# Measure DC current with current transformer

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.

• It's called zero flux transformer and needs external circitry to cancel the flux. Search for LEM module. Jul 23, 2020 at 6:43
• Hall-effect sensors will also work. This sounds like galvanic isolation is mandatory.
– user105652
Jul 23, 2020 at 7:00
• 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 Jul 23, 2020 at 7:10
• 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. Jul 23, 2020 at 7:12
• You can also use a magnetoresistive sensor. Jul 23, 2020 at 7:51

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:

• 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. Jul 23, 2020 at 8:39
• @Neil_UK Not when you make a normal current to voltage transformer by terminating with a resistor. Perhaps I should edit that in. Jul 23, 2020 at 9:01
• 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. Jul 23, 2020 at 10:09

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.