A fluorescent ballast for my son's aquarium blew a couple resistors. This part is built into the cover of the aquarium so I'd rather try to replace the blown components than the whole thing. Can anyone assist with determining the value of the resistors so I can try replacing them? I can't find any info using the ballast model number. The bulb is 8w.
If you replace the resistors they'll just blow up again. I bet you'll find both power transistors are shorted, particularly since they've used wimpy TO-92 transistors where big TO-220 cased devices were obviously the original design.
Personally I wouldn't even mess with it, you can get a suitable ballast circuit board out of a compact fluorescent lamp, or you can get a nice electronic ballast on ebay that'll work. I used a Triad ballast designed for
18W CFL's to replace the shorted chokes in my spa ozonator which has an 8W lamp.
The wattage doesn't quite work like that. Various classes of fluorescent tubes are rated at specific currents, and the length of the tube roughly determines the voltage across it and hence the wattage it runs at. The rating is just nominal, you can under or overdrive them by 10-20% or more at the expense of lifespan so it's really not too critical. There's a smaller version of this
which turns up from time to time which would work well for you, it's designed for 1-2 16W CFL's, that'd be ideal if you want to run either one or two lamps. You can't just run them in series, you won't have cathode heat and the tubes will fail soon. Either pull the ballast out of a 9W CFL, or find something similar to what I described. If you get stuck I think I have an extra one somewhere I'd part with.
I notice that some - but not all -electronic ballasts specifically mention "1 or 2" bulb operation. Since I have 2 single bulbs fixtures that will need new ballasts and I happened to have on hand 2 "2 bulb" electronic ballasts of more than sufficient wattage, any electrical reason why I could not make use of these? Thanks for any comments or wiring suggestions.
AFAIK, this 'problem' was originally one that was associated with non-electronic ballasts i.e. ones that were a pure line frequency transformer. To have the voltage at the output anything like the specified value, the load had to be known, and it was always recommended that if a bulb failed, it should be replaced ASAP, as the output voltage would increase with the reduced load, over-volting the remaining bulb(s). It's also important of course, not to overload such a device with too many bulbs, hence the wattage rating on it.
It's a little different with the electronic ballasts in that they are fundamentally a switch mode power supply, so in theory, you would expect them to give a reasonably stable output whatever the load, up to some specified maximum amount, where you would start to overload it. However, all the ones that I've seen, this side of the pond at least, only nod in the direction of a 'proper' switcher. The output is a high frequency square wave, modulated at line frequency, due to the fact that that there are no primary or secondary-side filter caps. There is also no kind of regulation applied to the design. So, the short answer to your question is that I think the only way that you will see if the ballasts that you have are suitable for continuous driving of a reduced load, is to stick a 'scope across the output, then load it with first one bulb, and then two, and see if there is any significant change to the output waveform, or brightness of the first bulb, when the second one is connected.
Actually, I though "sufficient" was the right word. I assumed that a ballast could be damaged if it was incapable of supplying sufficient current. If the ballast can supply more current, what is the harm?
[I glean that maximizing bulb life is an issue for precisely matrching ballasts in commercial applications but a minor concern in my house.]
Well, it might not be such a minor issue if you actually do it. Over-volting a 12v halogen by as little as 10% can significantly shorten its life, and when you start having difficulty getting them in a couple of years after the eco-bollox merchants have had the manufacture of them banned totally world-wide ...
If the ballast is rated to supply more current than is actually required, there is no harm going to come to the ballast itself. However, if there is no regulation of the output voltage other than by load and the ballast's output impedance, then you could significantly shorten the bulbs' service life, by having insufficient load.
I admit I don't know much about the use of electronic ballasts. It was my understanding that T12 bulbs were meant to be used with a magnetic ballast, either with or without a starter. For an electronic ballast, the bulbs are T8.
Is it also possible to use T12 bulbs with an electronic ballast? If not, you should be able to use your electronic ballast in the fixture by getting the correct bulbs. People in the electrical department of a Home Depot or Lowes type of store should be able to advise you.
You can get electronic ballasts for virtually any type of tube. I have tri-phosphor T8 types lighting the worktops in my kitchen driven off Osram dimmable electronic ballasts. They are more efficient than conventional ballasts therefore producing less heat and allow extremely long tube life. The dimming is achieved by a simple pot working in a low voltage circuit. Good starting characteristics too - near instant. Other benefit is some dislike the mains frequency flicker produced by some fluorescent lights. These ballasts run at approx 30kHz so no visible flicker or audible hum. They are expensive, however.
*Corduroy pillows are making headlines.
Dave Plowman email@example.com London SW