Diode Forward Voltage Test with a Digital Multimeter
In this range, the red probe connects to the internal positive power supply of the multimeter, and the black probe connects to the internal negative power supply. The connection of the two probes to the diode is shown in Figure 1. If measured according to the connection method in Figure 1(a), the diode under test conducts in the forward direction, and the multimeter displays the forward conduction voltage of the diode in mV. Normally, a good silicon diode has a forward conduction voltage of 500 mV ~ 800 mV, and a good germanium diode has a forward conduction voltage of 200 mV ~ 300 mV. If "000" is displayed, the diode has breakdown short-circuited; if "1" is displayed, the diode fails to conduct forward. If measured as shown in Figure 1(b), the reading should be "1", indicating that the diode is reverse-biased and cut off. If "000" or other values appear, the diode has suffered reverse breakdown.
This range can also be used to judge the quality of transistors and identify their pins. During measurement, connect one probe to a presumed pin, and touch the other probe sequentially to the remaining two pins. If both measurements show conduction or both show no conduction, then swap the two probes and test again. If both readings show no conduction or both show conduction, the transistor is good, and the presumed pin is the base of the transistor. If the red probe is connected to the base and the black probe conducts on both of the other two pins, the transistor is NPN type. Conversely, it is PNP type. Finally, compare the forward conduction voltages of the two PN junctions: the larger reading corresponds to the BE junction, and the smaller reading corresponds to the BC junction. The collector and emitter can thus be identified.
Transistor β Value Test First determine whether the transistor under test is NPN or PNP. Insert its pins correctly into the test socket for the corresponding type. Turn the function/range switch to the β range, and the β value can be read directly from the display. If "000" is displayed, the transistor is damaged.
Continuity Test Turn the function and range switch to the continuity beep position. Touch the two probes to the test points respectively. The buzzer will sound if a short circuit exists.
