Frequently Asked Questions about Resistance Measurements Using a Bench Multimeter

The benchtop multimeter is a commonly used resistance measuring instrument, but in actual applications, do you use the multimeter to measure resistance correctly? Below, Agitek will show you some common situations.

Q1. How to choose 2-wire or 4-wire?

Many multimeters include 2-wire and 4-wire measurement methods. How do you determine which method to use for a specific measurement?

The 2-wire method should generally only be used when measuring resistance values ​​above 10 ohms, since the effects of lead resistance can add significant error (1% or more).

Figure｜2-wire and 4-wire schematics

The 4-wire (also known as Kelvin) connection method provides a more accurate low resistance measurement solution by automatically eliminating the effects of lead resistance. In this configuration, the test current (I) is forced through the test resistor (R) via one set of test leads, and the voltage (VM) across the DUT is measured via the first set of wires (the sense leads).

Q2. What is bias compensation?

Offset compensation is a technique used to eliminate thermal voltage errors when measuring low resistances (typically less than 10Ω). It measures resistance by sourcing current, measuring the resulting voltage, and then using Ohm’s law to calculate the unknown resistance.

Figure｜Bias compensation mode

In the bias compensation technique, the source current is alternately turned on and off, and the voltage is measured during both parts of the cycle. When the source current is on, the measured voltage is the thermal error voltage and the voltage across the device under test, as shown in part b of this figure. When the source current is off, no current flows through the device under test, so the measured voltage is only the thermal error voltage, as shown in part b.

Therefore, the voltage measured in part c can be subtracted from the voltage measured in part b. By using this voltage VM and the source voltage during the on-part of the cycle, the correct resistance can be calculated. This technique works well as long as there is no change in thermal voltage errors between the on-off cycles of the source current.

Q3. Under what circumstances is the dry circuit mode applicable?

Figure｜Dry circuit mode

Dry circuit resistance limits the open circuit voltage to 20mV (typical levels are approximately 6 to 14V, depending on the range) to minimize physical and electrical changes in the measured joint. This low open circuit voltage does not penetrate the film, so the resistance measurement provided includes the resistance of the oxide film. Dry circuit resistance is only available in the 1Ω, 10Ω, 100Ω, 1kΩ, and 10kΩ ranges of the 4W function (maximum resistance 2.4kΩ).