It is convenient to measure the resistance using two wires, but it will cause measurement errors. This error can be almost eliminated by using a four-wire multimeter separated from the source terminal and the measurement terminal. Unfortunately, adding extra leads and connections increases the complexity of the measurement. You will need to connect the added leads and you may have to change the clips and probes when changing from voltage to resistance. Now, there is a new concept that allows you to make 4-wire resistance measurements with just two leads.
Figure 1 (left). The 2-wire resistance measurement produces an error due to the drop in the test lead voltage.
Figure 2 (right). The 4-wire resistance measurement eliminates the current in the voltage lead and eliminates this source of error.
Why use 4 wires to measure resistance?
The challenge of managing two leads is large enough, especially when measuring small components in tight spaces. If you want to use four leads to inspect small solder joints, soft connectors or chip resistors, you will face real challenges.
Switching the lead configuration may require replacement of the banana plug, resulting in measurement errors. It takes time to switch from a voltage probe to a Kelvin lead and then back. Why use 4 wires to measure the resistance?
Measuring voltage with two lines has little effect on measurement accuracy. The typical input impedance of the voltage input on the 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 current measurement is also not significant. Unfortunately, in resistance measurements, lead resistance can cause significant degradation in accuracy.
When measuring resistance, the multimeter switches the current source to the measurement loop. The current is driven through an unknown resistor and the voltage measured by the multimeter drops. If there are only two leads, as shown in Figure 1, the source current flows through the same path as the measured voltage drops. Measuring leads are not perfect conductors, they have a certain series resistance. By driving the current through the measurement leads, you can not only see the voltage drop in the unknown resistor, but also the voltage drop of each lead. Therefore, you measured the combined resistance of the positive, unknown, and negative leads.
If four leads are used, as shown in Figure 2, the source current measurement and the voltage measurement can be separated. The meter terminal is called the “source” (“Source”) for the current source and the “sensing terminal” (“Sense”) for the voltage input.
The series resistance in the source lead does not affect the current flow. There is almost no current flow in the measurement/sense leads because the input impedance of the meter is high. This means that there is no I x R voltage drop in the measurement leads. Therefore, you only need to measure the voltage drop in the unknown resistor due to the source current flowing through it.
Figure 3. The 2x4-wire resistor technology uses dedicated split banana jacks and leads to provide 4-wire performance using two wires.
Introducing 2x4 line resistance measurement
The patented split terminal socket on the Tektronix DMM series maintains the convenience of using two wires to measure resistance while providing the measurement performance of a 4-wire measurement method. The socket is fully compatible with standard 4 mm banana plugs. But internally, each socket is divided into two contacts: one source contact and one measuring contact. The specially designed test leads have two wires per line, which are also one source wire and one measuring wire. The leads are aligned with the contacts inside the socket to deliver separate source and measurement signals over the entire length of the leads.
At the distal end of the lead, clips and probes that keep the source signal separate from the measurement signal can provide 4-wire performance up to the component under test. Tektronix offers a range of fixtures and probes that provide up to 4 wires up to the connection point, including:
* Test probe
* Crocodile (Kelvin) clip
All of these accessories can be used to measure 4-wire resistance or voltage.
The 2x4 line technology available on the DMM series simplifies the task of accurately measuring resistance without having to change the cable configuration or handle a large number of cables.