Measuring PWM current? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: April 28, 2019, 09:16:11 pm
content: For a PWM voltage, is using a shunt and observing voltage with oscilloscope the only way to get even a ballpark current measurement?Thanks.

author: jmelson
author_rank: Super Contributor
timestamp: April 28, 2019, 11:15:54 pm
content: Quote from: iXod on April 28, 2019, 09:16:11 pmWell, in cases where you actually NEED to measure current, you can use an instrumentation amp.I used the AD620 in a PWM servo drive to measure the voltage across a small resistor (fraction of an Ohm).The actual voltage was above the ratings of the AD620, so I used a set of matched resistors to make a pair of voltage dividers. So, both inputs of the voltage dividers was seeing a 60 V square wave, but the difference between them was less than 1 V. It worked amazingly well, you needed to look VERY closely at the scope to see any trace of the common mode leaking through.Jon

author: iXod
author_rank: Frequent Contributor
timestamp: April 29, 2019, 01:13:31 am
content: But the output of the inst. amp will also be PWM, yes? Which will require... an oscilloscope to measure the current.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 01:28:37 am
content: you must use a true rms to dc chip like this one:https://www.analog.com/media/en/technical-documentation/data-sheets/AD636.pdf

author: iXod
author_rank: Frequent Contributor
timestamp: April 29, 2019, 02:31:02 am
content: Quote from: OM222O on April 29, 2019, 01:28:37 amBut PWM can't be accurately measured (or converted?) when using RMS means--the whole reason for my initial post.

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 03:07:22 am
content: if you have a look at the datasheet it claims it can calculate RMS for "complex AC waveforms" no mention of square wave in particular but one would safely assume it can convert it to rms? there's nothing inherently different with square waves that doesn't allow them to be converted to RMS

author: jbb
author_rank: Super Contributor
timestamp: April 29, 2019, 04:10:52 am
content: What kind of current do you want to measure? Average, min, max, RMS, peak to peak?There are many options. While you can fit a fast ADC and sample the crap out of it (ie the oscilloscope method) there are likely cheaper and easier ways.If you’re looking to solve a specific problem, we could be more helpful with more background.

author: iXod
author_rank: Frequent Contributor
timestamp: April 29, 2019, 04:34:49 am
content: I realize this forum is populated with engineers who build solutions. But my first approach is to use tools that exist.So my question is: is there a way to measure PWM current, real-time, without custom building a measurement and display tool?I’m leaning toward a scope and current probe as the most straightforward solution, although a scope is my first anti-requirement...Thanks.

author: oPossum
author_rank: Super Contributor
timestamp: April 29, 2019, 04:42:50 am
content: Range?Resolution?Accuracy?Average, min, max, RMS, peak to peak?

author: soldar
author_rank: Super Contributor
timestamp: April 29, 2019, 05:29:09 am
content: Quote from: iXod on April 29, 2019, 04:34:49 amYou have been asked questions and you are not answering them. It looks like either you don't even understand what you are trying to measure or you are unable to explain it.A PWM signal can be measured in different ways. What value are you looking for? Max instant current? Average current? RMS current?As long as you cannot articulate what you want it is very difficult to help you.Several people are trying to help you and asking questions which you are just ignoring. If you are going to ask for help the least you can do is cooperate with those trying to help you. This is an extremely simple question which is only made complicated by your vagueness.Please explain clearly what you are trying to do!

author: BravoV
author_rank: Super Contributor
timestamp: April 29, 2019, 07:27:54 am
content: Quote from: oPossum on April 29, 2019, 04:42:50 amAnother important one ... frequency, its a nasty little devil of it's own imo.

author: ogden
author_rank: Super Contributor
timestamp: April 29, 2019, 08:28:55 am
content: Quote from: iXod on April 28, 2019, 09:16:11 pmOther option would be to use current clamp-on probe. One shown below haveslowlow bandwidth (20KHz), yet it is considered as low cost and may do the job in the "ballpark" category. Obviously there are >=1MHz BW clamps, but for a price.https://www.picotech.com/accessories/current-probes-clamps/60-a-current-probe-bncDave have video regarding scope current probes:https://youtu.be/kmCvrGVtC0M?t=1

author: OM222O
author_rank: Frequent Contributor
timestamp: April 29, 2019, 10:25:16 am
content: he says "scope gives accurate RMS measurments" as if somehow the scope is doing something magical ... ANY half decent RMS to DC converter with enough bandwidth should be plenty good enough for this application.

author: Jeroen3
author_rank: Super Contributor
timestamp: April 29, 2019, 10:51:04 am
content: Your DMM should have no problem with PWM current. Unless you PWM at 5 hz or something ridiculous low.RMS might be off a bit, since part of the waveform is beyond it's bandwidth.Some multimeters can even do PWM voltage from VFD's, with an extra low pass. (Fluke 87v for example)

author: David Hess
author_rank: Super Contributor
timestamp: April 29, 2019, 12:51:02 pm
content: An oscilloscope with AC/DC current probe is the most universal method and can work to multiple MHz. A current shunt can be used with an oscilloscope and differential probe. At low frequencies, other ways are feasible like using a multimeter.