How to use a multimeter to measure resistance simply and accurately

Measuring resistance with two wires is very convenient, but it can introduce measurement errors. This error can be nearly eliminated by using a multimeter with 4 wires and separate source and measurement terminals. Unfortunately, adding extra leads and connections complicates the measurement. You need to connect the additional leads, and may have to change clips and probes when changing from voltage to resistance. Now, there is a new concept that enables you to make 4-wire resistance measurements using only two leads.

Why use 4 leads to measure resistance?

Managing two leads is challenging enough, especially when measuring small components in tight spaces. If you want to use four leads to check small solder joints, soft connectors or chip resistors, you are facing a real challenge.

Switching lead configurations may require changing banana plugs, leading to measurement errors. Changing from a voltage probe to Kelvin leads and back again also takes time. Why use 4 leads to measure resistance?

Measuring voltage with two leads has little effect on measurement accuracy. The typical input impedance of the voltage input on a multimeter is 10 megohms, so very small current flows in the leads and the resulting voltage drop in the leads are negligible. The effect of series lead resistance on current measurements is also not noticeable. Unfortunately, in resistance measurements, lead resistance can cause a significant loss in accuracy.

When measuring resistance, the multimeter switches a current source into the measurement loop. Current is driven through the unknown resistor and the multimeter measures the resulting voltage drop. If there are only two leads, as shown in Figure 1, the source current follows the same path as the measured voltage drop. The measurement leads are not perfect conductors and have some series resistance of their own. By driving current through the measurement leads, you can see not only the voltage drop in the unknown resistor, but also the voltage drop in each lead. Therefore, you are measuring the combined resistance of the positive lead, the unknown resistor, and the negative lead.

If four leads are used, the source current measurement and voltage measurement can be separated as shown in Figure 2. The meter terminals are called “Source” for current sourcing and “Sense” for voltage input.

The series resistor in the source leads has no effect on current flow. The measure/sense leads have almost no current flow because the input impedance of the meter is so high. This means there is no I x R voltage drop in the measure leads. So you are only measuring the voltage drop in the unknown resistor due to the source current flowing through it.

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Introducing 2x4-wire resistance measurement

The patented split-terminal sockets on the Tektronix DMM series maintain the convenience of measuring resistance using two wires while providing the measurement performance of a 4-wire measurement method. The sockets are fully compatible with standard 4 mm banana plugs. But internally, each socket is split into two contacts: one source contact and one measure contact. Specially designed test leads have two conductors per line, also one source lead and one measure lead. The leads align with the contacts inside the socket, delivering separate source and measurement signals over the entire length of the lead.

At the far end of the leads, clips and probes that keep the source and measurement signals separate can provide 4-wire performance right to the component under test. Tektronix offers a range of fixtures and probes that can provide 4 wires right to the connection point, including:
 Test probes
 Alligator (Kelvin) clips
 Tweezers

All of these accessories can be used to measure 4-wire resistance or voltage.

The 2x4-wire technology available on this series of DMMs simplifies the task of accurately measuring resistance without having to change cable configurations or deal with a multitude of cables.

Which multimeter is right for you?

The DMM series offers a variety of models to suit your needs and budget. The following multimeters offer 2x4-wire resistance measurement capabilities.