Shunt resistor

A shunt resistor is a resistor used to establish a known relation between current and a measured electric potential difference. For current sensing, it is placed in the measured current path, and Ohm's law givesThe voltage-sensing circuit should draw negligible extra current for this simple relation to describe the desired branch current. Its small series voltage drop is often called the burden voltage.

An original example uses $R_s=10\mathrm{m\Omega }$ and $I=5\mathrm{A}$. The sensed drop is50millivolts and Joule heating dissipatesReducing resistance lowers both the burden and dissipation, but also reduces signal relative to a fixed amplifier offset or readout noise. The nominal resistor value alone therefore does not determine measurement quality.

Lead and contact resistance can dominate a low-value shunt. If a10milliohm shunt is inadvertently measured together with2milliohms of current-carrying interconnect, interpreting the total voltage using only the nominal shunt value overestimates current by20percent. Four-terminal sensing separates the high-current connections from the voltage-sense connections. Analog Devices' LTC6115 data sheet illustrates these Kelvin sense paths and explains how interconnect resistance introduces error.^[ref]

A practical characterization drives known currents through the shunt while measuring the voltage at its sense terminals, then repeats after thermal settling. Temperature coefficient of resistance, self-heating and connection geometry affect the realized conversion. The amplifier's operating range and measurement calibration must be assessed alongside the resistor.

A shunt converts current into a local voltage signal; it does not inherently provide electrical isolation between the measured circuit and its readout. High-side current sensing and low-side current sensing describe alternative placements of that measurement relative to the load and reference conductor.

Burden voltage is the electric potential difference introduced between the terminals of an ammeter when an electric current flows through it. A shunt resistor makes a measurable signal by deliberately developing this drop, but the drop can also change the electric current being measured. In a real ammeter, leads, connections and a protection fuse can contribute as well as the shunt resistor.^[ref]

Consider an ideal1volt source driving a100ohm resistor. Before inserting an ammeter, Ohm's law gives10milliamperes. If the inserted ammeter has10ohms of total series electrical resistance,The ammeter could report its own current perfectly and still measure a current9.09percent below the undisturbed value. This is a change in the measured electrical circuit, rather than a display-rounding error.

Reducing the shunt resistor's electrical resistance reduces burden voltage but also reduces the signal presented to an amplifier. A feedback ammeter instead uses a transimpedance amplifier to keep its input near a reference potential. National Instruments illustrates the distinction with a low-voltage leakage-current measurement, where the burden voltage consumes a substantial fraction of the applied test electric potential difference.^[ref]

An experimental check uses several sensing resistors and records both the inferred electric current and the load's terminal electric potential difference. Dependence on the inserted electrical resistance reveals loading. Four-terminal sensing reduces errors from connection drops in the sensed signal; it does not remove the physical burden voltage across the shunt resistor.