Clamping high voltage?

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?



Reply to
Travis Hayes
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What I would look at doing is figure out how the starting pulse is generated, and putting a little less energy into it.

Reply to
Michael A. Covington

you can limit the hi voltage by using an inexpensive "calibrated slot" which will arc over at the voltage setting specified on the component..sometimes, you may hear a snapping sound on your tv or monitor..that sound is caused by the safety over-voltage limiter in the unit..look up tv parts..I may not know the correct name of the calibrated slot device..but you can ask about is available

Reply to

It is not the 5 KV that killed the bulb.

It is the amps at 120 volts.

Lower the current or buy a bigger bulb.

Reply to
Charles Elliot

Arc discharge lamps require current limiting ballasts when they are running after being started. Different types of lamps usually require different ballast voltage and current characteristics. The present lamp supply is serving as the ballast as well as generating the start voltage. In most cases, the lamp its self limits the start voltage. In other words, the start voltage is generated by a current limited source with high voltage compliance and the lamp starts to draw current at some point and then the start voltage stops rising. I think the problem is not a mis-matched start voltage, but a mis-matched running ballast voltage or current.


Reply to
Jim Meyer


Both will clamp all the way down to perhaps 25-50 V given sufficient current, which is not what *you* want, but what they were designed for.

MOV's - Siemens make them - can clamp; they can be had in many voltages and sizes up to hundreds of kV.


It may not be the starting voltage, that kills the bulb - more likely it is the Running Current that is regulated specifically to allow the expensive bulb to live, i.e. Too High for the El-Cheapo! (The starting voltage is probably current-limited anyway somwhere in the mA's and unless it actually flashes over and provide a carbonized track to short out the starter, I do not think a 25% overvoltage in a few seconds is killing it - it is probably even pulled down by the load).

Sounds worse than the problem.

Buy a new, proper, bulb and fit it!

Then Sell the projector and buy a new one that uses *cheaper* bulbs, having learned now what to look out for when buying projectors!

Reply to
Frithiof Andreas Jensen

Thanks to all for your good input. I agree that the best way to go would be to either spend the money for the proper bulbs, or to replace the projector with one which uses less expensive bulbs. I thought of MOVs at first, but my experience with them is that they are designed to dissipate a large amount of power for a very short amount of time, and would likely not survive long with a 1 second surge. That, and they tend to deteriorate over time and eventually fail as an open. The concensus seems to be that it is _not_ the starting voltage that would be causing the bulbs to fail, but rather an overvoltage during normal running. Unfortunately, I have not actually seen either of the bulbs in question; so far all of my information has been gleaned on the phone with him; I don't know any specifications of the bulb that he is trying to replace. Thanks again for your help.

Reply to
Travis Hayes

The cold-starting voltage and operating current of the lamp will depend on the combination of the lamp and ballast characteristics. The ballast has been designed for the lamp intended. It would have to be modified for another lamp with different characteristics and requirements, or a suitable ballast used.

Failure of these lamps can be explosive, so you shouldn't mess around without adequate eye protection.

What are the two lamp type numbers?


Reply to

Is your associate actually measuring terminal voltages, or just reading the manual?

The igniter can generally produce double the nominal breakover voltage, if a lamp is not present to clamp it (due to internal break-over).

Continuous operation with a damaged or removed lamp can overstress the igniter through overheating - it is normally disabled by the reduced voltage present when the lamp starts to discharge.


Reply to

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