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I have a Fluke 87V, which is a True-RMS DMM, and I would like to measure the current in my circuit. I have 2 motors being driven by a PWM so I wonder if the DC current or the AC current measurement of the DMM will be do the job for the PWM current. Is it there a recommended way to measure this kind of "mixed signal" using a regular DMM as a Fluke 87V?

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  • \$\begingroup\$ You might want to consider a Hall effect sensor and an oscilloscope, which will show you both AC and DC current components up to reasonably high bandwidths. A good digital scope will compute RMS and average currents for complex periodic waveforms with ease. \$\endgroup\$ – Adam Lawrence Jan 29 '14 at 18:37
  • \$\begingroup\$ That's a good idea, but I dont have a one at hand now... \$\endgroup\$ – mFeinstein Jan 29 '14 at 19:06
  • \$\begingroup\$ Too bad you didn't buy the 89V, which has AC + DC TRMS measurement capability. A closed-loop Hall sensor as suggested by @Madmanguruman will only run you ~$25 and might be all you need. \$\endgroup\$ – Spehro Pefhany May 29 '14 at 22:25
  • \$\begingroup\$ 89V? I just know the 87V, and there isnt any 89V at Fluke's website either.... \$\endgroup\$ – mFeinstein May 30 '14 at 2:05
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Is it there a recommended way to measure this kind of "mixed signal"?

Use a DMM with a AC+DC TRMS measurement capability?

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  • \$\begingroup\$ I fogot a "using a regular DMM" :P \$\endgroup\$ – mFeinstein Jan 29 '14 at 19:06
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What of sort frequencies, voltage levels?
(I'm not an AC power guy so if you blow something up...)
But I could imagine a shunt, and then a low pass and high pass filter. Just RC's if you can throw power away.. and move heat away. You'd have to work out the response functions.

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  • \$\begingroup\$ Well I guess that's not a bad idea for low power applications, but I was hoping to measure some 10A at least, and filtering this can be tricky... \$\endgroup\$ – mFeinstein Jul 29 '14 at 12:32
  • \$\begingroup\$ OK 10 A's, what's the PWM frequency? With a 0.1 ohm resistor you'd have a volt of signal to play with. Or 20 milli-ohm use the 200mV range on the DMM. \$\endgroup\$ – George Herold Jul 29 '14 at 12:45
  • \$\begingroup\$ It's more like a generic question, not specific to one particular case, so many people can benefit from this, even if they projects are different. Mine had a 3KHz PWM. \$\endgroup\$ – mFeinstein Jul 30 '14 at 13:09

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