Author Topic: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture?  (Read 5099 times)
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #15 on: March 16, 2015, 09:55:50 PM » Author: Alights
Lamp current was at .42A like stated above. .
to find wattage I did .42×120V=50.4W
« Last Edit: March 16, 2015, 09:57:22 PM by Alights » Logged
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #16 on: March 16, 2015, 10:15:44 PM » Author: themaritimegirl
Oh, sorry. I asked if you meant lamp or line current and you didn't say which, so I assumed the latter.

To get lamp power you have to multiply the lamp current by the arc voltage (about 100V for a 50 watt MV lamp), not the line voltage. Then you have to multiply the result by ~0.85, which is the inherent power factor of a discharge lamp. So we're talking lamp power of about 35 watts.

Strange though, that's rather low; no better than an F40T12 ballast. As Medved said, a 35 watt MH ballast should push around 0.5A. Do you know who made the ballast?
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #17 on: March 16, 2015, 10:23:44 PM » Author: Alights
ahh makes sense! I will remember that for future reference
Thankyou Trent

bummer the ballast under powers the lamp.it's an advance ballast kit

however the 50W MH ballast seems to overpower it, and I've been using it with 75/80w MV
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #18 on: March 16, 2015, 10:41:17 PM » Author: themaritimegirl
Yeah, the 50 watt ballasts are best suited to 75 watt lamps.
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #19 on: March 17, 2015, 12:26:56 PM » Author: Medved
I will repeat what was already said, but that error is far too common here:

For assessing the real power delivered to a lamp you have always study the lamp circuit (between the lamp and ballast), the mains current does not tell anything.

I know here are many statements (e.g. about 2xF40 lamps) like "That ballast draws 0.8A from the mains, so it is full power ballast" or so, but such conclusion is completely wrong: The fact the ballast consume 0.8A does not automatically mean it feed the lamps by the full 40W. The only thing it means it loads the mains by 0.42A, but it could still feed the lamps just by 25W.
OK, it does mean the total lamp power plus losses can not be more than 100W, but that information is not what was supposed to be judged...

Only with special cases the lamp current is equal to the mains current: Either not compensated series reactor ballast, or (way more rare case) just by a coincidence the mains and lamp current happen to match. The series reactors are quite common in the 230V area, but not in the uncompensated form.
And as the F40 arc voltage (~105V) is very close to the US mains voltage (120V), the high power factor (~0.95) F40 ballasts with the typical ~85% efficiency happen to have the lamp current not that far from the mains input current, but that belong to the coincidence case - if another ballast would be just normal power factor (~0.6) and feed the lamp by just 25W (common with the shop lites), the mains current would be similar 0.4..0.45A per lamp. And the conclusion "0.45A mains current => lamp fed by 40W" would be completely wrong.
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #20 on: March 17, 2015, 12:56:48 PM » Author: themaritimegirl
I know here are many statements (e.g. about 2xF40 lamps) like "That ballast draws 0.8A from the mains, so it is full power ballast" or so, but such conclusion is completely wrong: The fact the ballast consume 0.8A does not automatically mean it feed the lamps by the full 40W.

I've seen cases of such statements here, but the other way around. Many members deem the Advance Mark III Energy Saver ballasts to be reduced power because they are 0.73A (and probably because they have "Energy Saver" in the name), but I have confirmed through my own measurements of lamp current that they are full power.
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #21 on: April 21, 2015, 06:44:07 AM » Author: ggillis
I think I just recently seen a few 35 watt metal halide ballasts on E-bay,  if I see them again, I'll post the links.
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #22 on: April 21, 2015, 06:47:22 AM » Author: ggillis
The 50 watt mercury vapor ballast I have drives the lamps at approx. 60 watts  :-\  0.61A @ 99 volts
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #23 on: April 21, 2015, 03:35:57 PM » Author: themaritimegirl
That's exactly perfect. 99V by 0.61A is about 51 watts. You have to factor in the lamp's power factor, which throughout my testing I've found to be around 0.85.

The 50W MV lamp is rated for 0.60A.
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Re: will a 40 watt rapid start ballast work for a 50 watt mercury vapor fixture? « Reply #24 on: April 22, 2015, 08:09:03 PM » Author: ggillis
Oh yeah lol, I forgot about that  :-[
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