How to measure resistance with a digital multimeter
In the process of using a multimeter to measure resistance, engineers sometimes need to accurately measure small resistors less than 100 Ω, which often requires the use of techniques that can improve measurement accuracy. This article summarizes three common techniques for measuring resistance with a multimeter for technical personnel. Let's take a look together.
Four line measurement method
In the process of using a digital multimeter to measure resistance, technicians often use the four wire measurement method to improve the accuracy of testing small resistors less than 100 Ω. The so-called four wire measurement method is to separate the two current lines of the constant current source flowing into the measured resistance R and the two voltage lines of the voltage measurement terminal of the digital multimeter, so that the voltage at the measurement terminal of the digital multimeter is no longer the direct voltage at both ends of the constant current source.
In the process of using the four wire measurement method to accurately test the resistance of a digital multimeter, this method adds two more feeders than the usual measurement method and disconnects the connection between the voltage measurement terminal and the constant current source. Due to the disconnection between the voltage measurement terminal and the constant current source terminal, the constant current source forms a loop with the measured resistance Rx, feeder RL1, and RL2. The voltage sent to the voltage measurement terminal is only the voltage at both ends of Rx, and the voltage of feeder RL1 and RL2 is not sent to the voltage measurement terminal. Therefore, the feeder resistors RL1 and RL2 have no effect on the measurement results. The feeder resistance RL3 and RL4 have an impact on the measurement, but the impact is minimal. Due to the input impedance of the digital multimeter being much greater than the feeder resistance, the accuracy of measuring small resistance using the four wire measurement method is very high.
Four wire measurement with external constant current source measurement
The four wire measurement method mentioned above can certainly help engineers complete high-precision resistance measurement with a multimeter, but the accuracy of its constant current source current is crucial in the four wire measurement process. It is recommended to use a more stable external constant current source current here.
It should be noted that the magnitude of the applied constant current source current should be equal to the magnitude of the constant current source current of a digital multimeter. The external constant current source current we use consists of a high-precision reference voltage source MAX6250, an operational amplifier, and a current expanding composite tube, as shown in Figure 2. Temperature drift of voltage source MAX6250 ≤ 2ppm/℃, time drift Δ Vout/t=20ppm/1000h. During this measurement process, the current I should be taken as 800 μ A~1mA, R is the extremely low temperature drift wire wound resistance (if I=1mA, R=5k Ω), where the temperature drift and time drift of I are equivalent to the level of MAX6250.
Feeder resistance compensation measurement method
The feeder resistance compensation method is another common high-precision measurement method for measuring resistance with a multimeter. In the industrial field, if high-precision resistance testing is required, the three-wire connection method is often chosen to connect the measured resistance to the grounded wire. The principle of this testing method is shown in Figure 3. When using this technology for measurement, the current I is taken as 800 μ A~1mA, R is the extremely low temperature drift wire wound resistance (if I=1mA, R=5k Ω), where the temperature drift and time drift of current I are equivalent to the level of MAX6250.
