Principle of using a digital multimeter to measure resistance
There are hundreds of types of digital multimeters. According to the range conversion method, they can be divided into manual range digital multimeters, automatic range digital multimeters and automatic/manual range digital multimeters. According to their uses and functions, they can be divided into low-end popular types. (such as DT830 digital multimeter) digital multimeter, mid-range digital multimeter, smart digital multimeter, multi-display digital multimeter and special digital instrument, etc.; according to shape and size, they can be divided into pocket type and desktop type.
Principle of measuring resistance with digital multimeter
The functions of measuring voltage, current and resistance are realized through the conversion circuit part, and the measurement of current and resistance are based on voltage measurement. That is to say, the digital multimeter is expanded on the basis of the digital DC voltmeter. The converter converts the analog voltage that changes continuously with time into a digital quantity, and then the electronic counter counts the digital quantity to obtain the measurement result, and then the decoding display circuit displays the measurement result.
The logic control circuit controls the coordinated work of the circuit and completes the entire measurement process in sequence under the action of the clock. A digital multimeter (DMM) is an electronic instrument used in electrical measurements. It can have many special functions, but its main function is to measure voltage, resistance and current. As a modern multi-purpose electronic measuring instrument, the digital multimeter is mainly used in physics, electrical, electronic and other measurement fields.
How to measure resistance with digital multimeter
In the process of using a multimeter to measure resistance, engineers sometimes need to accurately measure small resistances less than 100Ω, which often requires the help of some technologies that can improve measurement accuracy. This article summarizes three common techniques for measuring resistance with multimeters for technicians. Let us take a look at them below.
Four-wire measurement method
In the process of using a digital multimeter to measure resistance, technicians often use the four-wire measurement method in order to improve the accuracy of testing small resistances less than 100Ω. The so-called four-wire measurement method is to separate the two current lines through which the constant current source current flows into the resistor R under test and the two voltage lines at the voltage measurement end of the digital multimeter, so that the voltage at the measurement end of the digital multimeter is no longer at both ends of the constant current source. direct voltage.
Four-wire measurement plus constant current source measurement
The four-wire measurement method mentioned above can certainly help engineers complete high-precision resistance measurement with a multimeter. However, during the four-wire measurement process, the accuracy of the constant current source current is very critical. It is recommended to use an additional more stable constant current source here.
It should be noted that the size of the external constant current source current must be equal to the size of the constant current source current of the digital multimeter. The external constant current source we use consists of a high-precision reference voltage source MAX6250, an operational amplifier and a current expansion composite tube. The temperature drift of the voltage source MAX6250 is ≤2ppm/℃, and the time drift ΔVout/t=20ppm/1000h. During this measurement process, the current I should be 800μA ~ 1mA, and R is the extremely low-temperature drift wire-wound resistance (if I=1mA, R=5kΩ), then the temperature drift and time drift of I are equivalent to the level of the 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, a three-wire connection method is often chosen to connect the measured resistance to the grounded wire. connected. The principle of this test method is shown in Figure 3. When using this technology for measurement, the current I is 800μA ~ 1mA, and R is the extremely low-temperature drift wire-wound resistance (if I=1mA, R=5kΩ), then the temperature drift and time drift of the current I are equivalent to those of the MAX6250 level.
