175w halide bulb on 250w ballast? revisit

[dz6t]

I saw a friend who is an electronic engineer running a 250w SE halide bulb on a 400w ballast. I was told that as long as the ballast is rated higher wattage than the bulb should be OK.
Can someone who has more information chime in please? Thank you very much

 

[planereefer]

They are correct, as long as the ballast is rated higher than the bulb. All is well, it just doesn't push the bulb as hot.

 

[s_kelley]

I'm confused .... So if I throw a 250w bulb into my 400w fixture it would be fine? Just doesn't seem right

 

[frank180reef]

seems odd but:
think of the bulb as a load/resistor. The balllist is a source that can supply the current to the supply the of 400watts (I=V/R).................The 400watts relates to the max current (I) I beleive...............

 

[jselzler1]

Almost correct (I=V/R) is Ohm's law. This states the current, measured in amperes, is equal to voltage differential divided by resistance. The ballast provides the voltage differential, the bulb is a resistor.

The correct relationship is (W = I * V), which states the power (watts) = current time voltage differential. A 250w bulb consumes 250w of power. If hooked to a 400w ballast the current draw of the bulb will be lower than if hooked to a 250w ballast. Because the current is lower, the resistance is lower (see Ohm's law) therefore bulbs temperature will be lower as resistance usually means heat.