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what are possible causes and solutions for low output voltage on a HID ballast? Answered

HV starting pulse is normal, but then output voltage drops to around 1.5-2VAC.  no obvious damage to PCB or components within.


Is that against a real HID lamp  ? Or a simulated load ? Or no load ? 

that's with no load, just the multimeter.....do i need to check the output voltage with the lamp connected?   

Yes, you have presented the PSU  with a fault condition ! Its whacked the tube with the starting pulse, fully expecting the thing to conduct and demand a lot of current at a low voltage ( I don't know what the tube parameters are), and you've not given it anything. Its probably shut down as a safety measure, but the lamp is pretty low impedance when its on as far as I can remember, so 1.5-2V might be nominal for all I know,

checked output voltage with the lamp connected, similar results, voltage was fluctuating around 1.84VAC....there's a brief blue flash in the arc tube when i power the ballast on.  the lamp only has about 200 hours on it... 


I assume the ballast strikes as soon as its powered ? Any way of testing it against another tube ? 200 hours may not be long, but it may still be screwed....


aha! i guess it was the lamp after all.... i didn't have another 400W metal halide to test but figured it wouldn't do too much damage to briefly connect a lower wattage lamp.  so i used a 175W mercury vapor lamp and it fired right up.  
for $30 my projector will be back in business....

thanks for your input steve! 

Fix ! AND you now know how a HID ballast works.
Result !


thanks for the info! i'll have to test this a bit more, i see... and i will report back 

Is this something that has stopped working after a long/short period in use, or something you're adapting for another purpose?


This is the ballast for the 400W metal halide in my homemade video projector that has stopped working after maybe a year and a half of minimal use. I used it infrequently until 3 months ago when i used it for approx 12 hrs/week.  Just the other day, i powered the projector on, and noticed that the arc tube had a brief, faint blue flash before going dead.  I figured with not much more than 200 hours on  the lamp, that the problem was not there, but with the ballast.  So I checked the voltage at the ballast output and found it incredibly low.  I would like to try to repair the ballast, or else i'm out $140 for a new one...although perhaps i could fit a low cost ballast kit into this one's heatsink/enclosure.....