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Is it possible to measure DC electrical currents with some type of split core current transformer? Or it is only for varying currents?

EDIT

Considering a zero-flux CT, could someone show me the basic external circuit that goes with it?

I want to measure the current out of a solar panel going to the battery.

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    \$\begingroup\$ It's called zero flux transformer and needs external circitry to cancel the flux. Search for LEM module. \$\endgroup\$
    – winny
    Jul 23, 2020 at 6:43
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    \$\begingroup\$ Hall-effect sensors will also work. This sounds like galvanic isolation is mandatory. \$\endgroup\$
    – user105652
    Jul 23, 2020 at 7:00
  • \$\begingroup\$ the first results when you google for "zero-flux transformer" encompass schematics; you might want to ask about what you don't understand about one of these than just asking for these schematics \$\endgroup\$
    – Marcus Müller
    Jul 23, 2020 at 7:10
  • \$\begingroup\$ I think current transformer is only for AC current. For DC, I think you can use AC712 Hall effect DC current sensor. Ref (1) elprocus.com/acs712-current-sensor-working-and-applications. Cheers. \$\endgroup\$
    – tlfong01
    Jul 23, 2020 at 7:12
  • \$\begingroup\$ You can also use a magnetoresistive sensor. \$\endgroup\$
    – Jeroen3
    Jul 23, 2020 at 7:51

2 Answers 2

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If you terminate your current transformer with a resistor to get a voltage signal out, a normal AC current transformer would saturate very quickly if you apply any DC current due to Vt=NAB where V is positive value (or negative, same effect) and as t goes on, B just keeps building until saturation. For AC, V keeps changing and nets out to zero.

In a zero flux current transformer, you use an amplifier to sense your output and drive a current in a secondary coil, wound in the opposite direction (or apply an inverse signal, take your pick) to cancel out the flux build up. This results in a new operating point where equilibrium is reached and the core won't saturate yet you can measure the current as a DC voltage which is proportional to your applied current in the secondary coil.

Here is an image borrowed from Hioki, a manifacturer of such modules: enter image description here

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  • \$\begingroup\$ B doesn't keep building until saturation, it's not a time integral. For any given DC current, there will be an H field, will generates a certain B field. In a typical AC transformer, the core permeability tends to be high enough that saturation will occur at a DC current which is a tiny fraction of the AC rated current. However, the flux is proportional to the current, not a time integral, like would be the current in an ideal inductor subject to a voltage. \$\endgroup\$
    – Neil_UK
    Jul 23, 2020 at 8:39
  • \$\begingroup\$ @Neil_UK Not when you make a normal current to voltage transformer by terminating with a resistor. Perhaps I should edit that in. \$\endgroup\$
    – winny
    Jul 23, 2020 at 9:01
  • \$\begingroup\$ It depends whether you apply a V, or an I. If you apply a V, I keeps building and so does B to saturation. If you apply an I, then it may or may not saturate depending on the value of I. Generally with a current transformer, we apply an I. \$\endgroup\$
    – Neil_UK
    Jul 23, 2020 at 10:09
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It's not possible to measure a DC current with a transformer. In the case of AC the changing magnetic field associated with the current in the primary induces a current in the secondary winding which is what you can measure.

As mentioned in the comments above if you need to detect a DC current you can use a Hall effect device which does not require a changing magnetic field see https://en.wikipedia.org/wiki/Hall_effect for a bit more detail.

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