# Measuring current using a Shunt Resistor & an Oscilloscope

#### FernandoDAS

Joined Jun 20, 2019
5
Hi people, I'm new using forums for this kind of purposes, so please forgive me if I can't explain myself well.

I have to measure the output current & voltage of an High Frecuency, High Voltage transformer, I can measure the Output Voltage using a High Voltage Probe and my Handheld Oscilloscope, but I still have to deal with the current, I want to use a Homemade Shunt Resistor (0.1ohms) and my Oscilloscope, but before doing it I tried by measuring that voltage with my Fluke Multimeter, I was increasing the input voltage & It was working well but at some point my Multimeter Screen started to fade & show random numbers, What happened? The voltage on the Shunt wasn't that high (around 2.5 volts) when it started to fail, it still works. I don't want to damage my Oscilloscope, so I need to now what causes that issue.

Here is my circuit diagram if you wan't to know the way I'm measuring.

Considerations:
-I know my multimeter doesn't measure at those frequencies, but the error is constant so I can just adjust it.
-I know my oscilloscope does work at those frequencies.
-I have no idea of the value of the output current & voltage, also by measuring another transformer the values are not consistent.

#### AlbertHall

Joined Jun 4, 2014
9,661
Just to get the silly cause out of the way - does your multimeter need new batteries?
What frequency is this running at?

#### FernandoDAS

Joined Jun 20, 2019
5
Just to get the silly cause out of the way - does your multimeter need new batteries?
What frequency is this running at?
It uses one 9 Volts battery, I just measured its voltage & it's around 6.2 volts.
What frecuency, the multimeter or the transformer?
The multimeter IDK, its datasheet says that it can measure from 45 Hz to 1K Hz.
The transformer is runing at 20 K Hz, also the signal is not a sin wave, it's some kind of square wave.

#### AlbertHall

Joined Jun 4, 2014
9,661
It uses one 9 Volts battery, I just measured its voltage & it's around 6.2 volts.
Fix that first and try again.

#### FernandoDAS

Joined Jun 20, 2019
5
Fix that first and try again.
Ok, I'll do it ASAP, but I've been using it since then & it has been working fine, I've measured voltage, current & some other thinks.
If something changes I'll tell you, thanks

Joined Jan 15, 2015
5,362
A few things I don't understand. When using a shunt you normally measure the voltage drop across the shunt. When measuring other than a true sine wave your meter needs to be a True RMS Responding meter, many lower cost DMMs are Average Responding RMS Indicating. Your high voltage probe (voltage divider) also needs to be able to respond accurately to what you show as a 20 KHz frequency as well as your meter. You show a 1000:1 probe but I don't see any load?

<EDIT> When measuring a nice clean true sine wave and cheap average responding RMS indicating will work as long as you are within the frequency response range of the meter. When measuring anything else like a square wave, modified sine wave, triangular wave, sawtooth wave or any other ugly strange looking wave you need a good quality RMS responding, RMS indicating, meter and again with a bandwidth to cover the waveform frequency, </EDIT>

Edited for clarification.

Ron

Last edited:

Joined Jan 15, 2015
5,362

#### FernandoDAS

Joined Jun 20, 2019
5
A few things I don't understand. When using a shunt you normally measure the voltage drop across the shunt. When measuring other than a true sine wave your meter needs to be a True RMS Responding meter, many lower cost DMMs are Average Responding RMS Indicating. Your high voltage probe (voltage divider) also needs to be able to respond accurately to what you show as a 20 KHz frequency as well as your meter. You show a 1000:1 probe but I don't see any load?

<EDIT> When measuring a nice clean true sine wave and cheap average responding RMS indicating will work as long as you are within the frequency response range of the meter. When measuring anything else like a square wave, modified sine wave, triangular wave, sawtooth wave or any other ugly strange looking wave you need a good quality RMS responding, RMS indicating, meter and again with a bandwidth to cover the waveform frequency, </EDIT>

Edited for clarification.

Ron
Hi, sorry it took me ages tu reply but I haven't been able to do those measures again.
I'm using a Fluke 115 multimeter to do that measure, honestly IDK how accurate it's, I know it doesn't work at those frequencies, but I wanted to know if the error was constant & it was (kinda) I'm attaching a graph showing the results.
How do I know if my probe works at those frequencies? I found the Datasheet & It only tells me that It works at 60Hz +/- 5%.
The meter I'm using to measure the output of my probe is a Fluke 123 Oscilloscope (it works at those frequencies).
There's no load at the output, atm I can't use the "normal" load that goes at the output.

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#### FernandoDAS

Joined Jun 20, 2019
5

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Joined Jan 15, 2015
5,362
One more time and per your original drawing. When using a shunt you measure the voltage drop across the shunt, below is an example of a current shunt.

This is not what your drawing reflects. Without a load there is not going to be any drop across the shunt. You will measure a voltage and that's about all. Additionally in your drawing you would want your ground on the other side of your shunt. So as drawn you are showing a 999 Meg 1 Meg voltage divider or simply put a 10000:1 divider. Beyond that I can't tell you anymore than the data sheets for the equipment you are using. The picture of a shunt I posted is merely an example of a typical current shunt.

Ron

#### cmartinez

Joined Jan 17, 2007
6,905