Kelvin Connections and Test Measurements
Making low-resistance measurements, under 1Ω, is subject
to sources of error including lead resistance and contact resistance. This
tech note describes these problems and how they are overcome.
1, Two-wire 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. This is known as a two-wire measurement.
In a two-wire measurement, the value of resistance is subject to the
resistance of the test leads. The lead resistance causes a small volt
drop, which can usually be considered negligible.
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 volt drop in addition to
the volt drop across the component (Figure 1).
The voltage measured by the meter will therefore not be the true value of
the voltage across the component.
2, Four-wire Connections
Given the limitations of the two-wire method, the
four-wire (Kelvin) method is generally preferred for low-resistance
These measurements can be made using a separate current source and
voltmeter (Figure 2).
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).
Although some small current may flow through the sense pair, it is usually
negligible (pA or less) because the impedance of the sense terminals is
The volt drop measured by the 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.
3, Degrees of Kelvin
Many test and equipment manufacturers use connections to
the device under test which are not in fact 'true' Kelvin, but 'semi'
This can best be illustrated in Figure 3a, 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.
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, Semi Kelvin Versus True Kelvin
When using a fixture, the fastest method of connecting
and disconnecting the component under test, while still maintaining a
true, four-wire Kelvin connection to the component, is to use Kelvin
Kelvin blades consist of two spring blades held in an insulating body
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
Fixtures made using spring probes have the advantage of
being easier to construct, easier to maintain, and they have a longer life
span 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 connection, they
should not be used when measuring a resistance of less than 1Ω.