5.3 Kelvin Connections
In testing many transformer parameters, such as winding resistance or inductance, it is necessary to measure electrical impedance.
The normal method of measuring impedance is to pass a test current through the unknown component, and to measure the resulting voltage produced across it. Dividing the voltage by the current gives the required value of impedance.
In making such measurements, great care must be taken not to include the impedance of the measuring leads in the result. A connection system that avoids such problems is shown below; it uses four wires and is often referred to as a Kelvin connection.
In this arrangement, the test current passes through the two ‘power’ leads, and the voltage is measured using the two ‘sense’ leads.
Provided that the sense leads are connected as close as possible to the body of the device under test, any additional voltage drop produced by the test current flowing through the impedance of the power leads is not measured.
The Kelvin connection therefore provides the most accurate means of sensing the voltage, and hence the impedance of the winding.
Ideally, all impedance measurements would be made using Kelvin connections. However, many terminals do not permit the use of four wires all the way to the body of the component under test.
In such cases, separate power and sense leads are used up to the base of the terminal, and the length of ‘common’ lead (from the junction of the power and sense leads, through the terminal and the component lead to the body of the component) should be kept to a minimum.
The ‘common’ lead length is shown as AA’ and BB’ in the diagram below.
The AT series testers provide all the connections required to take advantage of Kelvin measurements.
A test node may be considered to be a pair of connections consisting of one power and one sense terminal.
The following diagram indicates how to identify the node terminal function;
POWER terminals are all on the outside of the fixture area.
SENSE terminals are all on the inside of the fixture area.
IMPORTANT: Kelvin four terminal connections are generally advised to be used in all test fixtures; they are essential for transformers whose winding resistance’s are less then 1Ω.