teach you how to measure large-value resistance with a multimeter.
The maximum range of the resistance range of 1/2-digit and 41/2-digit digital multimeters is generally 20MΩ. For a 31/2-digit digital multimeter, using different resistance ranges can only measure resistance within the range of 0.1Ω～19.99MΩ; for a 41/2-digit digital multimeter, it can only measure resistance within the range of 0.01Ω～19.999MΩ resistance. When the measured resistance Rx≥20MΩ, the meter will display the overflow symbol "1". Experiments have proved that the range of the 20MΩ resistance range of a 31/2-digit or 41/2-digit digital multimeter can be extended to 100MΩ by using the following "parallel resistance method".
1. Measurement method
Prepare a resistance R1 of more than ten megohms in advance, and measure the resistance value R1 after setting the digital multimeter to the 20MΩ range, as shown in Figure 4-15(a). Then connect the measured resistance Rx in parallel to both ends of R1, and then measure the total parallel resistance R, as shown in Figure 4-15(b).
According to the calculation formula of resistors in parallel, it is easy to deduce that
1. Measurement example
The resistance to be tested is a high-resistance resistor Rx with an unknown mark, and R1 uses a resistor with a nominal resistance of 10MΩ. Using the 20MΩ resistance file of the DT830 digital multimeter, the measured resistance value of R1 is 10.05MΩ. After connecting Rx and R1 in parallel, measure with DT830, the total resistance value R=7.70MΩ is measured. Substitute into formula (4-13) to get
Judging from this, the nominal value of the measured resistance should be 33MΩ.
2. Precautions for measurement
(1) When the resistance value of the measured resistance Rx exceeds 100MΩ, the total resistance value R after parallel connection is very close to the resistance value of the selected standard resistance R1, and the digital multimeter itself has an error of ±1 character, which will cause measurement errors increase. Therefore, this method is not suitable for measuring resistances greater than 100MΩ.
(2) During the measurement operation, the measured resistance Rx and the standard resistance R1 should be connected in parallel and connected securely. If necessary, the two can be fixed with crocodile clips.
