Measuring PSU ripple. How do you do it?

Jonathan Bright · 15th July 2004, 07:58 AM

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?

Jonathan Bright · 15th July 2004, 08:42 AM

I must learn to post on my side of the world when the gurus are at their PC's on the other side.

Jennice · 15th July 2004, 09:15 AM

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

dmfraser · 15th July 2004, 03:53 PM

You use an oscilloscope with the input coupling set to "AC".

jackinnj · 15th July 2004, 04:13 PM

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 --

cunningham · 15th July 2004, 11:20 PM

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.

anatech · 16th July 2004, 12:11 AM

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

JensRasmussen · 16th July 2004, 10:26 AM

One measurement with a scope is worth a thousand equations!

\Jens

Mark25 · 16th July 2004, 10:36 AM

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;

anatech · 16th July 2004, 10:56 PM

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

15th July 2004, 07:58 AM	#1
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	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 Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.

15th July 2004, 08:42 AM	#2
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	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 Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.

15th July 2004, 09:15 AM	#3
Jennice 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
dmfraser 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

15th July 2004, 11:20 PM	#6
cunningham 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.


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16th July 2004, 12:11 AM	#7
anatech 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
JensRasmussen diyAudio Member Join Date: Jul 2002 Location: Denmark - Jutland	One measurement with a scope is worth a thousand equations! \Jens

16th July 2004, 10:56 PM	#10
anatech 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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