burned out halogen

I have a friend who moved into a house with a "fiber optic" pool light unit that wasn't working when he moved in. He went to the pool store and they gave him a replacement bulb -- it's a GE 24V 250W halogen lamp, i think its the same as this one: http://www.specialtyoptical.com/catalog/ge_elc_1755642.htm anyway -- the thing starts working but only lasts a couple of hours and burns out (the lamp is visibly grey on inside of bulb).
The one thing odd i noticed is that the open circuit voltage of the transformer is only 20V.
Possibilities? I'm thinking the open circuit voltage for a 24V light would be way above 24v(?)... Is it possible it's really a 12V system? Any other ideas? what else would cause the bulb to pop? These bulbs are pricey (~ $20) so I don't want to just keep popping them!
[halogen bulbs run better hot -- something about the tungsten atoms redepositing on the filament with high heat]
Thanks!
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Geek,
Are you sure that you got the correct bulb? That you measure less than 24v makes me wonder why you're using a 24v bulb. You're not going to get more voltage with a bulb in the circuit.
Dave M.
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David Martel wrote:

I believe there are some power supplies that might give more voltage under load, but I don't see them using one in this application.

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Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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It sounds as though it could be a heat problem. Is the unit well ventilated? Are there any dirty filters? A defective lamp socket or loose connection can cause excessive heat on the filament. Look for signs of burning or arcing on the lamp socket and the wiring that feeds it.
Are you sure that the proper bulb for the unit was installed?
It may be a 20 volt power supply to make the bulb last longer. Look around on the nameplate or inside to see if it mentions output voltage anywhere. 250 watts in a little MR-16 bulb is a lot of heat. That bulb is rated for 50 hours at 24 volts and is to be used with the socket base down or up only to full horizontal. The lamp socket should never be higher than the horizontal position.
John Grabowski http://www.mrelectrician.tv

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Halogen bulbs should not be touched by bare hands;the skin oils left on the envelope will cause early failure. Use gloves.
--
Jim Yanik
jyanik-at-kua.net
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A bulb run at less voltage will last longer , almost indefinatly at 5% -10% rated voltage. You have other problems, moisture hitting bulb, oil on bulb , a bad bulb , bad socket causing arching or overheating.
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Do not touch the bulb with your fingers. Use a clean gloves or rags. The oils on your hands cause them to burn out so soon. If you touch the lamp clean it with alcohol and let dry before installation. Read the directions with the lamp most say do not touch the lamp.
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