How to Measure Milliohm Resistances with a Cheap Multimeter

Importance of Milliohm Measurement

• High Currents: Small resistances matter in high current circuits.
  • Voltage Drop & Power Loss: Even a few milliohms can cause substantial voltage drops and power losses.
• Applications: Projects that may require such measurements:
  • Coilguns
  • Solenoids
  • Long battery wires

Problem with Traditional Multimeter Measurement

• Inaccurate Readings: Typical multimeters give inaccurate resistance readings in milliohm range.
  • Displays like 0 ohms or 0.3 ohms are common.
• Cable Resistance: Resistance of multimeter cables affects readings.
• Design Limitations: Most multimeters are not designed to measure very small resistances.

Solution: Four Wire (Kelvin) Measurement

• Concept:
  • Uses a constant current source of 1 ampere.
  • Measure voltage drop across the resistance (R).
• Ohm's Law: V = IR
  • Rearranged: R = V/I
  • Since current (I) is 1 ampere, R = V
  • Example: 50 millivolts across the wire = 50 milliohms resistance

Advantages of Four Wire Measurement

• High Accuracy: Removes effects of test wire and alligator clip resistances.
• Minimal Impact from Multimeter: High input impedance of multimeter ensures almost no current flows into it.

Practical Setup

• Equipment Needed: Bench power supply with current limiting.
• Steps:
  1. Set voltage to ~2 volts (low voltage is safer for short circuits).
  2. Set current limiting dial to zero.
  3. Connect multimeter to measure current directly across power wires.
  4. Increase current limit to get exactly 1 ampere.
• Measuring Resistance:
  • Connect current supply wires to the device.
  • Measure voltage across it.
  • Example: 47 millivolts = 47 milliohms

Alternative Without Current Limiting Power Supply

• DIY Constant Current Source: Using LM317 voltage regulator.
  • Diagram and specific resistor values needed.
• Real-World Adjustments:
  • Measure actual current output (may not be exactly 1 ampere).
  • Example: If output is 0.92 Amps, adjust calculation accordingly.
  • Formula: Resistance = Measured Voltage / Measured Current

Use Cases and Final Remarks

• Efficiency in Designs: Improves accuracy for projects like coilguns and solenoids.
• Broader Applications: Useful for any high-current circuits where precise resistance measurement is crucial.