If test devices are used to test products or calibrate systems, the change in resistance must often be determined by a low-resistance sensor many meters away. However, simply applying a known voltage across the thin-wire sensor cables, measuring the current and calculating the sensor resistance using Ohm's law (resistance = voltage/current (R = U/I)) does not work in this case. The nominal resistance and temperature coefficient of the sensor connections can distort the result considerably and cannot be calibrated out. However, the problem can be avoided.

Four-wire or Kelvin measurement as a solution

The falsification of the resistance measurement by the sensor leads can be prevented with a four-wire/Kelvin arrangement.

© Texas Components Corporation

The four-wire or Kelvin measurement is a widely used method for eliminating line-related errors when measuring the sensor resistance. For this purpose, the sensor is connected to a current source with a known current via the existing cable connections. Two additional conductors are then connected to the sensor to determine the voltage across the sensor using a high-impedance voltmeter or isolation amplifier. The known current and the measured voltage are inserted into Ohm's law - and that's it. With this four-wire arrangement and two additional simple voltage measurement wires, it is possible to avoid the resistance of the sensor connection wires being included in the equation.

Prevent self-heating of the sensor

In order to minimize the self-heating of the sensor and possible associated errors, the current should be as low as possible, but a sufficiently high voltage should be applied to the sensor. In most cases, the highest permissible voltage at the sensor is between 1 and 10 V.

A variant of the Kelvin circuit is used to measure the current through a load, whereby a measuring resistor is connected between the load and earth (low-side) or the source and load (high-side).

© Digi-Key Electronics

A variant of this technique is often used to measure the current of a load, for example a motor, where a 'low value' measuring resistor is connected in series with the load. A known resistor is used to measure load currents, usually in the order of 1 Ω or less, often in the mΩ range. Here, the voltage is measured using a differential isolation amplifier - which may need to be galvanically isolated from the rest of the circuit - and Ohm's law is applied again - this time in the version I = U/R.

Support components

During product testing or system calibration, the resistance value of a sensor often needs to be determined, so the tester needs to be aware of how the sensor's connecting leads can distort the measurements. This problem can be avoided with the four-wire/Kelvin method.

Chip resistor WSK1206R0100FEA with 0.01 Ω and 0.25 W in 3216 format for the connection of four conductors in current measurement applications

© Vishay

Sometimes the challenge is simply to be able to connect all four conductors at all. This problem can be solved with special components, such as the Vishay WSK1206R0100FEA Dale chip resistor. The component in 3216 size with 0.01 Ω and 0.25 W is designed for the connection of four conductors in current measurement applications.

The author

Bill Schweber, Digi-Key

© Digi-Key

Bill Schweber is an electrical engineer who has written three textbooks on electronic communication systems and hundreds of technical articles, commentaries and product descriptions.

Further information

Digi-Key: Fundamentals of Current Measurement: Part 3 - Funnel Amplifiers

Digi-Key: Reap the Benefits of RTD Temperature Sensors Without the Interface Complexity.

Digi-Key: Makers and Engineers: Get to Know Your Instrumentation Amplifier for Accurate IoT Data Capture