Four-terminal Sensing - Operating Principle

Operating Principle

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.