Hello all,
I've a friend who has enlisted my help in an area where I'm not too familiar. He has a power source that produces a 5kV pulse for about a second, after which it settles down to 120V line voltage. The supply is designed for starting and running a metal halide bulb. The 5kV pulse is to arc and get a plasma going, then the 120V sustains the arc until the supply is switched off. This bulb is the heart and soul of his LCD projector. The problem is that the bulbs themselves run $400 a piece and last for around
2000 hours. He is getting tired of buying these metal halide bulbs and is looking for possible replacements. He thinks he's found a source for bulbs that are of equivalent brightness and color temperature for about $30 a pop. The snag is that the starting voltage for these bulbs is only 4kV. He installed a fresh bulb, which started and ran fine, but failed on the second ignition. My guess is that the 20% higher starting voltage just stresses the bulb too much. Here is my question: He's come to me, asking how he can hold the start voltage to 4kV, while not impacting the voltage during normal operation. My experience is all in the sub-1kV range, so I'm not quite sure what to suggest. My personal opinion is that this is one of those penny-wise, pound-foolish ideas, but I'm looking for suggestions, none the less. I've thought of the following ideas, and rejected them for different reasons: 1) Gas discharge tube. Most that I've seen have a threshold in the hundreds of volts, while I need it to clamp to 4kV. 2) Spark gap. Seems to be very un-deterministic, and variable with time and repeated use. 3) Step-down transformer. A 1:0.8 turns transformer might do the trick, but at 5kV the insulation required would be a trick. Also remains the problem of switching it out of circuit during normal operation.Any pointers?
-Cheers,
Travis