The-Wolf said:
yes you can use a lower watt tube on a higher ballast, but you cant do it the other way around.
actually, I beg-to-differ
I believe, that as standard, the lamp should match the ballast.
Fluorescent lamps are negative-resistance, which means that as more current flows through them, the resistance of the lamp falls and allows even more current to flow (a run-away effect).
This is what a ballast is for; to limit that current. If you select a higher rated ballast, then that current-limiting point will be set higher (for a higher rated tube) and will therefore not limit the current to the lower rated tube until it is drawing as much current as the higher rated tube would, by which point it might have burnt out (or exploded) through overdriving. I guess the effect will depend on how big the difference between the ballast and tube ratings.
There are ballasts that can accomodate a selection of different lamps but these would be designed to do that by incorporating electronic current sensing, etc. I think they call these ones adaptable/universal ballasts.
So I would check the manufacturers description because I doubt that many aquarium ballasts are universal (to save cost).
Most electronic equipment (i.e. not negative-resistance) will draw as much current as it needs to operate and not 'run-away'. So a lower rated piece of normal equipment can be run from a higher rated power supply. But a ballast is a regulator, not a power supply. I think this is what you were meaning Wolf.
