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Author Topic: Finding AC Ripple in a Rectifier Chase Taylor Expert Film Handler Posts: 175 From: Troy, Alabama, USA Registered: Mar 2001 posted 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  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 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.  |  IP: Logged Jon Bartow Master Film Handler Posts: 287 From: Massachusetts Registered: Nov 1999 posted 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  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 02-10-2003 02:09 PM                     John, that is noteworthy. I never tried that, but it really makes sense.  |  IP: Logged David Graham Rose Expert Film Handler Posts: 187 From: Cambridge, UK Registered: Sep 2002 posted 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!  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 02-10-2003 03:02 PM                     David, I "toasted" a nice digital multimeter exactly the same way you did.  |  IP: Logged Rick Long Jedi Master Film Handler Posts: 759 From: Toronto, Ontario, Canada Registered: Nov 1999 posted 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.  |  IP: Logged Pat Moore Master Film Handler Posts: 363 Registered: Mar 2000 posted 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  |  IP: Logged Chase Taylor Expert Film Handler Posts: 175 From: Troy, Alabama, USA Registered: Mar 2001 posted 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  |  IP: Logged Jon Bartow Master Film Handler Posts: 287 From: Massachusetts Registered: Nov 1999 posted 02-11-2003 01:25 PM                     Bet you lunch you have a bad diode or a bad contactor. Jonathan  |  IP: Logged System Notices Forum Watchdog / Soup Nazi Posts: 215 Registered: Apr 2004 posted 04-08-2007 11:29 PM                 It has been 1517 days since the last post.  |  IP: Logged Jeremy Weigel Phenomenal Film Handler Posts: 1062 From: Edmond, OK, USA Registered: Mar 2007 posted 04-08-2007 11:29 PM                     quote: Jon Bartow 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. Is this measuring at the shunt or the DC leads coming from the rectifier?  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 04-09-2007 09:51 AM                     AC ccurrent ripple and inrush need to be measured across a calibrated standard so accurate results can be obtained. Osram used to loan out a calibrated current shunt standard for technicians to use. You need a storage scope to measure inrush current to any degree of accuracy. In many cases over the years I have found inrush current to be more the culprit than ripple in short lamp life. There are rectifiers out there that have inrush anywhere from X5 to X10 the operating current of the lamp. Excessive inrush is also easy to cure with a length if nichrome wire in series with the main capacitors. Short out the nichrome wire after ignition to keep ripple low. FWIW I use an ORC inrush/ripple meter kit to do my tests. On the inrush function it has a peak hold circuit to enable the peak DC inrush current to be displayed on it's analog meter.  |  IP: Logged

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