4-Wire Kelvin Testing

If you have used an ohmmeter to make resistance measurements, you may have heard of the terms “2-wire measurement” and “4-wire Kelvin measurement.” This article explains how ohmmeters measure resistance, the differences between 2-wire and 4-wire resistance measurements, and the special considerations for each.

How do ohmmeters work?

When using an ohmmeter to measure the resistance of a wire, the red meter lead is touched to each end of the wire and the black lead to the other end (figure 1). How does the meter measure resistance? What resistance is it really measuring? To understand how ohmmeters work, start with Ohm’s law:

Resistance = Voltage / Current (R = V/I)

To perform the measurement, the ohmmeter forces a known current through the wire, measures the voltage drop that develops, calculates the resistance using Ohm’s law, and displays the result. To do this, the ohmmeter must have a current source and voltmeter (see figure 1). Of key importance, is the point at which the current source and voltmeter connect.

Figure 1. Meters contain a current source (I) and a voltmeter (Vm).

Figure 2. A 2-wire measurement really measures the DUT resistance plus the meter lead resistance.

2-wire measurements

When making a 2-wire resistance measurement, the meter uses only two leads to connect to the device-under-test (DUT) as shown in Figure 2. This setup has the advantage of using just two wires to connect to the DUT, but the result is that the voltmeter measures the voltage drop across the DUT and the test leads.

Figure 3. A 4-wire measurement. Notice the meter has four connections.

Figure 4. A 4-wire measurement gives you control of where the voltmeter connects.

You may not need 4-wire with a Cirris tester

Many continuity testers require 4-wire testing to accurately measure resistances under 1 ohm. However, Cirris testers can accurately measure resistance down to 0.1 ohm. One reason Cirris testers excel at this capability is because even during 2-wire measurements, the testers use the 4-wire measurement principle internally to eliminate extraneous internal resistance from measurements providing a consistent, near-zero resistance at the face of the test points connectors. This configuration is invisible to the user and the benefit it provides requires no action of the part of the user.

When determining whether to use 2-wire or 4-wire measurements in an application, don’t forget to consider all the tolerances and variances that are factors in resistances measurement (see Setting Practical Resistance Specs for more informant on the topic). Connecting the DUT directly to adapters on Signature series testers eliminates much of the fixturing resistance and variances that occur when using adapting cables.

What you need to know before you can build a 4-wire test fixture

Proper “force” and “sense” points must be combined into 4-wire pairs. Not just any 2-points can be paired to create 4-wire fixturing. There are simple rules that tell you what points need to be paired. Don’t build it wrong! Check the 4-wire instructions for your tester in the corresponding user manual.

The loop resistance (the resistance from the tester through a source/sense 4-wire pair) must be less than 10 ohms.

Which Cirris Tester should I use?