# How do I place a long wire to make the EMI and inductance minimum?

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?

• 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. – DoxyLover Sep 12 '14 at 3:44
• That is a thousand times too large in value. Op wants sub milli ohms. – Andy aka Sep 12 '14 at 8:56
• Maybe using search terms like "ampmeter shunt" will lead to better results, there are tons of those on the market. – PlasmaHH Feb 9 '15 at 11:22