How Does a Shunt Resistor Work?

A shunt resistor works by measuring either direct or alternating current that flows in an electrical circuit through the voltage drop that is generated across the resistance. Also known as a current shunt or ammeter shunt, this precision device uses the principle of Ohm's Law, which is mathematically represented by the equation V = I x R, where "V" indicates voltage, "I" denotes current and "R" is resistance. The standard units for voltage, current and resistance are volts, amps and ohms, respectively.

The functionality of shunt resistors is generally indicated in terms of their current rating, output voltage, resistance drift, resistance accuracy and power rating or derating. Modern ammeters are typically designed with an on-board or pre-installed shunt resistor. This arrangement allows full current load to flow through the ammeter shunt, where the voltage drop that occurs across the resistance is measured by a built-in analog to digital converter. The electrical reading is then shown on a digital monitor.

Traditional ammeters, meanwhile, require an external shunt resistor. This set-up involves a parallel connection of the shunt resistor with the coil of an analog galvanometer, which is a type of an ammeter. This configuration is common in motor vehicles. The concept behind the application is that only a small portion of the measured current flows through the galvanometer while the rest passes through the shunt resistor. The low-resistance property of shunt resistors enables large currents to be measured without damaging the galvanometer.