The accuracy of normal digital multi meter on ohm is about 0.1-0.01.
So the minimum resistance we can measure is about 100-10 milliohm.
Considering the line resistance of probe, the accuracy will be very low when measure small resistor.
If the resistor to be measured lower than 10 milliohms, the measurement will be impossible.
Today, I want to make a circuit with several components.
With this circuit, make a normal digital multi meter to be a high precision milliohm meter.
LM317 provide a reference voltage of 1.25V.
With LM317, we can make an accurate constant current source.
The capacitor of 2.2uF as the filter of power input.
If the power source is battery, this capacitor could be omitted.
The resistor of 20 ohms as current-limiting resistance.
The high precision potentiometer of 100 ohms used to adjust the current cooperate with the 20 ohm resistor.
Let's assemble the circuit, first to install the DC socket.
Install the capacitor of 2.2uF
Give a heat sink to LM317, this could improve the temperature drift. The heat sink is optional.
Install the resistor of 20 ohms.
A jump wire to the LM317 adjust pin
Install potentiometer of 100 ohms.
Install 2-Pin terminal.
Finally, a jump wire for GND.
The circuit is simple.
Use the pins of components to make up the circuit, no need additional wire.
The duration of welding is a little long, you could jump to 2:13 to skip.
Clean the spared pins.
The constant current circuit is completed.
Let me show you how to measure milliohm by this circuit.
Here we need a pair of LCR clamp.
I have shared how to DIY these LCR clamp before.
If you are interested in it, you could check out it.
Connect the LCR Clamps to COM and mA socket.
Connect the other end of LCR Clamps to the output socket of constant current circuit.
5V Power Source or use a 3.7V Li-ion battery as Power Source
Connect the Power Source to the DC socket.
Switch to mA gear
Keep LCR Clamp separated with each other.
Adjust the potentiometer, make the reading on multi meter is just 100mA.
Connect the red banana plug to voltage socket.
Switch the gear to mV.
Let's connect the LCR Clamp to each other, the reading should be 0mV now.
Now, we could put the resistor between two LCR Clamp to measure it.
Let's measure this 10 milliohm alloy resistor first.
The reading is 1mV.
Since the constant current is 100mA.
We can calculate the resistance is 10 milliohm according to Ohm's law.
To make the conversion easily, just remember 10 times of the reading is the milliohm result.
Let's measure another 1 milliohm alloy resistor.
The reading is 0.1mV. 10 times is 1 milliohm.
The precision looks well.
Also, could be used to measure a larger resistor.
To measure a metal shell resistor of 1 ohm.
The reading is about 98.76mV, we can get the 10 times result is about 0.9876ohm.
Switch to ohm gear to do a comparison.
The reading is 0.99ohm approximate to 0.9876ohm.
Finally, I found a 0.5 milliohm alloy resistor which is the minimum resistance I have.
Let's check out whether it could be measured.
The reading is 0.05mV, 10 times result is 0.5milliohm.
The precision could reach 0.1milliohm.
But there is no 0.1 milliohm resistor in my hand to do further test.
I believe this precision have met the needs of most people.