Low Ohm Meter

Normally resistance meters work from one ohm to 10 Mohms.
How do you measure low resistance ohms?

Here I show how you can measure from 0.001 ohms to 1 ohms with a simple measurement device.

First we must generate 100mA constant current.

It is done with LM317 device, which is connected to current generator.
Using normal 10 ohm measuring resistance, we must adjust the current exactly to 100mA with P3 potentiometer.

currentgenerator.jpg

Them we must measure exactly voltage over the current generator,
It must do four fire measuring system.
It will go the current to measured resistance with two clips and conduct the voltage with other two clips.
Such clips costs 10e.

Kelvin_clips.jpg

It must measure the voltage with 10 times amplifier.
I selected Microchip amplifier MCP6642, which will work from full power supply to ground.

Still it will need offset calibration.

It is done to + input, so it could adjusted to zero ohms.

amplifier.jpg

The output voltage must go to voltmeter 0-1V
I did not found 1V voltmeter, so I build my own.

PIC16F886 works with 5V and the ADC works also with 5V, but it is easy to change range to 1.0 volt or actually 1.0 ohms.
LCD.jpg

There is also RS232 connection for the debugging the program.
There is 5V power input and power on switch.

There is also offset calibration resistor.

LOWOHMPCB.jpg

This is PCB for this project.
The size id exactly as of the LCD, so 80 *34 mm.

How it works?
I do no know, because it has not build yet.

Pekka OH3GDO

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