Power rating of flourescent ballasts?

[ivan747]

Stupid question here. Where does the current rating for a fluorescent tube ballast comes from?

If I have a ballast that claims to be 21W, and I place a 39W lamp, for example, what will happen?
-Will the ballast burn out?
-Will the ballast limit the current to the lamp? (I've learned that flouros are like LEDs in the sense that you need some current limiting for them not to auto destroy. And that limiting comes from an inductor in the ballast)

I'm specifically trying to figure out if I can switch a T5 bulb for a T5 high output bulb, the fixture has 21W marked on it, as well as the tube it came with. Of course, I could mod it with a different ballast if necessary but I'd rather keep it cheap (component level upgrades, perhaps), this is a very cheapskate project I'm working on.

Thanks !

[SeanB]

If it is a magnetic ballast it is simply a choke, which limits the current in the tube by it's reactance. In the USA it gets a little more difficult as most ballasts there are actually an autotransformer with a inductive ballast, so that you have a high enough voltage to strike the tube, then the current is limited by the ballast inductance.

But in all cases the current is roughly constant, and almost independant of the lamp type, as most flourescent lamps tend to want to keep a constant terminal voltage when struck of around 70- 130VAC, depending on lamp type, temperature and age.

With electronic ballasts the operation is similar, but many also incorporate sensing of lamp voltage and current, so might not start the higher power lamp, or it will switcvh off after a few seconds.

[ivan747]

So, if I understand correctly, power should be roughly limited by the ballast if it is not an electronic ballast. Also I will not get as much power as I should with a high output lamp but no damage will be done to the ballast.


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