merely in series with only one set of heaters active? I saw a system like this, it was a bodge and I never did understand how on earth it ever worked as the voltage had to break down two tubes not just one. If you try this sort of thing with neon's you either get nothing or a kind of brightness oscillation effect which is not really what was wonted. I also remember some old Woolworhs strips with two tubes and they end plugged into a bayonet. There was no choke and just a small little encapsulated blob. It worked but not when the weather was cold! Brian

[Roger Mills]

Can you point me at a suitable replacement for 18" T8's? They seem less plentiful than 24" tubes.

[Andy Burns]

Google showed me several

Plus numerous Amazon/eBay sellers ...

[Roger Mills]

I can't quite work it out. There are 6 wires coming out of the ballast. Two go to the bottom socket of the left tube, and two to the bottom right. The other two go one each to the top of the tubes. Finally there's a cross-connections between the other two pins on the top sockets. Whether this constitutes being wired in series, I'm not sure.

I've got a similar device at my holiday flat, and ISTR that when one tube failed, neither would light.

[Roger Mills]

Thanks, but the question to Adam was in response to his suggestion to use LED-based tubes, and that was what I was looking for. I'm aware that I can get bog standard fluorescent tubes ok.

[Dave Plowman (News)]

They can be odd, these small tubes. Have a battery one where the pins are wired in parallel. So no heater.

[ARW]

Erm. No I cannot.

I shall look into this bit lacking info.

[BrianR]

replying to ARW, BrianR wrote: I see it was some moths ago, but I have the same fitting. Did you ever find an 18" LED tube? I couldn't. I also can't find a ballast of the same size (length 240mm fixings).

Bian R

[Roger Mills]

No, I couldn't find any 18" LED tubes. I found some ordinary fluorescent tubes - but they didn't fix the problem.

I then bought a ballast from CP Lighting Ltd of Surbiton. It wasn't identical to the original, but it worked fine. I can't see the exact one on their website, but are several others which would probably do.

The one I bought was specified for 2 x 18w - 24w T8 tubes rather than 2 x 15w, but that didn't seem to matter.

It was slightly shorter and quite a lot wider than the original. Width wasn't a problem - there's plenty of room. I needed to bolt a short fish-plate to one end to make it the right length to fit the fixing points.

The connections on the new one were push-fit, and only worked with rigid conductors, whereas the captive wires connected to the old one were stranded. I got round that by using short rigid wires into the ballast, joined to the original wires with choccy blocks. [I think I used crimps on the mains input wires.]

I could probably have saved a few bob by just replacing the ballast - but anyway the thing now has a new lease of life!