Author Topic: Help with Readings  (Read 1071 times)

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Offline AustinTxBob

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Help with Readings
« on: January 23, 2016, 06:46:42 pm »
I built a cheap bench power supply and was going through it checking all of the voltages.  I hooked up the AC output to my DMM and my scope and am seeing something I don't understand.  So my DMM (both a BK2709 and also a Fluke 23) say ~6.95 VAC but on my scope I see ~19.8 Vpp.

What am I missing?

Thanks.
Bob
 

Offline bobcat

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Re: Help with Readings
« Reply #1 on: January 23, 2016, 06:50:20 pm »
Your scope is measuring Peak to Peak and your meter is measuring RMS or average. Set your scope to measure RMS and compare.
 

Offline Zero999

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Re: Help with Readings
« Reply #2 on: January 23, 2016, 06:51:09 pm »
The multimeter is displaying the RMS voltage, not the average which will be zero for an AC waveform with no DC component.

The oscilloscope is displaying the peak-to-peak voltage, which is twice the peak voltage.

For a sine wave, the peak voltage is equal to the RMS voltage multiplied by sqroot(2). It isn't a perfect sine wave so the results form performing this calculation will differ slightly.
 

Offline AustinTxBob

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Re: Help with Readings
« Reply #3 on: January 23, 2016, 06:55:51 pm »
Thanks.  RMS on the scope agrees with the ~6.9 VAC. 

I found some doc on RMS and the math now makes sense.  Thanks!
« Last Edit: January 23, 2016, 06:58:57 pm by AustinTxBob »
 


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