Method for Measuring In-Circuit Resistance with Digital Multimeters
If we first cross connect a resistor R1 between the V/Ω and COM sockets of the digital multimeter, that is, between the two probes, before measuring the online resistance, that is, pre connect a load resistor, and lower the test voltage of the digital multimeter in that resistance range. As long as the resistance value of R1 is selected appropriately, its maximum test voltage can be limited to below 0.3V (not greater than 0.3V). Considering that silicon tubes are commonly used both domestically and internationally, germanium tubes are extremely rare, and silicon tubes are still in a cut-off state at a voltage of 0.35V, the parallel effect of silicon tubes on the tested circuit can be ignored (silicon tubes can be regarded as open circuits). Therefore, this method can be used to measure the online resistance of transistors, which is the load voltage reduction measurement method. When measuring online resistance using this method, there should be a certain margin between the maximum test voltage of each resistance gear and the upper limit of 0.35V. Usually, the maximum test voltage should be less than or equal to 0.3V. Use the load voltage reduction measurement method to measure the circuit connection of online resistance.
Assuming the measured online resistance is RX, the displayed value of the digital multimeter is R, and the loaded resistance is R1 (take the measured value). Obviously, the relationship between R, RX, and R1 is R=R1. RX/(R1+RX), so the measured online resistance RX=R1. R/(R1-R) can be calculated from this equation. But what is the appropriate resistance value for the loading resistor R1 in each resistance range? The author conducted experiments using the circuit shown in Figure 3 to select the appropriate resistance value for R1. The connection is shown in Figure 3, and the experimental data is listed in the attached table. The open circuit voltage of each resistance range of the DT830A digital multimeter provided by the manufacturer is 0.65V or less than 0.7V.
It can be seen that the 200 Ω range of the DT830A digital multimeter has a loose requirement for the value range of R1. How can 2k Ω Zhu satisfy the requirement of R1 ≤ 1.76k Ω? Other high ranges have different values for R1. For ease of memory and use, R1=RO (or 0.1R0 ≤ R1 ≤ R0) is generally used for 200 Ω and 2k Ω ranges, while for resistance ranges above 2k Ω, 0.1R0 ≤ R1 ≤ 0.75R is usually used. The value of R1 cannot be too small, otherwise it will affect the measurement range of this resistance range. If R1 is too small, RX>>R1 will make the values of R and R1 very close, which will significantly increase the measurement error (because the digital multimeter itself has an error of ± 1 word).
Therefore, the lower limit of R1 is usually set to 0.1Ro. For the DT830A digital multimeter, as long as R1 is selected reasonably according to the above requirements, the maximum test voltage of each resistance range can be limited to below 0.3V, thus meeting the requirements for measuring online resistance. The load voltage reduction measurement method is also applicable to other models of digital multimeters.
Precautions for use
(1) The full-scale test voltage and open circuit voltage of different models of digital multimeters with different resistance ranges are different, so the range of values for loading resistor R1 should be determined by experiments.
(2) When operating, the load resistor R1 should be connected between the V/Ω of the digital multimeter and the COM socket, and the measured value of R1 should be read out by the digital multimeter in that resistance range before conducting online resistance measurement. It is not possible to connect the tested circuit in parallel with resistor R1 first, as this will cause the silicon transistor in the tested circuit to become conductive due to the high test voltage of the digital multimeter's resistance mode, resulting in significant measurement errors. So, this order cannot be reversed
