Measuring output power correctly - Page 1
Forum section: electronics / projects

author: DejanC
author_rank: Regular Contributor
timestamp: January 24, 2024, 04:09:34 pm
content: Hi,I have a large voltage spike and current spikes on my output. I am looking to measure this accurately. Can I just take the RMS voltage and RMS current values displayed by my scope and multiply them to get output power? I want to compare this to my input power from my two DC power supplies (which shows Volts, Amps and Watts simultaneously). Can I trust those readings on the input if the load is AC output?Anyone know of a HV full bridge rectifier circuit available for purchase for relatively cheap?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 24, 2024, 04:41:32 pm
content: you have power bar with ac line filters who does a good job eliminating spikes, or some line conditionnersfor measurement others will chime in

author: DejanC
author_rank: Regular Contributor
timestamp: January 24, 2024, 05:32:58 pm
content: If anyone has experience with measuring output power of messy AC voltage and current impulses, in a relatively cheap way, I would love to hear it.Thank you all

author: bdunham7
author_rank: Super Contributor
timestamp: January 24, 2024, 06:19:10 pm
content: Quote from: DejanC on January 24, 2024, 05:32:58 pmIf you can accurately measure both voltage and current with sufficient BW, the most straightforward method I know of is to use a scope with an appropriate MATH function, preferrably power analysis.The first step is to deskew your voltage and current inputs to make sure the time delay for each signal is the same. This may or may not be an issue depending on what you are measuring and how you are measuring it. For example, if you are looking at mains-frequency signals, even messy ones, and you are using a resistor to measure current and two identical probes on the scope, deskew probably won't matter.Next, you use the scope to multiply the voltage x current and the result is instantaneous power. What you can do with the instantaneous power MATH trace is dependent on further functions of the scope.Here's an example without any further math done on the power trace. It looks a bit messy because I haven't separated the traces, but it is the input power to a non-PFC SMPS that had a rapidly varying output.

author: DejanC
author_rank: Regular Contributor
timestamp: January 24, 2024, 06:26:02 pm
content: I tried to do this on my Siglent SDS1104X-E. It displays the MATH function (CH1*CH2, V*I) as a curve but does not output any values

author: bdunham7
author_rank: Super Contributor
timestamp: January 24, 2024, 06:50:59 pm
content: Quote from: DejanC on January 24, 2024, 06:26:02 pmIf you set CH2 units to 'A' (amps) and MATH as CH1 * CH2, your math trace should display as watts. Then if you go into measurements and select "Mean" with MATH as the source, you should get a display with the average power. I set up my SDS1104X-E with a simulated triac-drive signal via an ARB waveform as shown. The first is with the traces separated so that you can see everything better, the second is with everything overlaid and expanded to use the entire ADC range for the best measurements. It appears to work although I didn't test its accuracy.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: January 25, 2024, 08:38:26 am
content: Can you specify your application?QuoteDoes not give much insights.