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Ebay Rogowski current probe

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moffy:
I just received the following item from Ebay: https://www.ebay.com.au/itm/224784699913
and after some tests have come up with the following equivalent circuit see JPG.

Most of the capacitance appears to be the cable and shield and if I put 250 ohm resistors between OUTA and shield and OUTB and shield the response is fairly flat up to about 700kHz.
The output needs to be followed by an integrator of some description. Haven't measured the sensitivity yet, except to notice that loading the output reduces sensitivity, but that makes sense as the load current will tend to cancel the input current and reduce sensitivity.

moffy:
Update on sensitivity, the signal input is 100ma p-p:

1. No load: 20mv p-p @ 200kHz, 9mv p-p @ 100kHz, 4.5mv p-p @ 50kHz
2. 1kR between OUTA - OUTB: 19mv p-p @200kHz, 9mv p-p @ 100kHz, 4mv p-p @50kHz
3. 500R between OUTA - OUTB: 16mv p-p @200kHz, 8mv p-p @ 100kHz, 4mv p-p @50kHz bandwidth confirmed as approximately 600kHz

I have to take it back, loading does not appear to significantly change sensitivity. The differences could be due to measuring a moderately noisy scope trace by eye and the differences to the Q of the LC with and without loading and of course the 50R series resistance. 500R also produces a well damped response when a square wave current is used as the input signal.:)

moffy:
Just an update, I built my first version of the Rogowski coil amplifier and it seems to work but looks like it is dominated by 1/f noise. Since the signal is differential, P2:3-1, U1B inverts one side which is fed into U1A and summed with the other side, this helps eliminate common mode noise. Because of the nature of the Rogowski coil, sensitivity increases with frequency, U1A is an integrator. U2, the OP07, sets the output of U1A near to zero, a few mv doesn't matter. R7, R14/C3 form the feedback, connecting to R6 and the +ve input of U1A. C3 stops the DC control loop from oscillating by providing some feed forward at the low end and R14 limits the noise injected from U2 into U1A. The traces show the 1/f noise on the output. Trace is 5mv/div and excitation is 100ma p-p @ 500Hz. The response looks reasonably flat as a 1kHz square wave looks reasonably good. Not sure how to deal with the 1/f noise, except maybe a sharpish high pass filter @ 10Hz. Any suggestions would be appreciated.

Slh:
The high pass filter works well in my experience. I've built a few DC coupled ones and they've always had a lot of low frequency noise.

The best one that I've made used the cheapest, most rubbish amplifier available https://hackaday.io/project/190985-tl431-rogowski-coil-current-sensor and has the cleanest output signal thanks to the basic rc high pass filter.

moffy:

--- Quote from: Slh on June 02, 2023, 06:07:45 am ---The high pass filter works well in my experience. I've built a few DC coupled ones and they've always had a lot of low frequency noise.

The best one that I've made used the cheapest, most rubbish amplifier available https://hackaday.io/project/190985-tl431-rogowski-coil-current-sensor and has the cleanest output signal thanks to the basic rc high pass filter.

--- End quote ---

It looks like about a 3rd order high pass, and like we have similar projects, but I am impressed you wound your own! Thanks for sharing.

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