Four-terminal sensing
Encyclopedia
Four-terminal 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 ohmmeter
s and impedance analyzers, and in precision wiring configurations for strain gauge
s and resistance thermometer
s. 4-point probes are also used to measure sheet resistance
of thin film
s.
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.
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.
s, 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.
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
measuring technique that uses separate pairs of current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
-carrying and voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
-sensing electrodes to make more accurate measurements than traditional two-terminal (2T) sensing. 4T sensing is used in some ohmmeter
Ohmmeter
An ohmmeter is an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters make low resistance measurements. Megohmmeters measure large values of resistance...
s and impedance analyzers, and in precision wiring configurations for strain gauge
Strain gauge
A strain gauge is a device used to measure the strain of an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable...
s and resistance thermometer
Resistance thermometer
Resistance thermometers, also called resistance temperature detectors or resistive thermal devices , are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass...
s. 4-point probes are also used to measure sheet resistance
Sheet resistance
Sheet resistance is a measure of resistance of thin films that are namely uniform in thickness. It is commonly used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing, and glass coating. Examples of these processes are: doped semiconductor regions...
of thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
s.
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
Contact resistance
The term contact resistance refers to the contribution to the total resistance of a material which comes from the electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method...
.
Four-terminal sensing is also known as Kelvin sensing, after William Thomson, Lord Kelvin
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...
, who invented the Kelvin bridge
Kelvin bridge
A Kelvin bridge is a measuring instrument invented by William Thomson, 1st Baron Kelvin. It is used to measure an unknown electrical resistance below 1 Ω. Its operation is similar to the Wheatstone bridge except for the presence of additional resistors...
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
Crocodile clip
thumb|Standard clipthumb|Pair of standard clipsA crocodile clip is a simple mechanical device for creating a temporary electrical connector, and is named for its resemblance to an alligator's or crocodile's jaws...
, that connects a force-and-sense pair when it closes or slides onto a conductor is called a Kelvin clip.
Operating principle
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
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 thermometerResistance thermometer
Resistance thermometers, also called resistance temperature detectors or resistive thermal devices , are sensors used to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass...
s, 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.