Today, we are going to build our own milliohmmeter circuit. The milliohmmeter is a very useful tool used to measure minuscule amounts of resistance in resistors or other electronic components.
We will, like many of our other circuits, focus on utilizing existing resources to make this milliohmmeter circuit. The main component of which is the regulator ICs; as for the voltmeter for displaying the resistance, we are using a common digital meter we have.
Aside from that, we will be learning electronics by designing circuits and determining basic electronic component values. The resulting circuit would be functional while being simple to make and understand.
Ideal of Creating a Circuit to Measure Low Resistance
I once had a conversation with my friend who owns a cement factory. He told me about a type of ohmmeter that measures a very low resistance, around 0.000001Ω, called a microohmmeter. He used it to check for irregularity in the transformer or motor coils, as well as junctions, wires, and fuses.
Below is an example of an aftermarket milliohmmeter. It has 4 wires set up, allowing for the separation of current-supply and voltage-measuring wires, which improves accuracy.
6‑Gear Digital Milliohmmeter, 4‑Wire Measurement DC Low Tester
On Amazon.com (Affiliated link)
Anyway, this one is a little bit overkill for us; we just need a milliohmmeter to demonstrate basic electronic principles. Plus, as you will see later, putting together a milliohmmeter from scratch is not too difficult.
The Basics of How a Milliohmmeter Works
The basics of how a microohmmeter or milliohmmeter works are that it will apply a constant current to the measuring subject, then read the voltage across it before using both values to calculate the resistance.
The constant current ranges from 1A, 10A, 0.1A, etc. And the voltage the meter reads is in the µV or mV range. Therefore, the resulting resistance will be in the range of only µΩ to mΩ.
Turning Ideas into a Circuit
The idea for measuring the resistance is to use a constant current source and apply this constant current to the component we are testing. Then, use a voltmeter to measure the voltage across the component; the resulting voltage will reflect the resistance in a DC voltage. It may sound complicated now, but you will understand better by the end of this.
Current Source Circuit
First and foremost, we need a constant current source that can supply a stable 0.1A current. We have many choices here, including using transistors and ICs. At the moment, we have many 7805s and LM317s in stock, so we are going to use these two.
We are arranging them into a circuit in the same way as shown in the block diagram below.
We decided that the original power source is a 12V battery because it produces less signal noise. In addition, we could replace it with an equivalent linear power supply capable of supplying 1A or more current.
The next step is the 5V regulator circuit. We are using the L7805 for this job. And since the L7805 is a one-chip 5V regulator solution, we only need to add C1 and C2 filter capacitors to increase the efficiency and decrease noise.
The next part is quite interesting: the constant current source. For today, we are using the LM317. We have used this chip before to build an Ni-HM battery charger in the past. If you are interested, you can read more about that here.
Testing the Circuit
The step-by-step experiments and value justification are covered in the extended notes.
After assembling the circuit on a perforated board according to the schematic above, we will use an ammeter to measure the output current. The result from the reading should be a constant 0.1A. However, if this is not the case, we will need to adjust VR1 until the output current is exactly 0.1A.
Next, add the test resistor to the output and measure the voltage across it using a voltmeter. The result we get for a 0.1Ω resistor is 10.3mV (0.01V), which is in accordance with the calculation we did at the start.
However, this may sound confusing, as we measure voltage in units of V but read it as resistance in units of Ω. For example, when measuring a 0.5Ω resistor, the voltmeter will display 0.05 instead of 0.5 for 0.5Ω.
There are two methods of solving this problem.
First, increase the constant current to 1A. This will cause the voltage across the resistor to be the same as its resistance; in this case, 0.5V. But this might not be appropriate, because a higher current will cause the ICs to heat up more, and more energy will be wasted.
Second, we keep the constant current the same at 0.1A, but multiply the resulting voltage measurement by ten. With this method, we do not have to recalculate the resistance value for the circuit. It will make reading the value more convenient, as now 0.1 in the voltmeter is analogous to 0.1Ω resistance.
Amplify the Reading Voltage using CA3140 OP-AMP
The second method is better, so we are doing it that way using the CA3140 OP-AMP. This chip will increase the voltage by ten times. Our reason for using this IC is that it has a very high input impedance, resulting in less distortion.
Furthermore, the CA3140 requires a minimum of 4V to operate, so powering it with the 5V regulator already in the circuit is okay.
We adjust VR2 until the reading on the voltmeter is 0.1V when measuring a 0.1Ω resistor. Then, try to measure other low-resistance resistors. We tried a 0.5Ω one, and the reading showed correctly at 0.5V.
Nevertheless, we are not going to use this circuit that frequently; it is more akin to an experimental circuit. Also, it is only suitable for measuring resistance less than 3Ω because the output voltage of CA3140 is capped at 3V.
Conclusion
We hope that you had fun with this simple electronics experiment. With that said, this circuit could also be useful, such as for testing switch contacts or wires that carry large amounts of current.
For example, if you happen to have a 12V 10A DC motor and use a relay to control it, and you notice that the motor is not working at its full potential. This may be because of erosion at relay contacts, wires, or wiring junctures. At these potential failure points, the resistance would normally have been as low as 0.1Ω or less. However, erosion or loose joints can increase resistance to up to 0.5Ω. If this is the case, the current may be reduced to 24A rather than the usual 120A.
This circuit is not meant to be an alternative to an expensive, professional-grade microohmmeter. This circuit is but a way to learn about measuring low resistance values and basic electronics in general by way of solving simple electrical problems. We hope that this article is useful to you.
👇 Further experimental notes
This article presents the concept and overall structure.
The extended notes focus on the thinking process behind the design:
component selection, value justification, experimental order,
and small but important details that emerged during hands-on testing.
Self-taught and driven by curiosity since childhood. I solve electronic challenges by getting my hands dirty at the workbench. My approach is simple: experiment, learn, and repeat.
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