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Isolated current sense instrumentation
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JS:
The peoblem seems the choice of the transformer, you should look for a low turns ratio designed for low freq, not the case of the FIS155 which is 1:500 for 500kHz.

You could wind your own transformer, small core, M6 lamination should do. Not so many turns, easy transformer math applies here, use your desired output voltage and input current to pick a turns ratio and shunt.

JS
capt bullshot:
There are current transformers specially designed for your purpose. They are specially designed to give accurate phase and amplitude results at the power line frequency including harmonics to some kHz.
Typically, they have a 1:1000 (ballpark) ratio and a very high main (magnetizing) inductance (some 10H). In terms of accuracy, noise, linearity and immunity to external magnetic fields, they outperform any open loop Hall type sensor.

An example would be the datasheet I've attached.
eecook:

--- Quote from: capt bullshot on June 13, 2018, 06:43:31 am ---There are current transformers specially designed for your purpose. They are specially designed to give accurate phase and amplitude results at the power line frequency including harmonics to some kHz.
Typically, they have a 1:1000 (ballpark) ratio and a very high main (magnetizing) inductance (some 10H). In terms of accuracy, noise, linearity and immunity to external magnetic fields, they outperform any open loop Hall type sensor.

An example would be the datasheet I've attached.

--- End quote ---

I don't see how that is better than the one I am using (also designed for current sensing). It has an even worse turns ratio and inductance.
eecook:

--- Quote from: JS on June 13, 2018, 02:51:21 am ---The peoblem seems the choice of the transformer, you should look for a low turns ratio designed for low freq, not the case of the FIS155 which is 1:500 for 500kHz.

You could wind your own transformer, small core, M6 lamination should do. Not so many turns, easy transformer math applies here, use your desired output voltage and input current to pick a turns ratio and shunt.

JS

--- End quote ---

I will indeed have to seriously consider designing my own xfmr, but I will leave that for a 2nd iteration. At the moment it is not a possibility as the current sensing is part of a larger system and I have a time constraint for a working prototype.

In terms of the bandwidth I need, the FIS155 is the the only off-the-shelf part I found that met my spec. The price to pay is the turns ratio.

In any case, while the circuit could certainly be improved, it doesn't seem all that bad, does it?

Thanks again for your insight JS.

Cheers.
capt bullshot:

--- Quote from: eecook on June 13, 2018, 11:15:11 am ---
--- Quote from: capt bullshot on June 13, 2018, 06:43:31 am ---There are current transformers specially designed for your purpose. They are specially designed to give accurate phase and amplitude results at the power line frequency including harmonics to some kHz.
Typically, they have a 1:1000 (ballpark) ratio and a very high main (magnetizing) inductance (some 10H). In terms of accuracy, noise, linearity and immunity to external magnetic fields, they outperform any open loop Hall type sensor.

An example would be the datasheet I've attached.

--- End quote ---

I don't see how that is better than the one I am using (also designed for current sensing). It has an even worse turns ratio and inductance.

--- End quote ---

The CT's main inductance and the resistive part (sense resistor plus CT's internal copper resistance) form an L/C high pass. Its corner frequency is determined by (total R)/2*pi*L. As inductance increases by n² but resistance increases by n (assuming using the same wire, which is not true), it's easier to achieve low corner frequencies by increasing n (at first approach). Additionally choosing the appropriate core material gives a large increase in L without affecting R.
So this is why (by providing a much larger L) these line frequency CTs give better results than yours though they have higher R.

Edit: The CT whose datasheet I attached, has a main inductance of about 650H @100Hz and a resistance of 460 Ohm, put these values into your simulated circuit to see the results. For this particular CT and your chosen CT (which has 470mH only), the core material makes for the large difference in main inductance.

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