Teach you how to use a multimeter to measure large resistance skillfully
The maximum range of resistance range for 31/2 bit and 41/2 bit digital multimeter is generally 20M Ω. For a 31/2 digit digital multimeter, using different resistance ranges can only measure resistance within the range of 0.1 Ω to 19.99M Ω; For a 41/2 digit digital multimeter, it can only measure resistance within the range of 0.01 Ω to 19.999M Ω. When the measured resistance Rx ≥ 20M Ω, the instrument will display the overflow symbol "1". Experiments have shown that using the following "parallel resistance method" can expand the range of a 31/2 bit or 41/2 bit digital multimeter from 20M Ω resistance to 100M Ω.
1、 Measurement method
Prepare a resistance R1 of over ten megaohms in advance, set the digital multimeter to the 20M Ω range, and measure the resistance value R1, as shown in Figure 4-15 (a). Then connect the measured resistance Rx in parallel at both ends of R1, and measure the total parallel resistance R. Based on the calculation formula for resistance parallel connection, it is easy to derive.
Measurement examples
The measured resistance is an unidentified high resistance resistor Rx, and R1 is selected with a nominal resistance value of 10M Ω. Using the 20M Ω resistance range of the DT830 digital multimeter, the measured resistance value of R1 is 10.05M Ω. After connecting Rx and R1 in parallel, use DT830 to measure the total resistance value R=7.70M Ω. Substituting into equations (4-13) yields
Based on this, the nominal value of the measured resistance should be 33M Ω.
Measurement precautions
(1) When the resistance value of the measured resistor Rx exceeds 100M Ω, the total resistance value R after parallel connection is very close to the resistance value of the selected standard resistor R1. In addition, the digital multimeter itself has an error of ± 1 word, which will increase the measurement error. Therefore, this method is not suitable for measuring resistance values greater than 100M Ω.
(2) During the measurement operation, the measured resistance Rx should be securely connected in parallel with the standard resistance R1, and if necessary, the two can be fixed with a crocodile clamp.
