Have you ever measured in-circuit resistance with a multimeter?
Before describing the load voltage reduction measurement method, it is necessary to first introduce the principle of measuring resistance using the proportional method. The schematic diagram of measuring resistance using the proportional method is shown in Figure 1. The part inside the wireframe in the figure is the internal circuit of the multimeter. From the figure, it can be seen that connecting the measured resistor Rx to both ends of the multimeter is equivalent to connecting Rx in series with the reference resistor Ro and then connecting it between the V+pin and COM pin of the integrated block TSC7106. After turning the multimeter to the resistance mode, the reference power supply Eo of TSC7106 provides test current I to Ro and Rx, and the voltage drop VRo on Ro provides test voltage VRX, which serves as the reference voltage VREF for the integrated block TSC7106, and VRX is the input voltage VIN. The relationship between the input voltage VIN and the reference voltage is: VIN/VRO=VRX/VRO=RX/RO. From this equation, RX=RO/VRO.VRX and VRX=RX/RO.VRO are obtained. This is the basic principle of measuring resistance using the proportional method. It is not difficult to see from VRX=RX/RO.VRO that at the same electrical barrier of the multimeter, if the measured resistance is smaller, the test voltage at both ends will also be smaller. When a short circuit occurs, that is, when the multimeter displays "000" and the measured resistance RX=0, the test voltage VRX=0; On the contrary, as the measured resistance RX continues to increase, the test voltage VRX at both ends also increases. When the multimeter displays "1000", i.e. RX=RO, the test voltage VRX=VRO. When the measured resistance reaches RX=2RO, which is the full range, the overflow symbol "1" is displayed, and the test voltage VRX at both ends of the measured resistance is VRX=2VRO. When the tested resistor is open circuited, its test voltage reaches a maximum value of about 0.65V (typical value). Due to the open circuit voltage (no-load output voltage) of each resistance range of the DT830A digital multimeter being approximately 0.65V, it is not possible to directly measure the online resistance, as such a high test voltage is sufficient to make the silicon tube in the tested circuit (when measured in the forward direction) tend to conduct, thereby affecting the measurement results. According to the variation law between the measured resistance and the test voltage, it is not difficult to think that before measuring the online resistance, we first cross connect a resistor R1 between the V/Ω and COM socket of the digital multimeter, that is, between the two probes, that is, pre select 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). Given the widespread use of silicon tubes both domestically and internationally, with germanium tubes being extremely rare, and silicon tubes still being 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 considered as open circuits). Therefore, this method can be used to measure the online resistance of transistors, which is known as 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 range and the upper limit of 0.35V. Usually, the maximum test voltage is taken to be less than or equal to 0.3V. The circuit connection for measuring online resistance using the load voltage reduction measurement method is shown in Figure 2.
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.
