In this project I made a precise milliOhm meter, based on some circuit designs first published by Scullcom Hobby Electronics. I followed his circuit designs, then I designed my own PCB and developed a little code for an Arduino Nano.

The milliOhm meter uses a very simple principle: Ohm's law. We apply a know current (100 mA) on an unknown resistor, and we measure the voltage across the resistor. The know current and the measured voltage will tell us the resistance. The measurement is also known as 4-wire sensing or Kelvin sensing. The voltage measured by the AD converter is processed by an Arduino Nano and the data is sent to an OLED display as well as to the serial port.

Some tips:

Pay special attention to the polarity of the following components:

• All tantalum capacitors are polarized, so double check C2-C5, C8 and C9.
• LTC2400 and LT1634 don't have notches. The pin 1 is located where their numbering starts. Always double check this!
• The maximum resistance that you can measure is 5 Ohms. Don't go above this value, otherwise, you can damage the ADC!

Part list:

Actives:

• U1: LM2940-CS5.0
• U2: LT1634
• U3: LT3092
• U4: MAX680
• U5: INA106
• U6: LTC2400
• U7: ADR4540

Passives:

• R1-R7 and R9: 100R (0.1%) - R_Axial_DIN0204_L3.6mm_D1.6mm_P5.08mm
• R8: 56R (0.1%) - R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm
• R10: 10R (0.1%) - R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm
• R11: 499k (0.1%) - R_Axial_DIN0207_L6.3mm_D2.5mm_P7.62mm
• RV1: 100R - 3386P
• RV2: 100k - 3386P
• C1: 220 uF (electrolytic - d6.3, p2.5)
• C2-C5: 4.7 uF (tantalum - 3528)
• C6, C7, C10: 100 nF (ceramic - 0603)
• C8: 3.3 uF (tantalum - 3216)
• C9: 10 uF (tantalum - 3528)