Why can't you run two MH bulbs off one ballast

[BCOrchidGuy]

I've been told that MH bulbs need one ballast each, I was wondering why I couldn't run two 175watt bulbs off my 400watt ballast, yes both bulbs would be over driven but, I'm not sure the extra 25 watts would make that much difference.

Any ideas, comments etc?

Doug

[StirCrazy]

well it all comes down to the bulb design, MH bulbs have no current limiting device in them whats so ever so as they warm up there resistance will drop causing them to accept more power. so running a 175 off a 400 watt will probably result in a melt down at best. now as for running two 175 watt bulbs off one ballast there are a couple problems.

the first being as there is no current limiting device except for the ballast you have no way of insuring the power is split evenly between the two bulbs. in this situation which ever bulb heats up faster will draw more and more power which will cause it to heat up more and then draw more power (you can see where I am going).

the second problem is one ballast cannot supply enuf starting current for two bulbs.

all and all, I think this would be a dangerous thing to try.

Steve

[BCOrchidGuy]

Well for a short while I thought it was a good idea. Thanks.

Doug

[Bob I]

Originally Posted by StirCrazy

well it all comes down to the bulb design, MH bulbs have no current limiting device in them whats so ever so as they warm up there resistance will drop causing them to accept more power. so running a 175 off a 400 watt will probably result in a melt down at best. now as for running two 175 watt bulbs off one ballast there are a couple problems.

the first being as there is no current limiting device except for the ballast you have no way of insuring the power is split evenly between the two bulbs. in this situation which ever bulb heats up faster will draw more and more power which will cause it to heat up more and then draw more power (you can see where I am going).

the second problem is one ballast cannot supply enuf starting current for two bulbs.

all and all, I think this would be a dangerous thing to try.

Steve

Although the explanation is not fully correct, as resistance rises with temperature increase, I will not try my explanation. It is best left alone. If you do not have a supply of free bulbs, don't even think about it.

[Son Of Skyline]

If you do have old spare bulbs, I'd say give it a shot and see what happens. It's not like any of us have actually tried it (as far as i know) so who knows. The guys are probably right but I'd say its worth a shot if you've got the bulbs lying around.

[BCOrchidGuy]

I don't have the bulbs laying around I just thought I would ask. I thought maybe someone had said NO you can't run two bulbs off one ballast because they want to sell two ballasts.

Thanks for the input everyone

Doug

[StirCrazy]

Originally Posted by Bob_I

Although the explanation is not fully correct, as resistance rises with temperature increase, I will not try my explanation. It is best left alone. If you do not have a supply of free bulbs, don't even think about it.

um I think you better check up on that Bob as we were tought almost all conductive materials decrease resistance with increase in tempature, even had to do a few labs to prove it.

mind you the books we were using were probably written in the 60's... hmmm maybe the same ones you used

anyways don't try it, I saw the result of trying to run a 250 off a 400 watt ballast. just a waist of bulbs.

Steve