Film-Tech Forum ARCHIVE: Running bulbs below rated current...

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Posted by Michael Barry (Member # 237) on 08-27-2000, 01:22 AM:

It is a myth that running a xenon below its rated current will save the bulb's life, but can anyone provide technical information to explain why this is so?

Posted by Stefan Scholz (Member # 147) on 08-27-2000, 04:41 AM:

A bulb should be run at it's rated specs. This ensures maximum life. With under-current, you may prolong the life of incandescant bulbs, with loss of color integrity, but who uses tungsten filament bulbs except for xctrs or portable home gear, anyhow?
Xenons keep their color temperature, but beware, the arc, especially with horizontals gets instable, and the cathode point of best emission changes. Result will be a rather unstable arc, with significant flicker (that is any density variation on screen which is not 48 Hz or 72 Hz for 3 bladed shutters) on screen. After a while the cathode is totally uneven, and unable to create a stable point of electron emission. Even if you tend to use rated supply specs at that point, you have lost stable burning, and this will lead to premature bulb failiure.
In studio applications projectionists tended to burn bulbs at a very low current (c. 20A), the min. hold current during intermissions or projection pauses. This was done to avoid new ignitions with their related problems (sound inductions from ignited, unreliable striking etc.), but it was abolished very quick. The bulb life was decreased significantly due to the above mentioned mechanism.
Today's practise is to strike the bulb in the morning, and leave it running all day at rated current. This gives longest usable time, each ignition "eates" about 1 - 2 hrs of max. lifespan, no risks due to thermal extractions, stable focus and even illumination all day, asd well as extreme reliability. A DC arc will not extinguish until you turn off the AC side.

Posted by Gordon McLeod (Member # 33) on 08-27-2000, 08:38 AM:

Actually many places still use with outdetriment stand bye mode but in equipment nd with lamps designed for it
Ushio/Christie lamps designated HTP are designed for standbye operation and the Xebex/sanrex combinations were of lamphouse/rectifiers were designed around this feature as well with switched amounts of magnetic stabalization to keep the arc stable at lower currents.
In some cases to much exhaust vs low current has caused more lamp problems
All the 15k systems for large formatand all the small 16mm systems also have standbye built in