Diode Forward Voltage Measurement Using a Digital Multimeter
At 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.
When measured in the connection mode of Figure 1(a), the diode under test conducts in the forward direction, and the multimeter displays the forward voltage drop of the diode in mV. Generally, a good silicon diode has a forward voltage drop of 500 mV to 800 mV, while a good germanium diode has a forward voltage drop of 200 mV to 300 mV. If the reading shows "000", the diode is short-circuited due to breakdown. If the display shows "1", the diode fails to conduct in the forward direction.
When measured in the connection mode of Figure 1(b), the display should read "1", indicating reverse cutoff of the diode. If it shows "000" or other values, the diode has suffered reverse breakdown.
This range can also be used to verify the quality of triodes and identify their pins. During measurement, connect one probe to a presumed pin, then touch the other probe to the remaining two pins successively. If both measurements show conduction or both show no conduction, swap the two probes and repeat the test. If both readings are non-conductive or both conductive after swapping, the triode is intact, and the presumed pin is the base of the triode.
If the red probe is connected to the base and the black probe conducts when touching the other two pins respectively, the triode is NPN type. Otherwise, it is PNP type. Finally, compare the forward voltage drop of the two PN junctions. The junction with the larger reading is the base-emitter junction (be junction), and the one with the smaller reading is the base-collector junction (bc junction). Accordingly, the collector and emitter can be identified.
