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I'm using this current transformer to measure AC current in my application through a metering IC. This input from the secondary of the current transformer goes to a differential ADC in my metering IC.

There's some misunderstanding with the direction of current and the dot polarity shown in the image below:

enter image description here

Basic question first, since this is an AC current measurement, would the polarity of the differential signals matter (I1P, I1N) when I take the traces from the secondary and feed it to the input of the differential ADC?

Also, If the current flows from P2 to P1 (wire through the CT), how to comprehend the understand of the secondary current direction? Should I use some thumb rule to identify the direction of the current?

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  • \$\begingroup\$ If you populate it reversed the outputs are also reversed, meaning there is no change. \$\endgroup\$ Commented Nov 19 at 9:39
  • \$\begingroup\$ @Jeroen3, could you please explain, on how reversing the orientation of the CT also, reverses the outputs? \$\endgroup\$ Commented Nov 19 at 10:41
  • \$\begingroup\$ @Jeroen3 is saying that if you reverse the orientation of the CT (while keeping the current carrying conductor in the original orientation) then primary current direction has been reversed (as the transformer sees it) so the signal on the secondary side will be inverted. But, since the CT has been reversed the PCB will receive an inverse of an inverse and -1 x -1 = 1 so it will give the same output either way. \$\endgroup\$ Commented Nov 19 at 12:40

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The '*' should be the standard dot notation to identify starts of windings. Pin 1 secondary output should have the same voltage polarity as the P1 side of the hole for the one-turn primary. As currents cancel in the core, current flowing from P2 to P1 should create a secondary current from S1 to S2. 1

It does not matter if you're simply measuring current. It does matter which polarity is used if the meter computes both import and export of power.

As it's so easy to get current transformer connections reversed (ie install the core on the feed cable backwards), some metering ICs I've seen have a facility to reverse polarity, or ignore it if it's known that the application is import or export only. It's also quite straightforward to put a few 'do not fit' 0Ω components or jumpers on the board to swap it in the field if needed, or flip the orientation of the core on the primary wire.

If your meter can sense power, and is sensing it negative, then flipping the core orientation on the primary wire will flip the power reading to positive. Don't flip the primary orientation AND the secondary connections, that results in no swap at all.

1 And if the data sheet has it reversed, that is, dots for current rather than voltage, then just use one of the polarity swaps above.

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  • \$\begingroup\$ Can you please clarify on this scenario where I suppose, if I reverse the orientation of the CT alone without changing the primary wire orientation, I will still be having the same negative value of power, is it or will I have a positive value of power? \$\endgroup\$ Commented Nov 19 at 10:39
  • \$\begingroup\$ Basically, if I get negative power, swapping the transformer orientation solves the problem? \$\endgroup\$ Commented Nov 19 at 10:47
  • \$\begingroup\$ @Potionless absolutely, swapping the coil direction inverts the power flow result, that's the hardware solution for hardware that's behaving consistently. If your metering IC has a software facility to swap the orientation as well, and if its firmware writer was malevolent, and has some way of detecting that you've swapped the coil direction (snooping on you with a camera perhaps), and switches the IC polarity to confuse you, then you really are in trouble. Another gotcha - swapping both primary direction AND secondary connections gives you no swap at all. \$\endgroup\$ Commented Nov 19 at 12:35
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how to comprehend the understand of the secondary current direction? Should I use some thumb rule to identify the direction of the current?

You've chosen a transformer supplier that doesn't appear to have much quality or reliability (having read the scant data sheet). It mentions "same polarity" but, does that mean the currents are in the same direction or, does that mean the voltage polarities are the same?

As a matter of interest, both cannot be of the same polarity i.e. if the dots on the windings are for "same-phase" voltages then, the input and output voltage are in phase but, the input current necessarily has to be in the opposite direction to the output current for a resistive load. Image I created myself: -

enter image description here

Having said that, dots usually signify "same-phase" voltages but, the lack of provenance for the product does concern me.

Of course, if you are only interested in measuring RMS current then, the dots don't matter but, if you are trying to compute power (voltage × current) then it totally matters. So, if power is what you want to compute, be prepared to reverse the direction of the primary wire through the aperture or, swap the secondary connections.

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  • \$\begingroup\$ Can you just help me understand when you said, "but, the input current necessarily has to be in the opposite direction to the output current for a resistive load." ? \$\endgroup\$ Commented Nov 19 at 10:17
  • \$\begingroup\$ @Potionless I've added a little diagram to my answer that may help you with that. As you can see primary current flows in whilst secondary current flows out to the resistive load but, voltage polarities are identical as per the dots. \$\endgroup\$ Commented Nov 19 at 10:39
  • \$\begingroup\$ Just to clarify, after seeing the image, I have a question. In my question, the ground potential would not be the same for the primary and the secondary, right? But in the image, it depicts that primary and the secondary have the same reference, return or ground potential. Does it matter? \$\endgroup\$ Commented Nov 19 at 10:41
  • \$\begingroup\$ @Potionless the 0 volts is just for convenience of making the waveform images simpler and it doesn't matter that there is a connection. Yes many transformers don't make that connection (in order to achieve galvanic isolation between primary and secondary) but, making that common connection doesn't alter the polarities and magnitudes of voltages and currents. \$\endgroup\$ Commented Nov 19 at 10:45

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