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Old 15th July 2004, 08:58 AM   #1
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Default 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?
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Old 15th July 2004, 09:42 AM   #2
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Old 15th July 2004, 10:15 AM   #3
Jennice is offline Jennice  Denmark
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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
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Old 15th July 2004, 04:53 PM   #4
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Default Measure Ripple

You use an oscilloscope with the input coupling set to "AC".
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Old 15th July 2004, 05:13 PM   #5
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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 --
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Old 16th July 2004, 12:20 AM   #6
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Default 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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Old 16th July 2004, 01:11 AM   #7
anatech is offline anatech  Canada
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Default 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
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Old 16th July 2004, 11:26 AM   #8
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One measurement with a scope is worth a thousand equations!

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Old 16th July 2004, 11:36 AM   #9
Mark25 is offline Mark25  United Kingdom
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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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Old 16th July 2004, 11:56 PM   #10
anatech is offline anatech  Canada
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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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