Introduction: Simple Low Resistance Tester (Milliohmmeter)

By
About: Autistic person who's interests include utility cycling, recreational cycling, cycling safety, electronics, gardening, Arduino, and LEDs.

If you want to know the resistance of low resistance components such as wires, switches, and coils, you can use this milliohm meter. It's straightforward and inexpensive to make. It even fits in your pocket. Most ohmmeters are accurate down to 1 ohm, but this one is sensitive to low resistance in the range of milliohms or even microohms.

Step 1: Materials

R1: ~220-ohm resistor
R2: Unknown resistance
2x thin wires (e.g. mobile charger cords)
Rectangular shaped plastic box
5V source (eg. USB port, mobile chargers)
2x alligator clips
DC jack and connector (optional)
Solder
Hot glue
Multimeter with ohms and millivolts ranges (the lower the voltage range, the more sensitive the milliohm meter)
Calculator

Step 2: Drill Holes on Case

Drill the holes to fit the wires and leads.

Step 3: Soldering

Soldering can be done boardless. Just hot glue the parts to the box. If your power supply is bulky and you want it detachable, include the DC jack and connector.

Step 4: Using the Milliohmmeter

Before testing the unknown resistance, measure the resistance of R1. It should be close to 220 ohms.

To measure the unknown resistance (R2), attach it to the milliohm meter's test leads. Measure the voltage across R1 and R2. When measuring R2's voltage, measure it across R2 directly. Do not measure the voltage across the alligator clips because the contact resistance will add up the voltage drop and overestimate the resistance.

Based on Ohm's law, we know that R1 and R2 have equal current flowing through them. Because of this, we can use V2 and the current to calculate the unknown resistance.

R2 can be calculated as follows:
R2=V2/(V1/R1)

Where
V1=Voltage across R1
V2=Voltage across the unknown resistor
R1=Measured value of R1 (~220 ohms)

In the second picture, I used an ammeter as an example.

This link has more details about the low resistance tester:
http://www.robotroom.com/Measuring-Low-Resistances.html

Step 5: Measurements of Low Resistance Parts

Based on the calculations and expected values, this milliohm meter was reasonably accurate.

Since the voltmeter has a range down to 0.1 mV, it can measure down to 0.01 ohm. To increase the sensitivity, you can purchase a more sensitive voltmeter or use a lower resistor value. Because resistors are sensitive to temperature changes, the power rating needs to be higher.