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Tool to set up Picoscope for ease of RMS Current Measurement

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Tool to set up Picoscope for ease of RMS Current Measurement

Postby Michael Hayes » Tue Oct 27, 2009 9:08 am

I have been using the attached tool for the past few years and realised that others may have a use for it also.
Coincidently, the previous post is on a similar topic.
I do a lot of mains AC current measurements and often have to record waveforms. As anyone who has ever done this will know, it is a pain to start calculating a factor of Sqrt(2) to work out what the RMS value of a waveform is. There is no convenient notch on the curve where you can read off the RMS value so I created one!
The method is to introduce a custom range and build the conversion into this range and then further calibrate the range so that it displays directly in Arms.
All you then need to do is lay the Picoscope Ruler across the very peak of this waveform and it will directly read the RMS value of the current for you.
My normal set up is a Hioki 9018 Current Probe into a Bannana to BNC adapter and plug this directly into a Picoscope ADC-212.
The probe has adjustable ranges of 500A, 200A, 100A, 50A, 20A and 10A each giving a 200mV output. So for 500A rms you get 200mV rms to the Pico.
Start in Cell B1 and enter the maximum rms Current you want to display, then if you are using a CT, enter its primary ratio in cell B2, if not, just enter 5. Then enter the Probe Range with the drop down in Cell B4 now select Cell C15 and copy the text out of the Formula Bar then select Setting > Custom Ranges > Add and paste (Ctrl+v) into the scaling box, now enter Arms into the Units box. When you OK, you should see a new range of ±750Arms for example. Now on each channel, select the range drop down and at the bottom, under ±20 V you will see the newly created range. You should then do a check run against a meter to ensure you have the correct setup. You can also set a direct tigger (F4) in Arms for your waveforms.

It should be noted that the scaling is only valid for Sinusoidal waveforms (50 or 60Hz or other is irrelevant within the probe operating frquency) but is not valid for phase angle control or Variable Speed Drives etc but even with this limitation, is extremely useful and has saved me countless time and eliminated many "on the fly" calculation errors.

If you use a different probe or otherwise need to adjust the fixed ranges in Cell B4, you can edit the list by selecting Data > Validation > Settings Tab then modifying the list in the Source Box (XL 2003 under Windows XP)

Michael Hayes
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PicoRMSRangeCalculator.xls
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Michael Hayes
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