How to measure output power correctly - Page 1
Forum section: electronics / projects

author: DejanC
author_rank: Regular Contributor
timestamp: January 29, 2024, 02:49:28 pm
content: I have a device/circuit that produces pulsating voltage and current waveforms that vary in phase relationship and amplitude. I have an oscilloscope that has current and voltage probes. I also have a MATH function where I can multiply the two channels together to give Power (Watts). However, I am wondering about the accuracy of the calculated results. Any other way I can confirm these measurements or measure in another way, maybe using a rectifying circuit?Please advise.

author: macboy
author_rank: Super Contributor
timestamp: January 29, 2024, 03:30:21 pm
content: Is your current probe AC only or AC/DC?In either case, you will almost certainly need to align the phase of the voltage and current measuring probes in order to get accurate results. Using a non-inductive resistor load is one good way. You will need to check the manual for you oscilloscope to determine the procedure for this. It may refer to probe phase compensation or probe delay compensation.

author: DejanC
author_rank: Regular Contributor
timestamp: January 29, 2024, 03:37:33 pm
content: Current probes are DC to AC current probes (up to 50 MHz). Can you comment more how to use a resistor as a load? I need to do this after a rectifier circuit, correct? Any help with designing that part?

author: 2N3055
author_rank: Super Contributor
timestamp: January 29, 2024, 04:04:20 pm
content: As Macboy said, you need to align (deskew) phase of current and voltage measurement using deskew fixture.There is no simple, rigged way to verify your measurement, you just need to verify that scope probes are measuring correctly, and that scope works OK.What is the current probe?What scope is this?Not all scopes are created equal.

author: fourfathom
author_rank: Super Contributor
timestamp: January 29, 2024, 04:23:59 pm
content: What is the maximum frequency of these pulsating waveforms? Waveshape? Rise and fall times? This will affect the required precision of your de-skew.At relatively low frequencies, any non-wirewound resistor of the appropriate power rating will be a reasonable test load.

author: DejanC
author_rank: Regular Contributor
timestamp: January 29, 2024, 04:33:35 pm
content: I thought de-skewing is the process of calibrating your current and voltage probes, is that correct?Current probe: Micsig CP503Oscilloscope: Siglent SDS 1104 X-EMax frequency of pulsating wavesforms around 10 MHz, waveshape is irregular periodic. Rise and fall times I will need to measure...

author: zapta
author_rank: Super Contributor
timestamp: January 29, 2024, 05:52:53 pm
content: You want to match the delay of the current and voltage probes. You can try for example driving a resistive load and see how the current and voltage graphs match, timing wise.

author: mariahcarey
author_rank: Newbie
timestamp: February 16, 2024, 05:01:42 am
content: Quote from: DejanC on January 29, 2024, 02:49:28 pmIf the pulsating waveforms are periodic, you can use the oscilloscope's averaging function to average the power over multiple cycles. This can help smooth out fluctuations and provide a more stable measurement.

author: bdunham7
author_rank: Super Contributor
timestamp: February 16, 2024, 05:23:07 am
content: Quote from: DejanC on January 29, 2024, 04:33:35 pmDeskewing is adjusting the delay difference between channels so that the total signal delay is the same for current and voltage. If you have a low-inductance resistor as a load and a pulsed test signal, then you can measure both current and voltage and just adjust the channel skew in the scope menu until the traces are lined up. At 10MHz, you really have to start to worry about parasitics--stray capacitance and inductance--and you can judge your success somewhat by how closely your two waveforms resemble each other.