Four Magic DMM Methods You Should Know
Determining if a circuit or device is charged
The AC voltage digital block is very sensitive, even if there is a small induced voltage around it, it can be displayed. For this feature, it can be used as a test pencil. The usage is as follows: turn multimeter to AC20V gear, hang black probe in air, keep red probe in contact with measured route or device, then multimeter will display if displayed number is between a few volts and more than ten volts (different multimeters will have different displays ), which indicates that line or device is charged, and if display is zero or very small, this indicates that line or device is not charged.
Determine if a power line is active or neutral
This can be judged by method described above: a larger number is a live line, and a smaller one is a zero. This method requires contact with measured line or device.
No contact with measured line or device is required. Turn multimeter to AC2V gear, hang black probe in air, grasp red probe and carefully run tip along line, and if multimeter reads a few volts, it means that line is live. If display is only a few tenths of a volt or even less. This means that line is a null line. This method of judgment is not in direct contact with line. Not only safe, but also convenient and fast.
Look for cable breaks
When there is a break point in cable, traditional method is to use a multimeter to block and find cable break point section by section. This is not only a waste of time, but also damage to cable insulation. to a large extent. The cable disconnect point can be quickly found using DMM's inductive characteristic. First, use an electrical barrier to determine which core of cable is open. Then connect one end of broken core wire to AC220V, and then turn multimeter to AC2V gear position, hang black probe in air, hold red probe, and carefully run tip along line, at this time, if meter displays a few volts or a few tenths volts (different from different cables), if reading on meter suddenly drops strongly when moving to a certain position, write down this position: under normal circumstances. The breakpoint is between 10 and 20 cm before this position.
Using this method, you can also find a break in a resistance wire, such as a failed electric blanket.
For UPS power supply. The stability of voltage at its output terminal is an important parameter, and frequency of its output is also very important. However, it cannot be measured directly with DMM's frequency block because voltage that frequency block can withstand is very low. Just a few volts. At this time, a step-down transformer of 220V/6V or 220V/4V can be connected to output terminal of UPS power supply to reduce voltage without changing frequency of power supply, and then connect frequency block to output. transformer to measure UPS power supply Frequency.
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