Current Shunt Measurement

Formula

Description

A current shunt resistor converts current to a measurable voltage drop using Ohm's Law. The shunt resistance is chosen to be very small (milliohms) to minimize power dissipation and voltage burden on the circuit, while producing a voltage large enough for accurate measurement. Current sense amplifiers (INA219, INA226, MAX9611) amplify the millivolt shunt voltage for ADC measurement. High-side sensing (shunt between supply and load) is preferred for detecting ground faults but requires a differential amplifier that rejects the common-mode supply voltage.

Variables

• I — Current through the shunt (A)
• V_shunt — Voltage drop across the shunt (V)
• R_shunt — Shunt resistance (Ω)

Practical Notes

Common shunt values: 1-100 mΩ. A 10 mΩ shunt carrying 5 A produces 50 mV and dissipates 250 mW. For 1% current accuracy, the shunt must have better than 1% resistance tolerance and low temperature coefficient (< 50 ppm/°C). Four-terminal (Kelvin) connection to the shunt eliminates lead resistance error. PCB trace resistance can serve as a shunt for low-cost current sensing, but has ±20% tolerance and +0.39%/°C temperature coefficient.