Emergency measurement of online resistance using a digital multimeter
The following points should be noted when using a digital multimeter for emergency measurement of online resistance using the load drop measurement method:
(1) The full range test voltage and open circuit voltage of the resistance range of different models of digital multimeters vary, so the range of loading resistance R1 should be determined by experiments.
(2) When operating, the loading resistor R1 should be bridged between the V/Ω of the digital multimeter and the COM socket, and the measured value of R1 should be read by the digital multimeter in this resistance range before online resistance measurement can be carried out. It is not possible to connect the tested circuit in parallel with resistor R1 first, as this will cause the silicon tube in the tested circuit to tend to conduct due to the high test voltage of the resistance range of the digital multimeter, resulting in significant measurement errors. So, this order cannot be reversed.
(3) Due to the fact that in general circuits, the resistance value of the resistor parallel to the transistor emitter and collector junction is mostly k Ω to a few hundred k Ω, and there are very few tens of ohms. Therefore, when measuring online, the digital multimeter is usually set to the middle blocking position, which is 200k Ω (with a resolution of 0.1k Ω) or 20k Ω. If the measured R=R1. RX/(R1+RX) is O or very small, it indicates a short circuit fault (RX=0) or a high range in the tested circuit. At this time, low resistance (2k Ω gear) should be used for detailed measurement. If the measured R=R1. RX/(R1+RX) is very close to R1, it indicates that there may be an open circuit fault (RX=∞) or a low range in the tested circuit. High blocking (2W Ω gear) should be used for retesting.
(4) Online measurement generally rarely uses 200 Ω resistance range and 20M Ω range. Because loading resistor R1 in parallel with the measured resistor RX has actually expanded the measurement range of the resistance range and improved the ability to measure high resistance, using a 2M Ω range is generally sufficient. In addition, since the resolution of the 2k Ω range is 1 Ω, using this range is sufficient to determine whether there is a short circuit breakdown in the online transistor of the circuit that has been lost. In general, not only three loaded resistors can meet the needs of online resistance measurement. Taking the DT830A digital multimeter as an example, its 2k Ω gear takes R1=R0=1k Ω, 200k Ω gear takes R1=0.47RO=47k Ω, and 2M Ω gear takes R1=0.47R0=470k Ω. Of course, we can also use a 470k Ω potentiometer instead of the three loading resistors mentioned above.
(5) After measuring the online resistance, do not forget to remove the load resistor R1 that spans between the digital multimeter V/Ω and the COM socket in a timely manner to avoid affecting the normal use of the multimeter and causing accidents (when measuring high voltage).
