28V Halogen lamp leaks

Is the failure at first turn-on, during first use, after first use (i.e. detected by continuity check before next turn-on) or at next turn-on "after first use" ?

What are the lamps rated at (AC/DC?) and what are you using (AC/DC,voltage,regulation)?

I use an optical projector, simple 120 volt ac transformed down to suit 28v 150w Halogen lamp.

After a couple of years operation, and perhaps 3-4 bulbs, I started blowing lamps at about 1 per month. At $70 a bulb I was not a happy camper. There are apparently different duty cycle types you can buy to improve this however I am already buying the best available now. There also is a "change bulb" procedure which you must follow, which I did, however this still did not solve my problem.

Following discussions with manuf. reps, I have been told that the most probable cause is voltage fluctuation. The voltage supply must be stable, and must not fluctuate more than +/- 5% from rated. They also recommend the supply line is not shared with any other electrical device. On checking my supply, I was perhaps 5% above rated voltage and was sharing my circuit with other devices (work lamp, magnifier glass). I could not measure "spike" voltage.

Made changes to put in separate supply line. Also advised my operators to switch on the lamp once only at start of shift and leave on all day. Been 3 months with same bulb so, looks promising.

Good luck,

Jimmymac28

I've had 12v halogen sealed beam car lamps start smoking internally. They were all very old stock ( over ten years). My analysis was that the seal around the wire entries had shrunk slightly and admitted air, allowing the element to combust slowly. The light functioned ok, it just filled with smoke which made it useless as a light source. Perhaps you are getting old stock lamps? Here in Oz you get 3 months warranty on all parts, so I'd be asking for free replacement if it is only lasting one month

It just occurred to me that the stress of switching on and off, especially if the supply voltage is too high as it is here, is the most probable problem.

There are circuits that allow the lamp to switch on ONLY when the AC goes through the 0 point, reducing stress and making for a long life. No help for a DC circuit of course, unless you are inclined to design something.

Also pre-heating by say having a lower voltage on all the time, that only just makes the element glow low red hot, will also remove a lot of the switch on stress. A simple two switch circuit, the first one with a correctly sized resistance, would help considerably.

People forget that filament lamps have a very low resistance at switch on and this climbs rapidly as the element heats up, but that the stress from switch on is still very high.....

Of course if you see smoke or some "milky" coloring, then oxygen in the form of air has leaked in.....