Desktop multimeter is a common resistance measuring instrument, but in practical application, do you use the right method to measure resistance? Let’s take a look at some common situations.
How to choose line 2 or line 4?
Figure 2-line and 4-line schematic diagram
Many multimeters include 2-wire and 4-wire measurement methods. How to judge which method to use in specific measurement?
Generally, the 2-wire method should only be used when measuring the resistance value higher than 10 ohm, because the influence of lead resistance will significantly increase the error (1% or more).
The 4-wire (also known as Kelvin) connection method can provide a more accurate low resistance measurement solution by automatically eliminating the influence of lead resistance. In this configuration, the test current (I) is forced through the test resistance (R) through a set of test lines, and the voltage (VM) through the DUT is measured through the first bundle of lines (sensing lines). Although a small part of the current (generally less than 100Pa) may flow through the sensing line, it can generally be ignored in all practice. The voltage drop in the sensing line is negligible, so the voltage (VM) measured by the meter is basically the same as the voltage (VR) through the resistance (R). In this way, the accuracy of resistance value determined by 4-wire method is much higher than that determined by 2-wire method.
What is offset compensation?
Figure offset compensation mode
Bias compensation is a technique used to eliminate thermal voltage error when measuring low resistance (generally less than 10 Ω). It measures the resistance by providing current, measuring the voltage, and then using Ohm’s law to calculate the unknown resistance.
In the bias compensation technique, the source current is turned on and off alternately, and the voltage is measured in two parts of the cycle. When the source current is on, the measured voltage is the thermal error voltage and the voltage passing through the device under test, as shown in part B of this figure. When the source current is off, no current passes 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. The correct resistance can be calculated by using the voltage VM and the periodically open source. As long as the thermal voltage error between source current cycling switches does not change, this technique will be very practical.
Under what circumstances is dry circuit mode applicable?
Figure dry circuit mode
Dry circuit resistance limits the open circuit voltage to 20mV (typical level is about 6 ~ 14V, depending on the range), so as to minimize the physical and electrical changes in the measured contact. This low open circuit voltage does not penetrate the film, so the resistance measurement provided will include the resistance of the oxide film. Dry circuit resistance can only be used in the range of 1 Ω, 10 Ω, 100 Ω, 1K Ω and 10K Ω of 4W function (the maximum resistance is 2.4k Ω).
Editor in charge: GT