How good is the multimeter's three-stepped tube measuring?
It is very simple to use a multimeter to judge the quality of a triode. Here we take a digital multimeter as an example to introduce the judgment method (analog multimeters are too old and have a single function. Even African electronics enthusiasts are not willing to use them now).
First, let's talk about the internal equivalent circuit of the transistor, as shown in the figure. The left side of the figure is the NPN transistor, and the right side is the equivalent circuit of the NPN transistor. It can be seen that for an NPN transistor, its be junction (the PN junction between the tube base b and the emitter e) and bc junction (the PN junction between the tube base b and the collector c) are equivalent to two Diode with positive terminal connected. Similarly, the PNP transistor on the right side of the figure can be equivalent to two diodes connected to the negative pole.
From the above analysis, it can be seen that to judge the quality of a triode, just use the diode block of a digital multimeter to measure the quality of its be junction and bc junction.
Here we take the commonly used NPN silicon transistor as an example (germanium transistors are almost no longer used). When measuring, adjust the digital multimeter to the diode position (see the figure), touch the b electrode of the transistor with the red pen of the multimeter, and then, Use the black test lead to touch the e-pole and c-pole of the triode respectively. At this time, the reading displayed by the multimeter is the forward voltage drop of the PN junction. Its value is generally between ".500-.700". Then use the black test lead to touch the transistor. B pole, the red test pen contacts the e pole and c pole of the transistor. At this time, the reading displayed by the multimeter is the reverse voltage drop of the PN junction, and the displayed readings are all "1", so it can be basically considered that the transistor is good.
During measurement, if the reading of a certain junction is ".000" or the forward and reverse pressure drops are both "1", then the junction of the tube is damaged and the tube cannot be used.
For a good triode, use the diode block of a multimeter to measure the voltage drop between its c-e poles. Regardless of the forward voltage drop or the reverse voltage drop, the multimeter's reading should be "1". If the reading is ".000" ", the tube has been broken down, is bad, and cannot be used. If a certain value is displayed, it means that the penetration current Iceo of the tube is relatively large. Although it can be used, its working stability is poor and its use is not recommended.
