Emergency on-line resistance measurement with digital multimeter
Use a digital multimeter to emergency measure online resistance. The following points should be noted when using the step-down voltage measurement method:
(1) The full-scale test voltage and open-circuit voltage of the electrical barrier of different types of digital multimeters are different, so the value range of the loading resistor R1 should be determined experimentally.
(2) During operation, the loading resistor R1 should be connected between the digital multimeter V/Ω and the COM jack first, and the digital multimeter should read the actual measured value of R1 at the resistor before measuring the online resistance. You cannot 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 become conductive due to the high electrical blocking test voltage of the digital multimeter, thus losing the meaning of the measurement (producing a large measurement error). Therefore, this order cannot be reversed.
(3) Since the resistance value of the resistor connected in parallel with the emitter junction and collector junction of the transistor in a general circuit is mostly kΩ to several hundred kΩ, and there are very few tens of ohms, so when measuring online, the digital multimeter is usually placed in the middle block first. , that is, 200kΩ block (the resolution of this block is 0.1kΩ) or 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. In this case, a low barrier (2kΩ barrier) should be used for detailed measurement. If R=R1 is measured. If RX/(R1+RX) is very close to R1, it means that the circuit under test may have an open circuit fault (RX=∞) or the range is low, and the high blocking (2WΩ block) should be used for retesting.
(4) 200Ω electrical barrier and 20MΩ barrier are generally rarely used in online measurements. Because the loading resistor R1 is connected in parallel with the measured resistor RX, the measurement range of the electrical barrier has actually been expanded and the ability to measure high resistance has been improved. Therefore, generally using a 2MΩ barrier is sufficient. In addition, since the resolution of the 2kΩ block is 1Ω, it is enough to use this block to determine whether there is short-circuit breakdown in the online transistor that is disconnected from the circuit. Generally speaking, not only three loading resistors can meet the needs of measuring online resistance. Taking the DT830A digital multimeter as an example, the 2kΩ block is R1=R0=1kΩ, the 200kΩ block is R1=0.47RO=47kΩ, and the 2MΩ block is R1=0.47R0=470kΩ. Of course, we can also use a 470kΩ potentiometer to replace the above three loading resistors.
(5) After measuring the online resistance, be sure to promptly remove the loading resistor R1 connected between the digital multimeter V/Ω and the COM jack to avoid affecting the normal use of the multimeter and causing accidents (when measuring high voltage).
