Digital multimeter emergency online resistance measuring
Use a digital multimeter to measure online resistance in an emergency The following points should be paid attention to when using the load-drop voltage measurement method:
(1) Different types of digital multimeters have different full-scale test voltages and open-circuit voltages, so the value range of the loading resistor R1 should be determined by experiments.
(2) During operation, the loading resistor R1 should be connected between the digital multimeter V/Ω and the COM jack first, and the online resistance measurement can only be performed after the digital multimeter reads the measured value of R1 at this resistance barrier. It is not possible to connect the circuit under test first and then connect the resistor R1 in parallel. This will cause the silicon tube in the circuit under test to turn on due to the high test voltage blocked by the digital multimeter (large measurement error will be generated). Therefore, this order cannot be reversed.
(3) Since the resistors connected in parallel with the emitter junction and collector junction of transistors in general circuits are mostly kΩ to hundreds of kΩ, but rarely tens of ohms, the digital multimeter is usually set in the middle to block , that is, the 200kΩ block (the resolution of this block is 0.1kΩ) or the 20kΩ block. If R=R1 is measured. If RX/(R1+RX) is 0 or very small, it means that the circuit under test has a short-circuit fault (RX=0) or the range is too high. At this time, it should be carefully measured with low resistance (2kΩ block). If R=R1 is measured. RX/(R1+RX) is very close to R1, which means that the circuit under test may have an open circuit fault (RX=∞) or the range is too low, and it should be retested with high resistance (2WΩ block).
(4) 200Ω electric block and 20MΩ block are rarely used for online measurement. Because the load resistance R1 is connected in parallel with the measured resistance RX, the measurement range of electrical resistance has actually been expanded, and the ability to measure high resistance has been improved, so it is generally enough to use a 2MΩ block. In addition, since the resolution of the 2kΩ block is 1Ω, it is enough to use this block to judge whether there is a short-circuit breakdown of the online transistor without the circuit. In general, not only three loading resistors can meet the needs of measuring online resistance. Take the DT830A digital multimeter as an example, for the 2kΩ block, R1=R0=1kΩ, for the 200kΩ block, R1=0.47RO=47kΩ, for the 2MΩ block, R1=0.47R0=470kΩ. Of course, we can also use a 470kΩ potentiometer instead of the above three loading resistors.
(5) After measuring the online resistance, do not forget to remove the loading resistor R1 connected between the digital multimeter V/Ω and the COM jack in time to avoid affecting the normal use of the multimeter and accidents (when measuring high voltage).
