Emergency Measurement of In-Line Resistance with a Digital Multimeter
The following points should be noted when using the load-buck measurement method:
(1) The full-scale test voltage and the open-circuit voltage of the resistance block of different models of digital multimeter are different, so the value range of the loading resistor R1 should be determined by experiment.
(2) The operation should be first in the digital multimeter V / Ω and COM jack across the loading resistance R1 and the digital multimeter in the resistance block read out the measured value of R1 before the online resistance measurement. Can not be connected to the circuit under test and then connected in parallel with the resistor R1, this will be due to the high test voltage of the digital multimeter resistor block will be measured in the line of silicon tube tends to conduct and lose the significance of the measurement (resulting in a large measurement error). Therefore, this order can not be reversed.
( (3) As a result of the general circuit with the transistor emitter, collector junction in parallel with the resistance value of more than kΩ ~ hundreds of kΩ, and tens of ohms is rare, so the online measurement of the digital multimeter is usually set first in the resistance block, that is, the 200kΩ block (the resolution of the block for the 0.1kΩ) or 20kΩ block. If the measured R=R1.RX / (R1 + RX) for O or very small, then the measured circuit has a short-circuit fault (RX=0) or range is high, this time should be changed to a low resistance (2kΩ block) fine measurement. If the measured R=R1.RX / (R1 + RX) and R1 is very close, then the circuit under test may be open-circuit failure (RX=∞) or range is low, should be replaced with a high resistance (2WΩ block) retest.
(4) online measurement is generally rarely used 200Ω resistance block and 20MΩ block. Because the loading resistor R1 is connected in parallel with the measured resistor RX, it has actually extended the measuring range of the resistor block and improved the ability to measure high resistance, so it is generally sufficient to use 2MΩ block. In addition, because the resolution of 2kΩ block for 1Ω, with the block to determine whether the circuit is not online transistor short circuit breakdown is also enough. In general, not only with three loaded resistors can meet the needs of measuring online resistance. DT830A digital multimeter, for example, its 2kΩ block R1=R0=1kΩ, 200kΩ block R1=0.47RO=47kΩ, 2MΩ block R1=0.47R0=470kΩ. Of course, we can also use a 470kΩ potentiometer instead of the above three loading resistance.
(5) After the measurement of online resistance Chemo Zhi remember to remove in time across the digital multimeter V / Ω and COM jack between the loading resistance R1 to avoid affecting the normal use of the multimeter and accidents (when measuring high-voltage).
