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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, I haven't been able to find much info on the net. The particular bulb I'd like to run is mercury vapor rated at 135V arc voltage, 3.25 amps and 400W (intended to be run off 220V AC), but general electronic ballast ideas are also welcome!
-will it be enough to run it from 160V from the half bridge (I guess not) or should I step it up? -what should be the frequency and duty cycle? Is it dimmable by varying the pulse width (is the life decreased by running under rated wattage?) -is it accurate to measure the running current with DMM in series with the bulb? -the lamp was not designed for use with electronic ballast, will it be OK?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
-No, you need 220V or more for reliable running. Americans have to use step-up transformers in their mercury lamp ballasts.
-The frequency matters because if you hit an acoustic arc resonance, you can explode the lamp. The arc gets twisted out of shape and forced against the discharge tube wall, causing it to fail prematurely. You can view the arc through dark glass to see if it looks misshapen or snaking around.
The duty cycle doesn't matter: typically you use 100% and set the lamp current with a ballast inductor, just as if it was a 50Hz ballast. No, it is not dimmable: discharge lamps need to run at full power to keep the electrodes red hot, or else they will be destroyed by sputtering.
-Probably not unless your DMM is true RMS and the RMS converter works at high frequencies.
-If you avoid the above mentioned arc resonances, and don't try to dim it, maybe. These mercury vapour lamps have their arc resonances smack in the middle of the most popular SMPS frequencies, 30-100kHz, and I think HF mercury arc lamp ballasts are quite rare for that reason.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
> -will it be enough to run it from 160V from the half bridge (I guess not) or should I step it up?
The quickest route to getting it lit is probably to use a suitable line-frequency iron ballast in series with the lamp and run it directly off the mains. You might need to inject an HV pulse to ionise the gas and get it started though.
>-what should be the frequency and duty cycle?
50Hz if you say that it is not designed for use with an electronic ballast.
> -Is it dimmable by varying the pulse width (is the life decreased by running under rated wattage?)
You may be able to dim the lamp by reducing the pulse width of an electronic ballast to reduce the average current. I think high crest factors may shorten the lamp life though.
> -is it accurate to measure the running current with DMM in series with the bulb?
If driven from 50Hz then any true RMS reading DMM should show the arc current correctly. It is, however, unlikely to give a meaningful reading for the voltage measured across the lamp due to the negative resistance of the arc and non-sinusoidal waveshape.
If driven by an electronic ballast at high frequency, a ferrite CT and scope can monitor the lamp current. Most DMMs do not have sufficient bandwidth to give meaningful results above 400Hz.
> -the lamp was not designed for use with electronic ballast, will it be OK?
The worst that could happen is that the HID lamp explodes, so if you plan for this eventuality in terms of containing flying glass and Hg vapor it will be OK.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Thanks guys, I just found out that two MOTs primary in series, shorted secondaries give about the right inductance, so I'll use this to run it for now.
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Electronic ballast for HID bulbs
