Method of measuring online resistance with digital 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 principle of measuring resistance using the proportional method. The part inside the wireframe is the internal circuit of the multimeter. Connect the measured resistor Rx to both ends of the multimeter, which 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 input voltage VIN and reference voltage is: VIN/VRO=VRX/VRO=RX/RO
Obtain RX=RO/VRO from this equation VRX,VRX=RX/RO.VRO. 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 approximately 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.
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.
