When using a multimeter to measure the forward resistance of a diode, why are the different levels
A digital multimeter converts the measured resistance value into a digital signal through an A/D conversion chip and displays the resistance value. A pointer multimeter displays the value by causing the magnetic head to deflect. If we find that using a digital multimeter to test the resistance value of a diode in both directions has no resistance value, while using a pointer multimeter to test the resistance value of a diode in the forward direction has resistance value, there are mainly the following reasons:
Measurement of diodes in circuit boards
Firstly, the output voltage of a pointer multimeter and a digital multimeter in the resistance range is different. Generally, a pointer multimeter outputs a maximum voltage of 9 volts, while a digital multimeter outputs a maximum voltage of 3 volts. Moreover, the output voltage of each range is also different. When measuring, we choose different ranges, and the output voltage of a digital multimeter in the resistance range ranges from 1.0 volts to 3.0 volts. The output voltage of a pointer multimeter in the resistance range is generally higher than that of a digital multimeter. The output voltage of a pointer multimeter is greater than the voltage drop value of a diode, and the diode can conduct. However, a digital multimeter may sometimes be less than the voltage drop value of a diode, causing the diode to not conduct. The forward and reverse resistance caused by measuring diodes is infinite.
Secondly, the voltage drop characteristics of the diode are different, which can also cause deviations in the results of measuring the diode using a pointer multimeter and a digital multimeter. For example, silicon and germanium transistors generally have a voltage drop value between 0.3 volts and 0.6 volts, but some special diodes, such as high-voltage diodes, have a relatively large conduction voltage drop, usually reaching 0.7 volts or more. Our digital multimeter has a lower resistance range and cannot conduct the diode, so the resistance value will appear infinitely large during measurement.
When measuring the quality of a diode with a digital multimeter, it is best to choose the diode level. The diode level of a digital multimeter is usually around 2.6 volts, which is generally greater than the forward voltage drop of the diode. The diode can conduct in the forward direction.
If we want to measure whether a diode has leakage in the resistance range, we can choose the digital multimeter resistance range. At this time, the result should be that there is resistance in the forward measurement, and the resistance in the reverse measurement is infinite. The same goes for the pointer multimeter measurement. If a resistance is found in the reverse measurement, it proves that the diode may leak in the reverse direction. In this case, we need to use a specialized instrument to detect it. Using a multimeter to measure whether the diode has leakage is not accurate.
