Four-terminal sensing

(Redirected from Kelvin sensing)
Circuitry for Four-terminal sensing

Four-terminal sensing (4T sensing), 4-wire sensing, or 4-point probes method is an electrical impedance measuring technique that uses separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. 4T sensing is used in some ohmmeters and impedance analyzers, and in precision wiring configurations for strain gauges and resistance thermometers. 4-point probes are also used to measure sheet resistance of thin films.

The key advantage of four-terminal sensing is that the separation of current and voltage electrodes eliminates the impedance contribution of the wiring and contact resistances.

Four-terminal sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. Each two-wire connection can be called a Kelvin connection. A pair of contacts that is designed to connect a force-and-sense pair to a single terminal or lead simultaneously is called a Kelvin contact. A clip, often a crocodile clip, that connects a force-and-sense pair when it closes or slides onto a conductor is called a Kelvin clip.

Operating principle

Four-point measurement of resistance between voltage sense connections 2 and 3. Current is supplied via force connections 1 and 4.

When a Kelvin connection is used, current is supplied via a pair of force connections (current leads). These generate a voltage drop across the impedance to be measured according to Ohm's law V=RI. This current also generates a voltage drop across the force wires themselves. To avoid including that in the measurement, a pair of sense connections (voltage leads) are made immediately adjacent to the target impedance. The accuracy of the technique comes from the fact that almost no current flows in the sense wires, so the voltage drop V=RI is extremely low.

It is conventional to arrange the sense wires as the inside pair, while the force wires are the outside pair. If the force and sense connections are exchanged, accuracy can be affected, because more of the lead resistance is included in the measurement. In some arrangements, the force wires are very large, compared to the sense wires which can be very small. If force and sense wires are exchanged at the instrument end, the sense wire could burn up from carrying the force current.

The technique is commonly used in low-voltage power supplies, where it is called remote sensing, to measure the voltage delivered to the load independent of the voltage drop in the supply wires.

It is also common to provide 4-wire connections to low-value, high-power resistors. They are often known as "current-sensing shunts" rather than simple resistors in this case.

3-wire sensing

A variant uses three wires, with separate force and sense leads at one end, and a common wire on the other. Compensation for the voltage drop in the common wire is done by assuming that it is the same as in the force wire. This style is popular with resistance thermometers, also known as resistance temperature detectors or RTDs.

Another example is in the ATX power supply standard, which includes a remote sense wire connected to the 3.3V supply line at connector pin 13, but no sense connection for the ground wires.