How to accurately measure milliohms ???

[73sport]

All,
First time post - here we go...

I need a simply method to measure milliohms (from 1 to 500). Where most should be around .0045 ohms nominal

I have a DMM which can measure from 2.5V down to 1mv but resolution of ohms is only good to tenth.

The fact is I do not need to know the exact number rather, I need to do a comparison of good and bad. This is to check ground continuity.

The first item that comes to mind is a wheatstone bridge, a small accurate power supply and some good leads. These are available. Can someone give me some pointers on how to complete the rest of the project?

Thanks,
73sport (easy to remember)

 

[Klaus]

Milli Ohm measuring is usually done by passing a known current through the part and measuring the voltage drop across it. It has to be a reasonable size current to result in a measurable milli or micro voltage drop at very low resistances. It is a 4 wire device, 2 for the current and 2 for sensing.
They sell a milli ohm kit at the local electronic store which connects to one's digital meter, maybe you can find something like that. It would be simpler than building one from scratch.
Klaus

 

[Sebi]

Use the V-A method. Adjust the supply current limiter to 500mA, connect the resistor to output, and measure the voltage. R=U/I