enter image description here

We have a project where we have to measure if there is any current through the a cable, but having not common ground and knowing that the current could come direct or reverse direction. We thought about the chance of set a differential opamp, but if the current came reverse that would be a problem . We also thought about optocupler but we cannot limit the current so much.

Any idea is welcome,

Thanks in advance,


  • 1
    \$\begingroup\$ If you're able to put a sense resistor in the line like you drew, just check any of the analog chip vendors for their current-sense amplifier offerings. Most are not bidirectional, but a few are. \$\endgroup\$
    – The Photon
    Mar 24, 2017 at 16:28
  • \$\begingroup\$ How much current? AC/DC? What is the line voltage? \$\endgroup\$
    – Trevor_G
    Mar 24, 2017 at 16:34
  • 1
    \$\begingroup\$ You can consider using the ACS711 or similar. These are based on Hall effect rather than voltage drop in a sense resistor. Check it out and see if it meets your needs. \$\endgroup\$
    – user57037
    Mar 24, 2017 at 16:37
  • \$\begingroup\$ If you need to measure the current vs. just detect if there is any you can buy off-the-shelf non-contact AC and DC current sensors where you run the conductor through an aperture. I have used modules from LEM, they're very high quality and they have a wide range of products: lem.com They may be overkill for a current/no current sensor. \$\endgroup\$
    – John D
    Mar 24, 2017 at 16:43

2 Answers 2


A hall effect sensor can be used for what you appear to want. It produces a signal in the presense of a magnetic field and any current passing through a conductor produces a magnetic field: -

enter image description here

A saturable reactor is also something that can be used to detect the presense of current in a wire providing the wire carrying the current is wound close to the core of the saturable reactor.

  • \$\begingroup\$ Is this the same principle that they use in contactless multimeter? (which has a hook-like probe) \$\endgroup\$
    – ammar.cma
    Mar 24, 2017 at 16:34
  • \$\begingroup\$ @ammar.cma some might use this method but generally I guess a CT is used for AC current measurements. \$\endgroup\$
    – Andy aka
    Mar 24, 2017 at 16:35
  • \$\begingroup\$ seems like my only way is hall effect...but i got another problem, the current i measuring is in the milliamps order, so maybe...not suitable ? :( \$\endgroup\$
    – Carlos Hxc
    Mar 29, 2017 at 18:54
  • \$\begingroup\$ Form a ten turn loop if you can and this multiplies the field and makes it easier to detect or measure. \$\endgroup\$
    – Andy aka
    Mar 29, 2017 at 19:58

Quite a few ways.

  1. Hall effect sensors.

  2. Current transformers if it is a.c.. It can be as simple as a few windings over the wire.

  3. Thermal effect, like those used in airflow meter.

  4. Compass or magnetometer.

  5. Run the wire through a magnetic field and attach a needle to it. Essentsilly making a galvanic ammeter out of it.

..... I'm sure you can think of more.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.