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What is 2-wire and 4-wire?
If you've used an ohmmeter to make resistance measurements you've probably heard terms such as "2-wire measurement" and "4-wire kelvin measurement." This document explains how ohmmeters measure resistance, how 2-wire resistance measurements work, how 4-wire resistance measurements work, and the special considerations for each measurement type.
How do ohmmeters work?
When you use your ohmmeter to measure the resistance of a wire you touch one meter lead to each end of the wire and you get a resistance measurment (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. This equation says "Put a current through the wire, measure the voltage drop along the wire, and you can calculate the resistance of the wire."
Figure 1. A 2-wire resistance measurement.
Your ohmmeter forces a current through the wire, measures the voltage that develops, calculates the resistance, and displays the result. To do all this your ohmmeter must have a current source and volt meter (see figure 2). What's important is where the current source and volt meter get connected together.
Figure 2. Meters contain a current source (I) and a volt meter (Vm).
When you make a 2-wire resistance measurement your meter uses only two leads to connect to the device under test (DUT). Figure 1 shows a normal 2-wire test set up. This setup has the advantage of using just two wires to connect to the DUT but what is the actual resistance it's measuring? To measure just the resistance of the DUT you would want to measure just the voltage across the DUT. Figure 3 shows that the voltmeter is really measuring the voltage across the DUT and the test leads.
Figure 3. A 2-wire measurement really measures the DUT resistance plus the meter lead resistance.
Two-wire measurements actually measure the DUT resistance plus the test lead resistance. What should you do when you really want to measure only the DUT resistance?
Some ohmmeters have four connections; two come from the current source (sometimes called the "force" leads), and two come from the voltmeter (usually called the "sense" leads). With an ohmmeter like this you can do a 4-wire measurement as shown in figure 4. With four connections you choose where to connect the voltmeter so you are in control over exactly what resistance you want to measure (see figure 5). If you connect the meter directly to your DUT you will measure just the resistance of the DUT.
Figure 4. A 4-wire measurement. Notice the meter has four connections.
Figure 5. A 4-wire measurement gives you control of where the volt meter connects.
The disadvantage of 4-wire testing is it takes four connections to do the test but it does give you an accurate resistance measurement of the DUT without the resistance of the test leads.
Resistance measurements in your cable tester
Your cable tester basically contains a high speed ohmmeter, with a current source and a voltmeter. Normally you do 2-wire measurements; you use two test points per measurement. More advanced testers allow you to also make 4-wire measurements; using four test points per measurement. To make a 4-wire measurement on your tester you generally need to create custom 4-wire test fixturing that combines the force and sense lines near your DUT, canceling the fixturing resistance.
You may not need 4-wire with a Cirris tester
Many continuity testers require 4-wire testing to accurately measure resistances under 1 ohm. The Cirris easy-wire CR, Signature CH+, 1100H+/R+, 1000H+/R+ and Touch1 testers use internal four-wire connections to reduce the fixturing (lead) resistance of the tester. All Cirris testers that measure resistance have this feature. Also, adapters that plug directly into Signature series testers eliminate much of the fixturing resistance that often occurs with adapting cables. If you need your resistance measurement to be accurate to only 0.1 ohms you won't need to use 4-wire on your Cirris tester.
Why not just subtract fixturing resistance?
Fixturing resistance is sometimes referred to as a "tare value" that could be removed to meet a specification for maximum resistance in the DUT. While the tare value can be used to adjust your measurements, it's not as simple as it first appears. First the accuracy of the tester is reduced by the ratio of fixturing to DUT resistance. This means that a .1 ohm DUT measurement with 2 ohm of fixturing and a 2% tester accuracy has (2 + .1) ohm x 2% = 0.042 ohms of variation or 42% measurement error (adjusted measurement error = Tester measurement error X (fixture resistance + DUT actual resistance) / DUT actual resistance). In this example the threshold for a good cable would need to be adjusted to .1 x (100%- 42%) = 0.062 ohms.
There is a more serious danger if you "tare out" the fixture resistance. Try measuring the resistance of a piece of wire with your VOM. You will find that the resistance varies depending on how hard you hold your lest leads to the wire ends. This variation in resistance comes from the point of contact between the DUT and the fixturing. This resistance variation from measurement to measurement can add significantly to a learned resistance and will get worse as the mating connectors wear. The effect of this variation could be that resistance thresholds are set too high and defective cables are allowed to pass.
What does 4-wire testing buy you?
What 4-wire testing will cost you
What you need to know before you can build a 4-wire test fixture
Which Cirris tester should I use?
Important "Fixture Setup" Information
Signature 1000H+: All test points must be wired as 4-wire points. You must use the Signature AV4W-64 adapter. This adapter has 2 VME connectors on it. You must pair A1 with C1, A2 with C2 and so on, for each 4 wire connection on the adapter. As you build your fixture keep in mind that you can not skip over any points in the adapter. The test points used must be contiguous.
Signature 1100H+, 1100R+, and Touch 1 Testers: These testers are more flexible in the 4 wire mode than the 1000H+. These three testers let you use Signature Adapters in any combination you choose and you can mix 2 wire and 4 wire points in the same assembly. This means you can use 4-wire test points only where you wish to.
Keep in mind that you still must 'pair points' using one type "A" and one type "B" test point for each 4-wire pair. You can be sure of using one of each type point for each 4-wire pair by:
easy-wire CR Tester: You can mix 2 wire and 4 wire points in the same assembly on this tester also. Presently, 4-wire measurements cannot be made using "easy-wire adapters" that plug into the easy-wire transition boards.
Signature CH+: You can also mix 2 wire and 4 wire points in the same assembly with this model tester. Pair point A1 with Point B1, point A2 with B2 and so on.
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