How to diagnose and handle switch power supply faults in frequency converters?
A digital multimeter converts the measured resistance value into a digital signal through an A/D conversion chip, and then displays the resistance value. A pointer multimeter displays the value by deflecting the magnetic head. If in actual measurement, we find that using a digital multimeter to test the resistance range of a diode has no resistance value in both the forward and reverse directions, while using a pointer multimeter to test the resistance value of a diode has resistance value in the forward direction, there are mainly the following reasons:
Diode measurement in circuit board
Firstly, the output voltage of the resistance range of a pointer multimeter and a digital multimeter is different. Generally, the maximum output voltage of a pointer multimeter is 9 volts, while a digital multimeter generally has a maximum output voltage of 3 volts. Moreover, not only do they output different voltages, but when we choose different ranges for measurement, their output voltage also varies. The output voltage of the resistance range of a digital multimeter ranges from 1.0 volts to 3.0 volts. The output voltage of the resistance range of a pointer multimeter is generally higher than that of a digital multimeter. The output voltage of a pointer multimeter is greater than the voltage drop of the diode, and the diode can conduct. Sometimes, a digital multimeter is smaller than the voltage drop of the diode, causing the diode to not conduct. This can cause The forward and reverse resistance caused by measuring diodes is infinite.
Secondly, the voltage drop characteristics of the second stage transistor are different, which can also cause a deviation in the results of measuring the second stage transistor with a pointer multimeter resistance range and using a digital multimeter to measure the second stage transistor. For example, silicon and germanium transistors generally have a voltage drop value between 0.3 volts and 0.6 volts, but some more special second stage transistors, such as high-voltage diodes, have a conduction voltage drop of generally 0.7 volts or more. Our digital multimeter has a lower resistance range voltage and cannot conduct the diode, so the resistance value may appear infinite during measurement.
When measuring the quality of a diode with a digital multimeter, it is best to choose the diode gear. The diode gear of a digital multimeter is generally around 2.6 volts, which is generally greater than the forward voltage drop value of the diode, and the diode can conduct in the forward direction.
If we want to use the resistance range to measure whether there is leakage in a diode, we can choose the resistance range of a digital multimeter. At this time, the result should be a forward measurement with resistance value, a reverse measurement with infinite resistance value, and the result of a pointer multimeter measurement is the same. If a reverse measurement with resistance value is found, it indicates that the diode may have leakage in the reverse direction. In this case, we need to use specialized instruments to detect, and using a multimeter to measure whether there is leakage in this diode is not accurate.
