Measuring current using a Shunt Resistor & an Oscilloscope

FernandoDAS · Jun 21, 2019

Hi people, I'm new using forums for this kind of purposes, so please forgive me if I can't explain myself well, also if this is the wrong sub-forum please tell me ¡.

I have to measure the output current & voltage of a 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.

FernandoDAS · Jun 27, 2019

marconi said:
Re #8. The two waveforms tell you a lot. The equal mark:space ratio of the square wave means it contains only the fundamental sinusoid (20kHz) and odd harmonics of it ie: 60kHz, 100kHz, 140kHz,....

View attachment 50231

The waveform of the output voltage/current shows it is mainly comprised of the fundamental (20kHz) and third harmonic (60kHz).

View attachment 50232

Is your measurement question:

a. 'what is the effective root mean square(rms) value of this second waveform?' or

b. 'what is the amplitude of each component (fundamental, third, fifth, etcetera)? of this second waveform?'

I knew that would happen, because the transformer acts (kinda) like a filter.
I'm aware of how VRMS works, I know that I could get it's function & evaluate it to obtain such voltage (& then check if that matches with the value shown in my oscilloscope), also, knowing the values of those armonics is not really important to me.

My main issue is if there are risks if I measure the current using a shunt & my oscilloscope, because I did a measure using such shunt & my multimeter & at some point it started to show some random numbers & the display faded, I need to measure the waveform using the oscilloscope & I don't want to damage it.

Btw, no, it doesn't have a FFT function (& you don't know how much I hate that).

marconi · Jun 28, 2019

I think I better understand(?) your measurement problem now; how do you measure/detect a small ac current flowing in a high voltage circuit using an oscilloscope without risk of high voltage damage to the oscilloscope.

What one might do is provide galvanic isolation between the oscilloscope and the sensed circuit. A transformer does that as suggested earlier. You could also use a Hall effect current sensor. The problem I think you will find is discovering such a sensor which will measure small currents and provide sufficient galvanic isolation.

I once used this chip to measure ac current in a 240V circuit - it has 2.4kV of galvanic isolation but operates in the 0-10Amp range which is no use to you.

Allegro ACS723LLCTR-10AB-T Isolated Current Sensor IC 10A SOIC8 | Rapid Electronics

Allegro - ACS723LLCTR-10AB-T - Allegro ACS723LLCTR-10AB-T Isolated Current Sensor IC 10A SOIC8 - <p>The ACS723LLCTR-10AB-T is a high accuracy current sensor

www.rapidonline.com

What I suggest you do is do some research and also contact manufacturers of such Hall Effect Sensors, Allegro being one.

Sorry I cannot be more help - but it is certainly an interesting problem - to measure a tiny current, in a high voltage circuit, accurately and above all safely. Is this why you have been set the problem?

You might want to look at this:

Measuring small current (<1mA) at high DC voltages (~30KV)

I was wondering whether I could use a simple multimeter to measure small currents (<1mA) at high DC voltages (~30KV). From a cursory internet search I got three answers which were repeated ad na...

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