Overview of Kelvin Connections
Outlines of methods and techniques to obtain high accuracy readings of low resistances
Updated Jul 2024
Kelvin Connections and Test Measurements
Kelvin connections, also known as four-terminal sensing or Kelvin sensing, are a method used to measure electrical resistance with high precision, particularly in low-resistance scenarios.
This common technique is essential in minimising the errors associated with contact and lead resistances in measurement setups.
Here’s an overview of the principle, application, and advantages of Kelvin connections.
To measure the resistance of a component, a test current is forced through the component via a set of test leads.
The meter then measures the voltage across its terminals to give the component's resistance value.
1. Two-wire Connections
However, in any real-world measurement, the value of resistance is subject to the resistance of the test leads and the contact resistance of any connections used.
The lead resistance and contact resistance causes a small volt drop, which can usually be considered negligible if the UUT resistance is much higher than these “error” resistances.
The problem with the two-wire method is that, when measuring small values of resistance, typically 1Ω or less, the resistance of the test leads causes a relatively significant voltage drop in addition to the volt drop across the component (see right).
The voltage measured by the meter will therefore not be the true value of the voltage across the component that you are trying to measure.
Consider the equivalent circuit measuring a resistor R(uut)
- The source is set at a constant 1 Amp
- Assume the combined contact and lead resistance is 0.1 ohms
- When measuring R(uut) = 0.1 ohm resistor
V=IR, so
The UUT has a voltage drop of 0.1 Volts
The contact resistance + lead resistance has a voltage drop of 0.1 Volts
The DMM Voltmeter sees this combined as a 0.2 Volt drop.
- The DMM calculates the Resistance from R=V/I = 0.2 OHMS!
I.e., twice the actual value we are trying to measure.
Repeating the same, but when measuring a 10 ohm resistor
V=IR, so
The UUT has a voltage drop of 10 Volts
The contact resistance + lead resistance has a voltage drop of 0.1 Volts
The DMM Voltmeter sees this combined as a 10.1 Volt drop.
- The DMM calculates the Resistance from R=V/I = 10.1 OHMS!
I.e., 1% above the actual value we are trying to measure.
As the resistance of UUT you are trying to measure increases in its real value, the contact and lead losses become less significant, but they are always there.
2. Four-wire Connections
Given the limitations of the two-wire method, the four-wire (Kelvin) method is generally preferred for low-resistance measurements.
These measurements can be made using a separate current source and voltmeter.
With this configuration, the test current is forced through the test resistance via one set of test leads (power leads), while the voltage across the component under test is measured through a second set of leads (sense leads).
The current is the same in all of the current path, even if there is some voltage drop caused by the wire and contact resistances.
Although some small current may flow through the sense pair, it is usually negligible (pA or less) because the impedance of the voltage measuring device used is very high.
As the current is near zero in the sense lead path, the wire resistance and contact resistance cause no voltage drop in the voltage measuring path.
The voltage drop measured by the volt meter is therefore essentially the same as the voltage across the test resistance.
Consequently, the resistance value can be determined much more accurately than with the two-wire method.
Now consider this new circuit for a 4 wire Kelvin circuit measuring a resistor R(uut)
- The source is set at a constant 1 Amp
- Assume the combined contact and lead resistance is still 0.1 ohms
- When measuring R(uut) = 0.1 ohm resistor
V=IR, so
The UUT has a voltage drop of 0.1 Volts
The contact resistance + lead resistance has no voltage drop as no current flows in this sense path.
The DMM Voltmeter sees only the voltage drop across R(uut) as a 0.1 Volt drop.
- The DMM calculates the Resistance from R=V/I = 0.1 OHMS.
This time there is virtually no error due to the lead and contact resistance.
3.Semi-Kelvin connections.
Many test and equipment manufacturers use connections to the device under test which are not in fact 'true' Kelvin, but 'semi' Kelvin.
This can best be illustrated here where 'spring' probes are used.
It can be seen that the spring probe does not provide a true Kelvin connection, as the four wires are terminated at the probe receptacle, not at the point of contact to the UUT..
This will remove the effect of the wire resistance, but not remove any contact resistance.
If the contact resistance is low enough this may be an acceptable compromise.
Additional factors such as physical placement, pin separation and topology may make a semi kelvin solution acceptable for use.
In order to be 'true' Kelvin, each 'power' and 'sense' lead needs to make the connection directly to the test component lead and as close as possible to the test component itself.
4. True Kelvin Connections
As previously explained, true Kelvin provides the most ideal connection method when measuring resistances <1Ω.
However, when designing a test fixture, the mechanical aspect of the connection method must be considered.
In this case, spring probes may provide an alternative to Kelvin blades.
However, the current through the component under test must then also pass through the spring probe itself, introducing an additional, undesirable voltage drop.
Fixtures made using spring probes have the advantage of being easier to construct, easier to maintain, and they have a longer lifespan than Kelvin blades, which are subject to wear from the action of inserting and removing the test component.
However, because spring probes can only offer semi-Kelvin connections, they should not be used when measuring a resistance of less than 1Ω.
5 LCR meters / AT tester and Compensation.
Most LCR meters (and the Voltech AT testers) allow you to perform Short and Open compensation to further remove the effect of leads on a measurement.It would appear at first that such compensations would remove the effect of Lead and contact resistance for you.
However, it is important to realise that contact resistance can vary widely between EACH test unit connection.
This would in reality, be different and unrepeatable, and of the order of 20 mohms to 150 mohms between each separate fit of a component even with seemingly “good” contacts
4 Wire / Semi Kelvin
The problem with any fixed “one time” Short compensation is that it will only remove the contact/lead resistances seen at time of compensation. As this will change with every subsequent UUT insertion it will only ever remove a fixed offset from your real measurements.
4 wire / True Kelvin
As explained above, the contact resistance in the Sense line changes on each insertion
In True Kelvin, however, the zero current flow in the Sense path means that,regardless of the instantaneous level of constant resistance, the associated contact resistance voltage drop, will never be seen by the high impedance voltmeter.
6 Conclusion
Advantages of Kelvin Connections
High Accuracy: By eliminating the influence of lead and contact resistances, Kelvin connections provide highly accurate resistance measurements.
Low Resistance Measurement: Essential for applications requiring the measurement of very low resistances, which is difficult with two-wire methods.
Repeatability: Ensures consistent and repeatable measurements, which is critical in quality control and manufacturing processes.
In conclusion, Kelvin connections are an invaluable technique in precision electrical measurement, particularly for low-resistance applications. By using four terminals to separate the current supply from the voltage measurement, they eliminate errors due to lead and contact resistances, thus providing more accurate and reliable results. This method is indispensable in fields requiring high-precision measurements and continues to be a fundamental practice in electrical engineering.