When using a multimeter to measure the forward resistance of a diode, why in each range )
A digital multimeter converts the measured resistance value. It uses an A/D conversion chip to transform the analog signal into a digital signal and then displays the resistance value. An analog multimeter, on the other hand, shows the value by the deflection of the magnetic head. If in actual measurement, we find that when using the resistance range of a digital multimeter to test a diode, there is no resistance value in both the forward and reverse directions, while when using an analog multimeter to test the diode, there is a resistance value in the forward direction, there are mainly the following reasons:
Measurement of the diode on the circuit board
Firstly, the output voltages of the resistance ranges of the analog multimeter and the digital multimeter are different. Generally, the maximum output voltage of an analog multimeter is 9 volts, while the maximum output voltage of a digital multimeter is usually 3 volts. At the same time, not only are their output voltages different, but when we select different ranges during measurement, the output voltages are also different. The output voltages of the resistance ranges of a digital multimeter range from 1.0 volt to 3.0 volts. The output voltage of the resistance range of an analog multimeter is generally higher than that of a digital multimeter. The output voltage of the analog multimeter is greater than the voltage drop value of the diode, so the diode can conduct. However, sometimes the output voltage of the digital multimeter is less than the voltage drop value of the diode, causing the diode not to conduct. This will result in an infinite resistance value in both the forward and reverse directions when measuring the diode.
Secondly, the different voltage drop characteristics of the diodes will also lead to deviations in the measurement results when we use the resistance range of an analog multimeter and a digital multimeter to measure diodes. For example, the voltage drop values of silicon diodes and germanium diodes are generally between 0.3 volts and 0.6 volts. But for some special diodes, such as high-voltage diodes, their forward conduction voltage drops are relatively large, generally reaching more than 0.7 volts. Since the voltage of the resistance range of our digital multimeter is relatively low and cannot make the diode conduct, the measured resistance value will appear to be infinite.
