This is topic Finding AC Ripple in a Rectifier in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.


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Posted by Chase Taylor (Member # 778) on 02-10-2003, 11:57 AM:
 
What’s the best way to see if you have AC ripple coming off of your rectifier. Can you use an Oscilloscope?

Equipment Used:
Rectifier: Kneisley M95rtx2x1
Lamphouse: Xenex L-2730
 

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Posted by John Pytlak (Member # 331) on 02-10-2003, 12:38 PM:
 
The safest way is to let some of the direct "spill" light from the lamphouse fall on a photocell with about a 500 ohm load resistor, and observer the output of the photocell on an oscilloscope. The cell's output should be some DC steady state voltage, with any ripple in the light output superimposed.

See page 18 of the Osram publication "Technology and Applications, XBO Theatre Lamps" for the technique.

Unless you know what you are doing, trying to measure current ripple by directly connecting a scope to the line (e.g., across the ammeter shunt) is potentially dangerous . "Clamp around" current meters can measure RMS AC on a line carrying high DC current with ripple, but may be fooled by irregular waveforms.
 

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Posted by Jon Bartow (Member # 241) on 02-10-2003, 01:04 PM:
 
Kneisley rectifiers are usually very good at having little or no AC ripple.
Quick, semi-accurate way to check:
use a true-RMS multimeter and check the DC voltage then the AC voltage across the DC leads. (connect your leads AFTER the bulb is lit) The AC measurement should be less than 1% of the DC measurement.
You can also use a scope across the ammeter shunt AFTER the bulb has lit, but If you don't know what you are doing you could say goodbye to your scope.
Why do you want to check AC ripple? Is your image flickering?

*Warning to anyone reading this: Connecting any measurement device (multimeter, scope, etc) to the DC leads and then igniteing the bulb is an excellent way to destroy (possibly violently) your scope/meter and possibly injure/kill yourself.
IF YOU DON'T KNOW WHAT YOU ARE DOING DON'T DO IT! THIS STUFF CAN KILL YOU! *

Jonathan
 

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Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 02:09 PM:
 
John, that is noteworthy. I never tried that, but it really makes sense.
 

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Posted by David Graham Rose (Member # 1450) on 02-10-2003, 02:40 PM:
 
Hi All

I once tried measuring the a.c. ripple from my recifier, since my image was flickering. Imagine my dismay when the extract momentarily cut out, causing the contactor to jump out and then back in again. Luckily, at the time I had just got up to retrieve my cup of coffee from the rewing bench as the extract came back on again and my brand new 200MHz 'scope was blown to smithereens welding my leads to the anode and cathode clamp. £900 quid down the drain. Hey ho. This is one of the reasons why I still insist on drinknig coffee whilst on the job!
 

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Posted by Paul G. Thompson (Member # 655) on 02-10-2003, 03:02 PM:
 
David, I "toasted" a nice digital multimeter exactly the same way you did.
 

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Posted by Rick Long (Member # 213) on 02-10-2003, 04:17 PM:
 
Ditto. I was working on one of those horrible little 900-watt Irems, the ones with the orange covers that are such a bitch to open. The bulb would ignite and then go out right away. Diodes checked ok.

I decided to observe the voltage just after striking. I reasoned that the .1uf cap across the DC output should shunt the r.f. fairly well and cause no damage to my meter right?

Wrong.

Had to use one of those cheap minature V.O.M's for a week or so while I waited for my Fluke to comeback from the repair depot for repair and re-calibration.

I am wondering why you suspect ripple. A check of the bulb should give you a pretty good indication, especially if you notice that the anode or cathode is oddly deformed.
 

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Posted by Pat Moore (Member # 380) on 02-10-2003, 08:57 PM:
 
Jon's method works well enough to measure voltage ripple and indicate whether you have a problem or not. Just don't leave the meter leads attached to the DC output longer than necessary just in case the system does decide to have another ignition cycle. Goodbye meter.

You can do a similar test of current ripple with a clamp on AC/DC ammeter. Ripple is the AC component in the DC reading. If you read 100A DC and 5A on the AC setting, your ripple is around 5%. This is a quick test if you have the gear handy.

Pat
 

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Posted by Chase Taylor (Member # 778) on 02-11-2003, 09:49 AM:
 
Thanks alot for the help guys. Yes the image is flickering and the anode and cathode do not look odd. Just trying to track down the problem. Once again thanks alot for the help. I will report back to tell you the results, and promise to be very very very careful
 

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Posted by Jon Bartow (Member # 241) on 02-11-2003, 01:25 PM:
 
Bet you lunch you have a bad diode or a bad contactor.
Jonathan
 

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Posted by System Notices (Member # 2357) on 04-08-2007, 11:29 PM:
 

It has been 1517 days since the last post.

 

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Posted by Jeremy Weigel (Member # 4208) on 04-08-2007, 11:29 PM:
 

quote: Jon Bartow

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Quick, semi-accurate way to check:
use a true-RMS multimeter and check the DC voltage then the AC voltage across the DC leads. (connect your leads AFTER the bulb is lit) The AC measurement should be less than 1% of the DC measurement.

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