Measuring PSU ripple. How do you do it? - diyAudio
 Measuring PSU ripple. How do you do it?
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 15th July 2004, 07:58 AM #1 diyAudio Member   Join Date: Feb 2004 Location: Queensland Australia Measuring PSU ripple. How do you do it? Just a simple one. How do you measure the ripple on the output of a capacitor smooothed PSU? Hopefully the ripple voltage will be in the millivolt range but the rail voltage will usually be several tens of volts. Can you do this with a DMM or do you need a CRO? __________________ "It was the Springtime of the year when aunt is calling to aunt like mastodons bellowing across primeval swamps." P.G. Wodehouse.
 15th July 2004, 08:42 AM #2 diyAudio Member   Join Date: Feb 2004 Location: Queensland Australia I must learn to post on my side of the world when the gurus are at their PC's on the other side. __________________ "It was the Springtime of the year when aunt is calling to aunt like mastodons bellowing across primeval swamps." P.G. Wodehouse.
 15th July 2004, 09:15 AM #3 diyAudio Member   Join Date: Jun 2003 Location: Planet Earth G'day, mate Whichever way you want. Just make an AC measurement. If you're not certain about the capabilities of your measuring device, put a capacitor of maybe 100uF or something in series, charged to the PSU output voltage. With the fairly high input impedance of your measurement equipment, you'll have no problems measuring the AC ripple fairly accurate. Potential problem is that not all DMM's measure "True RMS", but assume that they're seeing a sine shape AC, which is by no means the case for this type of measurement. this gives a faulty read-out of the signal. Jennice __________________ I get paid to break stuff. My g/f gets paid to play with children. Life is good.
 15th July 2004, 03:53 PM #4 diyAudio Member   Join Date: Dec 2003 Location: Orange County, CA Measure Ripple You use an oscilloscope with the input coupling set to "AC". __________________ Dan Fraser
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Quote:
 Originally posted by Jennice Potential problem is that not all DMM's measure "True RMS", but assume that they're seeing a sine shape AC, which is by no means the case for this type of measurement. this gives a faulty read-out of the signal. Jennice
Even relatively inexpensive DMM's will measure a.c. voltages to a few hundred hertz -- the problem arises if you are trying to measure the ripple in a switching power supply -- but you'll use a scope for this anyway.

if you need a good RMS measuring instrument, consider getting a used HP3403C -- or an analog 3400 --

 15th July 2004, 11:20 PM #6 diyAudio Member   Join Date: Jun 2004 Location: Knoxville math?? What ever happened to mathmatics?? If you know how much current the power supply outputs, and you know what the capacitor values are, and the frequency of AC Line with rectifier, 50-60Hz or 100-120Hz(full wave), then you should be able to calculate the ripple voltage. I think it is I(load) / (C * f) but I am not sure. You may want to look up the formula.
 16th July 2004, 12:11 AM #7 diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Crest Factor Hi jackinnj, Jennice was refering to "crest factor". The ratio of peak voltage to rms voltage. Any deviation from a sine wave has an amount of crest factor. This can be approximated and corrected for if you really know your meter. BTW, inexpensive meters I have calibrated often don't meet their own published specs, or hold cal. very well. Many fail out of the box! I guess it depends on how accurate you want your readings to be. -Chris
 16th July 2004, 10:26 AM #8 diyAudio Member     Join Date: Jul 2002 Location: Denmark - Jutland One measurement with a scope is worth a thousand equations! \Jens
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Quote:
 Originally posted by JensRasmussen One measurement with a scope is worth a thousand equations! \Jens
ditto that ! I wonder what a meter would make of the lower trace here;

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 16th July 2004, 10:56 PM #10 diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Oh yeah! Especially a digital 'scope that will do the math & tell you the answer. Mark25, I think many meters would just make an answer up. -Chris

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