Shunt Resistor
A very low-value, high-precision resistor that converts current into a measurable voltage. By Ohm's law, 30 A flowing through a 1 mΩ shunt produces 30 mV. The measurement is simple and cheap, but a loss arises in the resistor: in the same example the power dissipated is 0.9 W.
The shunt is the basis of functions such as current-sensing stop and current limiting; the driver measures the current through the motor by amplifying the voltage across this resistor.
Choosing the value is a balancing act. A higher-value shunt gives a larger signal that is easy to read, but it also increases the loss; the loss is directly proportional to the square of the current and to the resistance value. A lower-value shunt reduces the loss but requires a good amplifier to separate a millivolt-level signal from noise.
- Temperature stability: If the resistance of the shunt changes with temperature, the measurement drifts as well; that is why special alloys with a low temperature coefficient are used.
- Four-terminal connection: The sense terminals are taken separately from the power terminals (Kelvin connection); otherwise solder and trace resistance enter the measurement.
- No isolation: A shunt measurement shares the reference of the power circuit; if an isolated measurement is needed, a Hall-based sensor is used.
The accuracy of the measurement is the accuracy of the driver's protection threshold; a drifting shunt reading can be the overlooked cause of early or late tripping.
Where this term is used: DC Motor Driver Selection Guide