A split core current transformers is currently clamped around a live wire. One end of the transformer's output wires are connected to the analog pin of an Arduino and the other end has a 2.5V bias created by tapping between two 470k ohm resistors connected across GND and +5V of the Arduino.

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If the current transformer is removed and A0 is connected to the 2.5V point, Arduino gives a steady reading of 512 which is expected..

However when the current transformer is connected as described and A0 is connected to the other end of the current transformer (as described), Arduino readings of pin A0 will fluctuate around between 503 and 519 even when the clamped wire is unplugged or even when removed.

What is causing the current transformer to still give a reading?

Additionally, if we have 2-3 split core current transformers about 1-3 inches apart from each other, will the readings of each current transformers be affected?

  • \$\begingroup\$ Does the "spurious" reading reduce when the clamp is closed? What range of readings do you get when operated properly with a decent load current? \$\endgroup\$
    – Andy aka
    Mar 21, 2014 at 13:15

1 Answer 1


The secondary of the transformer may pick up some 60Hz signal from the mains even if mains is not actually threaded through it. The signal would be weaker than if the mains were threaded through the core. Try moving the mains wire closer and further from the transformer. If you move it further, the signal should reduce. You may be picking up the signal from the wiring in the building.

Suppose, you have one of these transformers clamped on a wire W1, while another wire W2 is close by but not going through the transformer. The transformer may sense both of these wires. The signal from W1 will be stronger than from W2, though.

  • \$\begingroup\$ Is there a way to shield the transformer from sensing the other wires? \$\endgroup\$
    – Nyxynyx
    Mar 21, 2014 at 5:35
  • \$\begingroup\$ Thought I'd also mention: If you're measuring power, you may well have the split-core clamp near other transformers, from which it may be subject to their fluctuating magnetic field. \$\endgroup\$
    – gwideman
    Mar 21, 2014 at 8:37

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