# How to measure the transistor?

Last Update Time: 2019-10-30 10:34:17

1. Measurement of ordinary signal transistor (no built-in resistance).

The diode block of the multimeter is usually used to distinguish the tube type, polarity and quality of the transistor. It is best to blow the transistor off the motherboard when measuring.

If any of the multimeter pens are fixed to one pin of the transistor and then the other pen is connected to the remaining two pins of the transistor, we can measure it twice. According to this method, three cycles, six times of measurement results can be obtained, and all the measurement work can be completed.

We first analyze the possible results of these six measurements.

Through the study of the switching principle of the transistor, we already know that when there is no conduction voltage on the base of the transistor, the resistance value between the CE of the NPN and the EC of the PNP is infinite. Therefore, the pressure drops between CE pins of normal NPN or EC pins of normal PNP should be infinity 1 by using multimeter open circuit. That is to say, the results of these two measurements are 1.

Through the study of transistor structure, we already know that the BE of NPN transistor is equivalent to a diode (B pole positive E pole negative), BC is also equivalent to a diode (B pole positive C pole negative), and the BE of PNP transistor is equivalent to a diode (B pole negative E pole positive) and BC is also equivalent to a diode (B pole negative C pole positive). When the order of the multimeter is exactly in line with the direction of the tube, it should be able to measure a value of about 650. When the order of the multimeter does not conform to the direction of the tube, it should be measured to infinity 1. That is to say, when the meter pen happens to measure the positive and negative electrodes of the above two diode, the result should be 1 time and 650 times respectively.

When the gauge pen happens to measure the pressure drop between the c pole and the E pole, it should not be feasible: the twice representation of the multimeter is 1. To sum up, all six measurements should be carried out with the results shown in Table 1.

 Order PNP NPN Red one, lower left. In the middle of the dark. 1 650 Black, lower right. 1 650+ In the middle of Red one. One black, one left. 650 1 Black, lower right. 1 1 Red, down to the right. One black, one left. 650+ 1 In the middle of the dark. 1 1

In these two measurements with readings, we find that there is a pen and the pole it touches is fixed. The fixed pole that this watch pen touches is the base pole of the transistor. If this pen is a red pen, then the transistor is NPN type: if this pen is black pen, then the transistor is PNP type.

After judging the base pole, it should also be judged according to the size of the two measured values. The emitter is slightly larger and the collector is slightly smaller. It is important to note that this difference is small (often only 0.01V), the quality of the multimeter will be measured twice equal numbers, at this time need to replace the more accurate multimeter.

If the accuracy of the multimeter is not enough to distinguish the collector from the emitter, if the transistor is a direct plug transistor with long pins, it can also be judged by wetting method. Although the size of transistor package used in notebook is too small (patch SOT23), it is difficult to judge conveniently by wetting method, but readers should still master this method in order to better understand the conduction principle of transistor.

Wetting method is specially used to judge the collector and emitter of direct insertion transistor after judging the base electrode. The principle is to simulate the conduction of the tube. For NPN, touch two pins other than the base with a watch pen, and the reading of the multimeter should be 1. Wet the fingers with the spit first, and then touch the electrodes connected to the base and the red pen with the F finger at the same time. If there is a reading and the reading is reduced regularly, then the red pen is connected to the collector of NPN.

2. The measurement of a digital transistor.

The measurement process of digital transistor with built-in resistance is different from that of ordinary signal transistor. Because of the existence of built-in resistance, the voltage drop of PN junction cannot be measured by using multimeter when measuring the emission junction of transistor [red base pole (left lower pin), black emitter (right lower pin)] and collector junction [red base pole (left lower pin), black emitter (middle pin)].

Therefore, the method mentioned above cannot be used to measure the quality of the digital tube, and the auxiliary "bias power supply" should be used at this time.

3. The auxiliary bias power supply is used to measure the signal transistor and the digital transistor.

The auxiliary bias power supply can be used to simulate the conduction process of the transistor to measure the ordinary signal transistor and the built-in resistance digital transistor, and can also be used to roughly judge its quality on the board side (please refer to Section 3.5.8 for the manufacture of the bias power supply and the assembly of the multimeter). When the auxiliary bias power supply is used to turn on the transistor on the board side (especially note that the process of changing the reading number of the multimeter from the larger value to the smaller value is the process of changing the digital transistor from cutoff to on), the voltage drop between its CE or EC can be seen from the multimeter (which means that the transistor has been turned on).

For the common NPN signal transistor, the pressure drops between the CE measured by the multimeter (red Chinese pin, black right lower pin) is about 0.500V instead of 0V. For the common PNP signal transistor, the pressure drops between the EC measured by the multimeter (black middle pin, red right lower pin) is about 1.500V instead of 0V. For the digital NPN transistor, the music drops between the CE measured by the multimeter (red middle pin, black right lower pin) should be less than 0.060V instead of 0V. For the digital PNP transistor, the pressure drop between the EC measured by the multimeter (red middle pin, black right lower pin) should be less than 1.300 ≤ 0.7V instead of 0V. Readers should master the difference between the use of bias power supply common signal transistor and digital tube through the actual measurement, rather than memorizing the specific data listed by the author, because these data will be different according to the specific tube.

These measured data also show that the performance of the digital transistor is much better than that of the ordinary signal transistor.

If you want to know more, our website has product specifications for transistor, you can go to ALLICDATA ELECTRONICS LIMITED to get more information