【Four Techniques to Efficiently Use a Digital Multimeter】
1. Determine if a Circuit or Device is Live
The AC voltage range of a digital multimeter is very sensitive; it can display even small induced voltages. Based on this feature, it can be used as a test pencil. The method is as follows: set the multimeter to the AC20V range, keep the black probe in the air, and touch the red probe to the circuit or device being tested. If the multimeter displays a voltage between a few volts and a dozen volts (different multimeters may show different values), it indicates that the circuit or device is live. If the display shows zero or a very small number, it indicates that the circuit or device is not live.
2. Distinguish Between Live and Neutral Wires
First method: You can use the above method to determine: the wire with the higher displayed number is the live wire, and the one with the lower number is the neutral wire. This method requires contact with the wire or device being measured.
Second method: This method does not require contact with the wire or device. Set the multimeter to the AC2V range, keep the black probe in the air, and gently slide the red probe along the wire. If the display shows a few volts, it indicates that the wire is live. If it shows only a few hundredths of a volt or even less, it indicates that the wire is neutral. This method is safe and convenient as it does not require direct contact with the wire.
3. Locate Breaks in Cables
When a break occurs in a cable, the traditional method is to use the multimeter in resistance mode to check segment by segment for the break, which is time-consuming and can significantly damage the cable’s insulation. By utilizing the inductive characteristics of a digital multimeter, the break in the cable can be quickly located. First, use the resistance mode to determine which core wire of the cable has an open circuit, then connect one end of the broken core wire to an AC220V power source. Next, set the multimeter to the AC2V range, keep the black probe in the air, and gently slide the red probe along the wire. If the display shows a few volts or a few hundredths of a volt (depending on the cable), and if the display suddenly drops significantly at a certain position, note that position: generally, the break is located about 10-20cm before this position.
This method can also be used to locate breaks in heating wires of faulty electric blankets.
4. Measure the Frequency of UPS Power Supply
For UPS power supplies, the stability of the output voltage is an important parameter, and the output frequency is also crucial. However, you cannot directly measure the frequency with the frequency mode of a digital multimeter, as it can only handle low voltages of a few volts. In this case, you can connect a 220V/6V or 220V/4V step-down transformer to the output terminal of the UPS to reduce the voltage without changing the frequency of the power supply. Then connect the frequency mode to the output of the transformer to measure the frequency of the UPS power supply.
(Source: Preface CNC Technology)