DIY Milli-Ohmmeter: Build a Precise Low-Ohm Tester

If you're interested in a DIY attachable milli-ohmmeter, why not check out this article and try making a constant current source along with it?

Usually, LM317 is used a lot for making constant current sources, but the current from an LM317 isn't very stable. The measured resistance can deviate quite a bit, and the range of resistances you can measure is also limited. In reality, the circuit is basically just using the volt-ampere method to measure resistance, except here the current is known and stays constant! So you can use a comparator (a type of op-amp connection) as a constant current source.

That way, you can start designing the circuit. If you don't have a TL431, you can replace it with an LM7805. The circuit diagram looks like this:

Brief principle: A standard voltage is applied to the non-inverting input of the comparator, and the voltage across the sampling resistor is fed into the inverting input. If at any moment the current starts to rise, the voltage at the inverting input will also rise, exceeding the non-inverting input voltage. At that point, the comparator reduces the base current of the transistor, lowering the current through it. The transistor acts like a variable resistor here, so a heatsink is necessary.

Once the circuit is connected, you can start debugging: the VCC voltage should be above 7V so the LM7805 can work stably, but it shouldn't be too high, or the transistor will overheat. Around 9V is a good choice here.

First, use a multimeter in current mode to short terminals 1 and 2, then adjust the potentiometer to set the current to 100mA (you can choose your own value; if you increase the current, the sampling resistor's power rating should also be increased to prevent resistance changes from temperature fluctuations). Connect the resistor to be tested directly between terminals 1 and 2, and the multimeter in millivolt mode should be connected inside the current terminals.

At this point, you can start measuring resistances:

If you don't have a precise low-value resistor, you can test with a resistor labeled 330 milliohms:

As shown in the figure above, the measured voltage drop across the resistor at 100mA is 31.9893mV. According to Ohm's law: R = U / I. The current I is the known constant 100mA, so the resistance is: R = 31.9893mV / 100mA = 319.893 milliohms

You can then check the accuracy using other instruments.

As shown above, another measurement reads 320.2 milliohms.

This battery internal resistance tester shows 319 milliohms.

The accuracy of an attachable milli-ohmmeter mainly depends on the following factors:

1. The precision of setting the constant current and the accuracy of the multimeter measuring voltage.

2. The stability of the LM7805 (including its voltage divider resistors and potentiometer; using TL431 is even better. Of course, using a dedicated voltage reference is the most effective).

3. Temperature drift of the sampling resistor. (If the sampling resistor changes too much with temperature, the constant current will inevitably be unstable.)

4. It's best to use Kelvin clips.

The end.