Introduction to Common Types of Measuring Power Plugs with Multimeters
Transformer type
This power supply method is generally used in appliances such as radios, recorders, speakers, black and white televisions, and early remote-controlled color televisions.
The power plug is directly connected to the power transformer or standby transformer, and some may add a power switch in the middle.
When measuring, turn on the power switch and use a multimeter to measure the two pins of the power plug. At this time, the primary coil of the transformer inside the machine is measured, and the resistance value is generally between a few hundred to two thousand ohms.
Switching power supply type
This type of electrical appliance is now the most common. Various mobile phone chargers, power adapters, and most LED lighting fixtures belong to this type
When measuring, the Rx 1K or Rx 10K range of the pointer type multimeter can be selected. At the beginning, there will be a small resistance value, and then gradually increase. It is similar to the scenario of measuring small capacity electrolytic capacitors.
However, this measurement is limited to pointer meters. Due to the low output voltage of most digital meters, it is difficult to break through the cutoff zone of the rectifier bridge, so it is possible that the resistance value cannot be measured.
Simple electric heating appliances
Electric heating appliances mainly include electric heaters, electric kettles, and rice cookers. They usually use mechanical temperature control switches to control the electric heating wire for heating. As we usually measure at room temperature, the temperature control switch will be in the on state, so the measured resistance is the electric heating wire resistance.
The resistance value of this type of electric heating wire is usually below 100 Ω. The term "simple" in the subheading mainly refers to rice cookers. Because the control circuit of some multifunctional rice cookers needs to be powered on in order to work, even if measured directly with a multimeter, the resistance value may not be measured.
Resistance capacitance voltage reduction type
The resistance capacitance voltage reduction type is generally used in low-end low-power appliances, such as LED bubble lights, small flashlights, or internal chargers for shavers.
Due to the small capacity of the capacitor, it is difficult to measure with a multimeter. But there will be a discharge resistance of several hundred K to 1M Ω on the capacitor, so the measured resistance will be the serial resistance value between this resistance and the load. Its value will be slightly greater than the discharge resistance.
For the measurement of the above four situations, the first and third can be measured using a pointer or digital meter, while the second and fourth must be measured using a pointer meter.
Due to the diverse power supply designs nowadays, it is difficult to accurately determine the quality of electrical appliances solely by measuring the power supply head. But in most cases, the measurement results can serve as a basis for whether the tested electrical appliance can be powered on for trial operation, which can minimize accidents caused by accidental power on.
