current comparator

Thread Starter


Joined Aug 1, 2010
I need to close a switch whenever a current in a wire exceeds the current in another by a multiple (probably 3)
eg if current in wire A is 3amps and current in wire B is >1amp then close switch. This needs to be checked only once every 3 secs or so.
Am I right in thinking that if I run both wires through identical coils but put A through once and B through 3 times then that will make the comparison between two equal currents?
BTW the currents are 240v. Max in A could be 100amp, B 16amp.
If someone could point me to an off the shelf comparator module which will accept the inputs from the two coils I would be very grateful.


Joined Apr 24, 2007
This is just off the top of my head. I hope that someone else will chime in if there's a fatal flaw in what I suggest, here.

You could insert a small resistance in series with each wire and use the voltage across them to sense the current. Then you could compare the voltages, with appropriate scale factors if necessary, and go from there.

For the wire with 100 Amps max, you'd need to use a very small resistance, with a large-enough power rating. If it's a sinusoidal 100-Amp-peak current, then the power dissipated in a resistor R would be about 5000*R. Even just 0.1 Ohm would dissipate 500 Watts.

But you could use two 0.01 Ohm resistors in parallel, for example, to get .005 Ohm, which would need to be able to dissipate 25 Watts (12.5 Watts per resistor; Use resistors that can do at least 25W each.). The only single resistor that I noticed in the catalog that would do the job (and dissipate <= half its rated power) is an Arcol surface-mount of 0.0005 Ohms, 8 Watts, which would need to dissipate 2.5 Watts for 100-Amps-peak AC. And I'm not sure what its maximum current rating is.

Anyway, assuming you can find resistors or parallel combinations of resistors that would handle the power dissipation, to sense the currents and convert them to voltages for you, then you could probably connect the inputs of two opamp amplifiers, one across each current-sense resistor, and get two voltages to compare.

You would probably want to set the gains of the opamp amplifiers so that when one amplifier output voltage was greater than the other, it would satisfy your current-multiple condition. Then a simple circuit using a comparator should be able to provide a signal that could be used to close your switch (relay, or whatever).

If you only want to check it every three seconds or so, you'd need to add that too. And I'm sure there are a few other details that would need to be worked out.

Alternatively, you might be able to wrap a current-sensing coil around each wire, or even just put an inductor near each wire, and sense the currents that way. Someone else will probably be able to give you estimates of what might work for that.


Last edited:


Joined Apr 20, 2004
One suspects there are a number of current transformers that would perform the function very well, and with no contact necessary to the conductors. Any number os techniques would be able to determine the currents with perfect safety.

Thread Starter


Joined Aug 1, 2010
Thanks for you input y`all. I am getting a clearer picture.
BTW my timezone IS GMT, I`m about 200m west of Greenwich