Film-Tech Forum ARCHIVE: Lamp AC

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Posted by Antonio Marcheselli (Member # 373) on 07-19-2002, 04:40 AM:

Hi everyone

Since we are experiencing problems with many xenon bulbs, our tech connected an oscilloscope on the rectifier output to monitor the current quality.
Result: we have about 1V AC on the output. Our older lamp has three point (perhaps it is not the right term, hope that it is clear!) that show an AC in the current.

So, is is 1V within the specs?

Bye
Antonio

Posted by Jean-Michel Grin (Member # 1360) on 07-19-2002, 05:13 AM:

Antonio,

What kind of rectifier are you using in your cinema ?
Conventional like a IREM or an Electronic model ?
What is the wattage of your lamp ?
I've been experienced the same problem with a 7000 watts Osram Lamp and a IREM Variable Plunger (I'm not sure of the therm) rectifier. We wanted to replace the condensators at the output of the unit but, finaly we have exchanged this conventional IREM rectifier (is was too noisly) for a electronic ADDE BBR-180 rectifier and we got less trouble.

Sorry I can't tell you nothing more...

Good Luck !

Jean-Michel Grin - Europlex Cinemas (Switzerland)

Posted by John Anastasio (Member # 607) on 07-19-2002, 05:59 AM:

It's not so much the voltage of the ripple as it is the % of the operating voltage of the lamp. If the lamp is running at 20 volts and you have 1 v of ripple, you've got 1/20, or 5% ripple. If you run at 40v, then the ripple voltage is 2.5% At what voltage does that lamphouse run? It seems that 1v p/p at normal lamphouse operating parameters is rather high, but maybe one of our experts can jump in here....

Posted by John Pytlak (Member # 331) on 07-19-2002, 06:13 AM:

Usually, it's the amount of CURRENT ripple that is measured. Excessive current ripple causes flicker in the light output, and quickly erodes/damages the lamp electrodes. In countries with 60Hz electrical power, the 60Hz ripple "beats" with the 48 cycle (2-blade shutter) or 72 cycle (3-blade shutter) frequency to produce a very noticeable and annoying 12Hz. flicker.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
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Posted by Steve Guttag (Member # 268) on 07-19-2002, 08:23 AM:

IREM is rather noted for having one of the highest ripple in the industry. They offer a filter unit (read: admission of guilt) to improve it. If you have IREMs and you don't have the AFU, then get it...be prepaired, it costs a bit.

Steve

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"Old projectionists never die, they just changeover!"

Posted by Antonio Marcheselli (Member # 373) on 07-19-2002, 11:31 AM:

We have a 3000W Osram lamp, G3 Irem rectifier. I do not have specs right here and I don't know what is the nominal voltage.

Antonio

Posted by John Pytlak (Member # 331) on 07-19-2002, 12:36 PM:

Some links to information about excessive current ripple:
http://www.xenonbulbs.com/pdf_manuals/xenon_bulb_info/xenon_bulb_failure_analysis.pdf
http://www.ltilamps.com/techinfo/arc_instab.html

The Osram publication "XBO Theatre Lamps" says:

"A better method is to estimate the ripple by means of the light. Using a fast enough photocell and an oscilloscope, it is possible to ascertain whether the ripple is in the correct order of magnitude by MEASURING THE LIGHT, without using a great deal of equipment. With a ripple of between 5 and 10% the light ripple is about 50% higher, as the luminous flux is proportional to the lamp current to the power of 1.5."

In other words, let a bit of the direct light from the lamphouse (no shutter moving) shine on a properly loaded silicon photocell connected to an oscilloscope. Look at the "steady state" light output and the amount of light ripple using the scope.

Posted by Antonio Marcheselli (Member # 373) on 07-19-2002, 01:35 PM:

John,

Great trick the to found the lamp instability. Thanks!

Bye
Antonio

Posted by John Anastasio (Member # 607) on 07-19-2002, 05:21 PM:

I've actually done that little trick using a mirror to reflect a bit of the lamphouse "spill" onto the solar cell and just looking at the results on a scope connected to the audio chain. You could not only see the flicker, you could hear it.

Posted by Steve Guttag (Member # 268) on 07-19-2002, 05:32 PM:

This "trick" is also good for looking at shutter efficientcy.

Steve

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"Old projectionists never die, they just changeover!"