Sometimes my job can become quite interesting, due to either crappy quality control from an equipment manufacturer or some really bizarre karma. The job I'm currently wrapping up is a good case in point.
First, bear with me as I give some background on what a "Xenon Power Supply" is. Modern cinema projectors use a Xenon Short-Arc lamp as a light source. Wattages of the lamp
range from 500 - 7,000 watts (up to 10,000 watts for "IMAX"). The average cinema uses either 2k or 4k lamps. The lamps operate as a constant-current arc, at low DC voltages. A typical case such as this one is 2,000 watts, 70-85 amps and roughly 24-35 volts DC.
Early designs for these power supplies were based on a transformer/rectifier combination, not unlike a typical arc welder/forklift battery charger. (In fact, the very first supplies for cinema use were modified forklift supplies made by Christie Electric.) The advantages are simplicity and ease of repair (most common failure being the diodes), but have the disadvantages of weight, noise, and heat generation.
Modern supplies, "switchers", like the ones in this caper, are built like computer supplies on steroids, using switching PWM technology and much smaller (and lighter) transformers. Light weight, fairly efficient, but not repairable in the field.
That brings us to the caper at hand:
The room is equipped with two projectors using identical lamps. Part of the job was to provide a cutoff when the fire alarm was activated. Easy enough, I just had relays interrupt the 120vac control voltage to each power supply. I had replaced the old supplies as one was going bad (early generation switcher) and the other was the old magnetic style without a housing!!
(Leaving exposed terminals at the supply feed of 208v 3 phase.)
Both of the new supplies worked right off the bat, and my tests of the fire alarm cutoffs went well. I also test ran some film for about a total of 3 hours running time on both units.
The first hint of trouble was this past Tuesday. The fire alarm folks had scheduled the final test for 6am. I go in with my helper, we fire up both lamps and get ready for the test. About ten minutes later, the lamp on #1 projector goes out. Huh. I try to relight it, no dice. The control lines were still hot, and the breaker for the supply was still on. I crawl under the floor to the power supply and it has a fault lamp lit, indicating a bad module (the main power module.) Oops, defective supply. No big deal, we only needed to have one to prove the fire cutoff works. So we do the test with the #2
It shut down, just as advertised. But it wouldn't relight after the alarm was reset!!
Geez!!! Out comes the Fluke..good control voltage to the supply but it wouldn't turn back on!! Now I'm getting a bit torqued. So we kill the breakers to it, I recheck everything, no dice. To heck with it, I gotta go back to change out #1 anyway.
On a whim, about an hour later, just before we left, we tried #2 again. It fired right up as if nothing was wrong!!
Fast forward to today. We go back and replace the #1 supply with a new one. Fire it off, it works!! Yea! Let's let it cook for an hour and call it a day. At the hour, I tell my helper to shut the lamp off, to let it cool for ten minutes and then we'll turn it back on. He tells me it won't shut off!!
Sure enough, the only way to kill the lamp is to shut off the breaker!!
Out comes the Fluke again. Lamp control leads read as .02 volts OFF, 119.6 volts ON. All normal there. But the power supply's "GO" light stays on anyways. Disconnected the control leads, same result. Power supply says "GO" and lamp lights.
So now the factory (record so far 0-for-2) gets to send me another
Did I mention that these power supplies cost $3,800 EACH?
Did I also mention that they sent the first replacement Next Day Air? (52lbs and oversize.) From Omaha, Nebraska to Southern California?
Just glad I'm not paying the bill for all that freight.
The saga will continue.........
edited smiley code
[This message has been edited by mxslick (edited 11-17-2006).]