Banana plugs|Simple and accurate resistance measurement with a multimeter

Banana plugs|Simple and accurate resistance measurement with a multimeter


Measuring resistance with two wires is very convenient, but introduces measurement errors. This error can be almost eliminated by using a multimeter with 4 wires and separate source and measure terminals. Unfortunately, adding extra leads and connections increases the complexity of the measurement. You'll need to connect additional leads, and may have to replace clips and probes when going from voltage to resistance. Now, there is a new concept that enables you to make 4-wire resistance measurements with just two leads.
Why use 4 wires to measure resistance?
Managing two leads is challenging enough, especially when measuring small components in tight spaces. If you want to inspect small solder joints, soft connectors or chip resistors with four leads, then you have a real challenge.
Switching lead configurations may require changing banana plugs, resulting in incorrect measurements. It also takes time to change from voltage probes to Kelvin leads and back again. Why use 4 wires to measure resistance?
Using two wires to measure voltage has little effect on measurement accuracy. The typical input impedance of a voltage input on a multimeter is 10 megohms, so the very small current flow in the leads and the resulting drop in lead voltage are negligible. The effect of series lead resistance on the current measurement is also not significant. Unfortunately, in resistance measurements, lead resistance can cause a significant drop in accuracy.
When measuring resistance, the multimeter switches the current source into the measurement loop. A current is driven through an unknown resistance and the voltage measured by the multimeter drops. If there are only two leads, as shown in Figure 1, the source current takes the same path as the measured voltage drop. The measurement lead is not a perfect conductor and has its own series resistance. By driving current through the measurement leads, you can see not only the voltage drop in the unknown resistance, but the voltage drop across each lead. Therefore, you are measuring the combined resistance of the positive lead, the unknown resistance, and the negative lead.
If 4 leads are used, as shown in Figure 2, the source current measurement and voltage measurement can be separated. The meter terminals are called "Source" for current source and "Sense" for voltage input.
Series resistance in the source lead does not affect current flow. There is little current flow in the measurement/sensing leads because of the high input impedance of the meter. This means that there is no I x R voltage drop in the measurement leads. So you just measure the voltage drop across the unknown resistor due to the source current flowing through it.