How the current shunt calculator works
A shunt is a resistor of very low value placed in the current path; the current is the voltage across it divided by its resistance. Industrial shunts are rated by current and voltage drop, such as 100 A / 75 mV, while board-level current sense resistors are given in milliohms.
The calculator works from either. It gives the resistance, the voltage and dissipation at your current and at the highest current, the resistance error from tolerance and temperature coefficient, and the amplifier gain and current per ADC code when you enter the converter’s full-scale input.
Rating and resistance
A 100 A / 75 mV shunt drops 75 mV at 100 A, so R = 75 mV / 100 A = 0.75 mΩ. At 50 A it drops 37.5 mV and dissipates 1.9 W; at 100 A it dissipates 7.5 W. The dissipation heats the shunt, and its resistance changes with its temperature coefficient: 50 ppm/K over a 40 K rise is 0.2 %. Manufacturers state how far below the rated current a shunt should run continuously; follow their derating.
Choosing the voltage drop
A larger drop is easier to measure accurately but wastes power and heats the shunt. 50, 60, 75 and 100 mV are common for industrial shunts. On circuit boards, drops of 10 to 100 mV at full current are typical, chosen so the amplifier’s offset voltage stays small against the signal. Below about 10 mV at full current the offset and the thermoelectric voltages at the connections become a noticeable part of the reading.
Connections and amplifier
Measure at the shunt’s sense terminals, not at its current terminals. A four-terminal (Kelvin) connection keeps the resistance of the screw contacts and the cable out of the measurement. The amplifier’s offset divided by R is a current error that dominates at low currents: 100 µV on 0.75 mΩ is 0.13 A.
Enter the full-scale input of the amplifier or ADC to see the gain that brings the highest current to full scale, and the ADC bits to see the current per code. The ADC resolution calculator and the instrumentation amplifier gain calculator take the design further.
Frequently asked questions
High side or low side?
A low-side shunt, in the return to ground, is easy to measure but lifts the load’s ground. A high-side shunt keeps the ground intact but needs an amplifier that tolerates the common-mode voltage of the supply. Current sense amplifiers are built for the high side.
When is a Hall sensor better?
When the current is high, the circuit must be isolated, or the dissipation of a shunt is unacceptable. A closed-loop Hall transducer adds no heat in the conductor and gives isolation, at a higher cost and offset.
How does a current transformer compare?
A current transformer only measures alternating current and is limited by its burden; the current transformer burden calculator checks that. A shunt measures DC and AC alike.