The maximum range of the resistance range of the 31/2-digit and 41/2-digit digital multimeters is generally 20MΩ. For 31/2-digit digital multimeters, using different resistance ranges can only measure resistance in the range of 0.1Ω to 19.99MΩ; while for 41/2-digit digital multimeters, it can only measure resistance in the range of 0.01Ω to 19.999MΩ. resistance. When the measured resistance Rx≥20MΩ, the meter will display the overflow symbol "1". Experiments have shown that the following "parallel resistance method" can be used to extend the range of the 20MΩ resistance range of a 31/2-digit or 41/2-digit digital multimeter to 100MΩ.
1. Measurement method
Prepare a resistance R1 of more than ten megohms in advance, turn the digital multimeter to 20MΩ and measure the resistance value R1, as shown in Figure 4-15(a). Then connect the measured resistance Rx to both ends of R1 in parallel, and then measure the total resistance R in parallel, as shown in Figure 4-15(b).
It is easy to derive from the calculation formula of resistors in parallel
1. Measurement example
The resistance to be measured is an unidentified high-resistance resistance Rx, and R1 is a resistor with a nominal resistance of 10MΩ. Using the 20MΩ resistance range of the digital multimeter, the measured resistance value of R1 is 10.05MΩ. After connecting Rx and R1 in parallel, measure again, and measure the total resistance value R=7.70MΩ. Substitute into equation (4-13) to get
From this, it is determined that 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 word, which will make the measurement error. increase. Therefore, this method is not suitable for measuring resistances with a resistance value greater than 100MΩ.
(2) During the measurement operation, the measured resistance Rx and the standard resistance R1 should be connected in parallel and firmly, and the two can be fixed with alligator clips if necessary.
