Troubles & Prevention of Digital Multimeters
① Most damages to digital multimeters are caused by incorrect measurement range selection. For example, if the resistance range is selected while measuring AC mains power, the internal components of the multimeter will be instantly damaged once the probes touch the mains supply. Therefore, always verify that the measurement range is correct before taking measurements with the multimeter. After use, set the selector switch to the AC 750 V or DC 1000 V position. This prevents damage to the digital multimeter even if you mistakenly select the wrong parameter during the next measurement.
② Some digital multimeter failures occur when the measured voltage or current exceeds the instrument's range. For instance, measuring mains power on the AC 20 V range can easily damage the AC amplifier circuit inside the multimeter and disable its AC measurement function. Exceeding the rated range when measuring DC voltage may also lead to internal circuit faults. When measuring current, an over-range condition usually only blows the built-in fuse without causing further damage. Therefore, when measuring voltage with an unknown approximate magnitude, start with the highest range. After obtaining a rough reading, switch to a lower range to get a more accurate value. If the target voltage far exceeds the maximum range of the multimeter, use dedicated high-voltage probes. This applies to measuring the CRT anode high voltage and focus high voltage of black-and-white or color televisions.
③ Most digital multimeters have a maximum DC voltage range of 1000 V. Generally, measuring DC voltages below 1000 V will not damage the meter, while voltages above 1000 V carry a high risk of damage. However, the maximum measurable voltage varies between different digital multimeter models. Resistor voltage division can be adopted for voltages exceeding the meter's range. In addition, when measuring high DC voltages from 400 V to 1000 V, ensure firm probe contact with the measurement point without any shaking. Otherwise, besides inaccurate readings and possible multimeter damage, the display may go blank in severe cases.
④ Never measure resistance on an energized circuit.
