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For one of my projects I need to measure if there is a DC current going trough the thick power cable. The current is DC, and my method of measurement has to be non-invasive.

The current going trough the cable will be anywhere from 1A to 50A. Also the measurements don't have to be precise (at all), I just need to know if there is current being drawn (flowing trough cable) or not.

I've found several solutions using current shunts, hall effect sensors but all of them have some upsides and downsides.

I was wondering if someone could suggest a cheap/simple solution for Non-Invasive high power DC current measurement?

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  • \$\begingroup\$ Current clamp meters use ferromagnetic core transfomers to sense the current through a wire. Similar to hall effect. But essentially its a iron 'ring' (toroid) with insulated copper windings wrapped around the 'ring'. They work great for medium currents (10 - 100A) but a bit inaccurate for small currents (mA) \$\endgroup\$ – crowie Aug 29 '16 at 16:29
  • \$\begingroup\$ One of the options was to have hall effect sensor sitting in the slit in the toroid, which would give nice readings. But after a lot of searching, I couldn't find a premade toroid manufacturer, only cores with hall sensors embedded in them, costing around $10-15. Which is too expensive for the application where I need just to sense if the DC current is being drawn or not. \$\endgroup\$ – Calculon Aug 29 '16 at 16:52
  • \$\begingroup\$ @crowie: He's measuring DC. A CT won't work on DC. \$\endgroup\$ – Transistor Aug 29 '16 at 21:59
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If you have access to both ends of the cable, then the voltage drop across the cable is easy to measure.

I have been known to use a the screen of an RF co-axial cable as a heavy current conductor (some cables are built with a lot of copper in the screen), and use the inner to sense the voltage at the far end. This gives a nicely screened differential voltage at this end to measure. An op-amp used as a comparator to sense the mV can be very inexpensive.

An improved Hall sensor method would be to use a pair of Hall sensors, one attached to each side of the cable, facing in opposite directions. This way, the magnetic field due to the current would add, and any external fields would cancel, yeilding a much better signal to interference ratio.

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  • \$\begingroup\$ If you go the route of a hall sensor, you'd probably have more luck using an integrated current sensor, like this one \$\endgroup\$ – Brendan Simpson Aug 30 '16 at 13:24
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Since the current is DC and you're not allowed to connect directly to anything (what I interpret you mean by "non-invasive"), a Hall effect sensor is the obvious option. The current will make a circular magnetic field around the cable, which the Hall effect sensor detects.

You could make some mechanical contraption based on the principle of a compass needle, but I'd go with the Hall effect sensor.

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  • \$\begingroup\$ I've found few hall effect sensors from Melexis that should do the job. But I was just wondering if there is a better way to just sense if there is current flowing trough the cable or not. \$\endgroup\$ – Calculon Aug 29 '16 at 16:49
  • \$\begingroup\$ A hall sensor 'just senses' if there is current flowing. \$\endgroup\$ – Neil_UK Aug 29 '16 at 18:49
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Is measuring voltage invasive?

If you monitor the output voltage the cable resistance will act as a current shunt and indicate the current from a stable power supply.

If the power supply dips with current, it's internal resistance or ESR would have to be included with the cable (both wires) to determine if current is flowing.

Otherwise measuring the voltage difference of each end for + or - will indicate actual voltage drop but a filter cap may be needed for noise.

Testability should always be an up-front design requirement and not an afterthought, FWIW.

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  • \$\begingroup\$ This is a good suggestion, but in this case I would be measuring the voltage over the batteries. So the voltage will always be in some range that doesn't have to be directly proportional to the current flowing trough that cable. But I didn't mention that this system has batteries, so my bad. \$\endgroup\$ – Calculon Aug 29 '16 at 16:42
  • \$\begingroup\$ A DMM can easily measure the voltage drop in mV on either wire, when current flows. \$\endgroup\$ – Tony Stewart EE75 Aug 29 '16 at 16:49
  • \$\begingroup\$ I'm trying to embed this into my solution. Using a DMM for that is not an option. Using precision differential op-amp, fighting with the noise from the environment and etc. seems like too much trouble for a very simple thing. Hall-effect sensor on the other hand seems like a most likely solution. \$\endgroup\$ – Calculon Aug 29 '16 at 16:55
  • \$\begingroup\$ Since source and load is very low impedance series R to shunt C of any amplifier can work easily. Worst case, ferrite beads around the signal pair will improve transient CMMR better than any active device at high di/dt and dv/dt with Twisted pair from each end. Hall sensors have gap loss with a 5mm chip. so calibration may be needed for threshold<1A \$\endgroup\$ – Tony Stewart EE75 Aug 29 '16 at 17:01
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You can try an experimental approach. Can you wrap a non-ferromagnetic ring around the cable? The magnetic field produced by the cable will be contained within this ring, which will become an annular magnet. If you create a little slit on the surface of this ring, the magnetic field will leak and will loop over the slit. By pouring magnetic particles on the slit, some will stick to it. If there is current, there is magnetic field. If it is strong enough, it might do the trick.

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  • \$\begingroup\$ That is very similar to what people usually do with Hall Effect sensor and small toroid. But the problem is that I cannot find a decent supplier of the toroids with the slits in them to put the hall effect sensor there. They usually sell sealed solutions with price tag of $15 and up, which is totally fine, but too expensive for this application. \$\endgroup\$ – Calculon Aug 29 '16 at 17:07
  • \$\begingroup\$ Get a little tube (like those of cheap water meter), put alcohol and magnetic particles inside, place it close to the cable surface as possible (the magnetic field is strongest there) and see if the particles arrange themselves in the liquid. \$\endgroup\$ – tozheneznayu Aug 29 '16 at 17:21
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Many smartphones and tables have a built-in magnetometer. I remember seeing a few applications that use them to measure the magnetic field (for example, this one). You could use one of them to measure the magnetic field generated by the DC current. I don't know the sensitivity of this method but it should be more than enough to measure the magnetic field generated by currents in the range you want.

Or, if you are using a microcontroller, there are some cheap breakout boards with magnetometer chips.

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