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Old 26th March 2006, 11:29 AM   #1
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Default measuring ac ripple with DVM?

Hi all,
I am a newbie to this site with limited eclectronics knowledge. I need to know how to measure ac ripple on dc p/s o/p using a DVM.

How accurate is this method compared to scope?

thanks.
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Old 26th March 2006, 12:52 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
most DMM on Vac scale have a high input impedance. Check your spec sheet. You need to be sure you do not overload the DC rating due to the very high DC across the input when measuring AC voltages.

You can set the DMM to high scale Vac and measure the voltage to ground across the smoothing caps.
At highest scale it will (should) read zero.
Progressively reduce your scale until you get a reading.

This is an average (a very weird average due to rms scaling for the normal sampling and DC averaging) reading. The peak to peak will be about 3 times the rms reading. This compares closely to the peak to peak obtained from a scope.

It is very poor at showing spiky type pulses that only a scope can show but for ripple which is a sort of asymmetric saw tooth it does OK.
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Old 31st March 2006, 07:19 PM   #3
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Default Re: measuring ac ripple with DVM?

Quote:
Originally posted by basildogann
measure ac ripple on dc p/s o/p using a DVM.

How accurate is this method compared to scope?

thanks.
Scope method unless it's a modern one ( LeCroy) or so can work it out on a side bar or like Lissajous patterns has to workd geometrically. I'm old fashioned and still use Lissajous for certain measurements.

As AndrewT mentions...weird current measuremts can be haphazard......One pays the price for a true rms meter with good b/width which incidentally is the norm measurement.

Futuristically, If one is advancing into PFC development or anything to do with efficiency measurements, one soon realises a good dvm with true rms readings coupled with good b/w is a sound investment.

For some time I felt a duffer when I realised that my output readings where nearly better than my input ones......beware of Normal DVMS non rms !!

richj
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