How to Measure a Semiconductor with a Digital Multimeter
1. Diode
The open-circuit voltage of the diode of the digital multimeter is about 2.8V. The red test lead is connected to positive and the black test lead is connected to negative. The current provided during measurement is about 1mA. The displayed value is the approximate value of the forward voltage drop of the diode, and the unit is mV or V. The forward voltage drop of silicon diodes is about 0.3~0.8V. The forward conduction voltage drop of germanium diode is about 0.1~0.3V. And the forward voltage drop of diodes with higher power is smaller. If the measured value is less than 0.1V, it means that the diode has broken down, and both the forward and reverse directions are conducting at this time. If both the forward and reverse directions are open, it means that the PN section of the diode is open. For light-emitting diodes, the diode emits light when measured in the forward direction, and the tube voltage drop is about 1.7V.
2. Transistor
The transistor has two PN nodes, the emitter node (be) and the collector node (bc). Just measure it according to the method of measuring diodes. During the actual measurement, the forward and reverse voltage drops must be measured between every two pins. A total of 6 measurements are required. 4 of them show open circuit and only two show the voltage drop value. Otherwise, the transistor is bad or a special transistor. (Such as band-stop transistors, Darlington transistors, etc., can be distinguished from ordinary transistors by model). In two measurements with numerical values, if the black test lead or the red test lead is connected to the same pole, the pole is the base. The smaller measured value is the collector node, and the larger measured value is the emitter node, because the base has been judged. Correspondingly, the collector and emitter can be determined. At the same time, it can be judged: if the black test lead is connected to the same pole, the transistor is PNP type; if the red test lead is connected to the same pole, the transistor is NPN type; the voltage drop is about 0.6V, it is a silicon tube, and the voltage drop is about 0.2V, it is germanium Tube.
3. SCR:
The anode, cathode and control electrode of the thyristor are open circuit. Based on this, the anode pin can be determined and whether the thyristor has broken down. There is also a PN node between the thyristor control electrode and the cathode, but there is a protection resistor between the high-power thyristor control electrode and the cathode. The displayed value during measurement is the voltage drop on the resistor.
4. Optocoupler
One side of the optocoupler is a light-emitting diode, and the voltage drop during measurement is about 1V. The other side is a triode. Some only lead out c and e. The measurement is cut off in both forward and reverse directions. If all three pins are lead out, the measurement characteristics are the same as the triode above. (Mostly NPN tubes). When using a multimeter to make the diode forward, use another multimeter to measure the conduction voltage drop of triode c to e, which is about 0.15V; disconnect the multimeter connected to the diode and the triode c to e is cut off, indicating that the optocoupler is good.
