A Very Simple Circuit to measure Low Resistance Values.
With a "Direct Resistance Readout in Ohms".

You MUST use Two Seperate Batteries.
One for the DMM and one to supply power to the LM317LZ.

I Recommend the LM317LZ, which is the 100 mA, T0-92 version of the Normal LM317.
But you can also use the LM317, in the T0-220 Package, if you want.

The Trimpot MUST be Set PRECISELY to deliver 100.0 mA out or better,
to get Truly Accurate Resistance Measurements.
So you need a VERY ACCURATE Milli-Amp Meter to adjust this Correctly.
(And like Any Test Equipment, This Calibration should be Re-Checked once a year or so.)

** My Calibration Meter is a 4 1/2 Digit DMM, "So 100.00 mA setting Capability".

The Meter Leads MUST Connectly DIRECTLY Across the Resistor or Wire to be Tested.
DO NOT CONNECT IT to the Clip On Leads, that connect to the Resistance under test,
As this will give FAULTY Readings, as a Result of Contact Resistance.

The Display will read out Directly in Ohms, Between 0.001 up to 1.999 Ohms.

AS A OUALITY TEST:
I connected my 100 mA current across a Single Conductor of 22 AWG Bare Copper wire
that was about 16 inches long, "but exactially 12 inches long between my Meter Test Leads".
And it measured 0.016 Ohms.

Looking at my Copper wire Chart, it tells me: 22 AWG has 16.46 Ohms per 1000 feet.
So that is 0.01646 Ohms per Foot.
So I AM HAPPY with this result!

I could also create a Milli/Micro-Ohm Resistance Meter,
But the real Problem would be the Accurate Calibration of the current source.




"Back to My Projects Page."


 Low Ohm Meter Schematic.

 PCB for the 100mA Current Source.
This is a Small Circuit and can be built without a PCB,
But this PCB is Available, If Wanted.

 My Completed Unit.
You Could use a DMM instead of this Panal Meter, But you will Probably Lose One Decimal point of Sensitivity.
The Front Decal was created in a small Cad Program and Reverse Printed on "Transparency Film".
Than Sprayed with "3M's Spray Mount Adhesive", let dry and than applied to the front.
Because it is Reverse Printed, the writing Can't rub off.



All Imformation in this Article is "Copyright protected".

 Chemelec 
 *Copyright © 2010*