How to place a long wire to make the EMI and inductance minimum

current measurementhigh-currentinductanceinterferencesensor

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?

Best Answer

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.

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