Emergency On-Line Resistance Measurement with a Digital Multimeter
(1) The full-scale test voltage and open-circuit voltage of the resistance range vary across different digital multimeter models, so the value range of the loading resistor R1 should be determined through experiments.
(2) During operation, first connect the loading resistor R1 across the V/Ω and COM jacks of the digital multimeter, and obtain the actual measured value of R1 using the multimeter's resistance range before performing online resistance measurement. Do not connect the circuit under test first and then parallel the resistor R1-this is because the higher test voltage of the multimeter's resistance range may cause silicon diodes in the measured circuit to conduct, rendering the measurement meaningless (causing large measurement errors). Therefore, this sequence must not be reversed.
(3) In most circuits, the resistors paralleled with the emitter-base junction or collector-base junction of transistors typically range from kilohms to several hundred kilohms, while resistances in the tens of ohms are rare. Therefore, for online measurements, the digital multimeter is usually first set to the medium resistance range, i.e., the 200 kΩ range (with a resolution of 0.1 kΩ) or the 20 kΩ range. If the measured value R=R1+RXR1⋅RX is 0 or very small, it indicates a short circuit fault in the measured circuit (RX=0) or an excessively high range, and a lower range (2 kΩ range) should be used for detailed measurement. If R=R1+RXR1⋅RX is very close to R1, it suggests a possible open circuit fault in the measured circuit (RX=∞) or an excessively low range, and a higher range (2 MΩ range) should be used for remeasurement.
(4) The 200 Ω and 20 MΩ resistance ranges are rarely used for online measurements. Since the loading resistor R1 is paralleled with the measured resistor RX, the measurement range of the resistance range is effectively expanded, and the capability to measure high resistances is improved, so the 2 MΩ range is generally sufficient. Additionally, the 2 kΩ range has a resolution of 1 Ω, which is adequate for determining whether an online transistor in the measured circuit is short-circuited or damaged. In most cases, using three loading resistors can meet the needs of online resistance measurement. Taking the DT830A digital multimeter as an example, the 2 kΩ range uses
, the 200 kΩ range uses
, and the 2 MΩ range uses
. Of course, a 470 kΩ potentiometer can also be used to replace these three loading resistors.
(5) After completing the online resistance measurement, do not forget to promptly remove the loading resistor R1 bridged between the V/Ω and COM jacks of the digital multimeter to avoid affecting the normal use of the multimeter or causing accidents (e.g., during high-voltage measurements).
