KelvinConnections

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FOUR-WIRE KELVIN CONNECTIONS

Making low-resistance measurements, under 1ΩΩ,, is subject to sources

of error including lead resistance and contact resistance. This tech

note describes these problems and how they are overcome.

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 resis-

tance 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 compo-

nent (Figure 1). The voltage measured by the meter will therefore not be the

true value of the voltage across the component.

Figure 1. Two-wire test connection showing leadresistance affecting measurement accuracy

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2© 2002 Voltech Instruments. All rights reserved.

Four-wire Connections

Given the limitations of the two-wire method, the four-wire (Kelvin) method is gener-

ally preferred for low-resistance measurements. 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 high. 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.

Degrees of Kelvin

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 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.

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Figure 2. Diagram showing 4-wire Kelvin connection

3© 2002 Voltech Instruments. All rights reserved.

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4

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.

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 blades. Kelvin blades consist of two spring

blades held in an insulating body (Figure 3b).

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.

Figure 3a.. Semi Kelvin connection Figure 3b. True Kelvin connection

© 2002 Voltech Instruments. All rights reserved.

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 connec-

tion, they should not be used when measuring a resistance of less than 1Ω.

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5© 2002 Voltech Instruments. All rights reserved.

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Note: While every care has been taken in compiling the information for this publication, Voltech Instruments cannot accept legal liabil-ity for any inaccuracies. Voltech Instruments reserves the right to alter product specifications without notice and whenever necessaryto ensure optimum performance from its product range.

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