I'm going to measure high current. After seeing that there is not a lot of low valued sensor resistors in the market and the available ones are very expensive, I decided to make my own current sensing resistor by using copper wire.

I wrote down a small program code to play with the parameters to find out a satisfying result (10mV voltage drop at maximum current). I ended up at 2mm cable in 23mm length.

Resistivity of the material : 16.8       mOhm.mm^2/m    (20 C)
Resistivity at amb. temp.   : 18.7464    mOhm.mm^2/m
Ambient temperature         : 50         C
Temperature coefficient     : 0.003862   K^(-1)
Wire diameter               : 2          mm
Wire resistance             : 0.133333   mOhm
Maximum current through     : 75         A
Voltage drop at max. cur.   : 10         mV
Power loss at max. cur.     : 750        mW
REQUIRED WIRE LENGTH        : 22.3444    mm

This wire will be connected at the bottom of an H-Bridge which will run at 50kHz frequency.

My demands are:

  1. The cable inductance must be low enough to not damage the H-Bridge MOSFETs.
  2. The rest of the circuit includes a lot of different opamps, and a TL494. I don't want the electromagnetic waves radiated by this wire to disrupt the circuit function.
  3. The cable should consume smallest volume possible.

How should I place this wire? Should I make a winding in which half of the turns are in one direction and the other are in the other direction, so that they magnetic fluxes cancel out each other?

  • \$\begingroup\$ Here's a DigiKey link for 0.13 ohm 3-7W resistors: digikey.com/product-search/…. They start at US$1.55 single quantities. They are wire-wound so there will be some inductance; I have no idea if it would affect you. \$\endgroup\$ – DoxyLover Sep 12 '14 at 3:44
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    \$\begingroup\$ That is a thousand times too large in value. Op wants sub milli ohms. \$\endgroup\$ – Andy aka Sep 12 '14 at 8:56
  • \$\begingroup\$ Maybe using search terms like "ampmeter shunt" will lead to better results, there are tons of those on the market. \$\endgroup\$ – PlasmaHH Feb 9 '15 at 11:22

I would imagine skin effect will disrupt your calculations quite a bit: -

enter image description here

The above picture tells me that at 50 kHz (sinewave of course), the skin depth of a copper wire will be 0.3mm - as you are using 2mm thick wire for your experiment, most of the current will be travelling around the outer perimeter of the wire and your cable resistance will be significantly greater and that's just at 50kHz. If the signal you hope to measure is a square/pulse wave (with harmonics at 100k, 150k, 200k, 250k, 300k etc, the resistance for these higher frequencies will be increased. At 150kHz, the skin depth will be about 0.1 mm.

How should I place this wire?

I wouldn't bother because it probably won't meet your expectations. Picture taken from here.

  • \$\begingroup\$ @hkBattousai Perhaps using a PCB track would be more effective? They're very thin and you could cut a plain copper PCB and tailor it to your needs. \$\endgroup\$ – Guillermo Prandi Sep 12 '14 at 0:45

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