Introduction to the Measurement of the Quality of the Three-stage Tube by Multimeter
Firstly, let's talk about the internal equivalent circuit of the transistor, as shown in the figure. The one on the left is an NPN transistor, and the one on the right is the equivalent circuit of the NPN transistor. It can be seen that for NPN type transistors, their be junction (the PN junction between the base b and emitter e of the transistor) and bc junction (the PN junction between the base b and collector c of the transistor) are equivalent to diodes with two positive terminals connected. Similarly, the PNP transistor on the right side of the diagram can be equivalent to two diodes with negative terminals connected.
From the above analysis, it can be seen that to judge the quality of a transistor, it is only necessary to measure the quality of its BE and BC junctions using the diode block of a digital multimeter.
Taking the commonly used NPN type silicon transistor as an example (germanium transistor is almost no longer used), when measuring, adjust the digital multimeter to the diode mode (as shown in the figure), use the red probe of the multimeter to contact the b-pole of the transistor, and then use the black probe to contact the e-pole and c-pole of the transistor respectively. At this time, the reading displayed by the multimeter is the forward voltage drop of the PN junction, which is generally between ". 500-700". Then use the black probe to contact the b-pole of the transistor, and the red probe to contact 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 reading is "1". This way, It is generally believed that the transistor is good.
When measuring, if the reading of a certain junction is ". 000" or the pressure drop in both directions is "1", then the junction of the tube is damaged and the tube cannot be used.
For a good transistor, measure the voltage drop between its c-e terminals using the diode block of a multimeter. Regardless of the forward or reverse voltage drop, the reading on the multimeter should be "1". If the reading is ". 000", the transistor has been broken down and cannot be used. If there is a certain value displayed, it indicates that the penetration current Iceo of the tube is relatively high. Although it can be used, its working stability is poor and it is not recommended to use it.
