Techniques and Methods for Measuring Transistors with a Multimeter
Discrimination of transistor electrodes and tube types
(1) Visual inspection method
① Identification of pipe type
Generally, whether the pipe type is NPN or PNP should be distinguished by the model marked on the pipe shell. According to ministerial standards, the second digit (letter) of the transistor model, A and C represent PNP tubes, B and D represent NPN tubes, for example:
3AX is a PNP type low-frequency low-power transistor, and 3BX is an NPN type low-frequency low-power transistor
3CG is a PNP type high-frequency low-power transistor, and 3DG is an NPN type high-frequency low-power transistor
3AD is a PNP type low-frequency high-power transistor, and 3DD is an NPN type low-frequency high-power transistor
3CA is a PNP type high-frequency high-power transistor, and 3DA is an NPN type high-frequency high-power transistor
In addition, there are internationally popular 9011-9018 series high-frequency low-power tubes, with the exception of PNP tubes for 9012 and 9015, all of which are NPN type tubes.
② Discrimination of Tube Poles
Commonly used small and medium-sized power transistors have metal circular shells and plastic packaging (semi cylindrical). Figure T305 introduces three typical shapes and electrode arrangement methods.
(2) Using a multimeter to determine the resistance range
There are two PN junctions inside the transistor, which can be used to distinguish the three poles e, b, and c using a multimeter resistance range. In the case of fuzzy model labeling, this method can also be used to distinguish the pipe type.
① Discrimination of base
When distinguishing the transistor electrode, the base electrode should be confirmed first. For NPN tubes, connect a black lead to the assumed base and a red lead to the other two poles. If the measured resistance is small, it is about a few hundred to several thousand ohms; When the black and red probes are swapped, the measured resistance is relatively high, exceeding several hundred kiloohms. At this point, the black probe is connected to the base electrode. PNP tube, the situation is opposite. When measuring, when both PN junctions are positively biased, the red probe is connected to the base electrode.
In fact, the base of low-power transistors is generally arranged in the middle of three pins. The above method can be used to connect the black and red probes to the base respectively, which can not only determine whether the two PN junctions of the transistor are intact (similar to the measurement method for diode PN junctions), but also confirm the tube type.
② Discrimination between collector and emitter
After determining the base electrode, assume that one of the remaining pins is the collector electrode c and the other is the emitter electrode e. Use your fingers to pinch the c and b electrodes respectively (i.e., use your fingers to replace the base resistance Rb). At the same time, contact the two probes of the multimeter with c and e respectively. If the tube being tested is NPN, use a black probe to contact the c pole and a red probe to connect the e pole (opposite to the PNP tube), and observe the pointer deflection angle; Then set the other pin as the c-pole, repeat the above process, and compare the deflection angle of the pointer measured twice. The larger one indicates that the IC is large, and the tube is in an enlarged state. The corresponding assumptions for the c and e poles are correct.
2. Simple measurement of transistor performance
(1) Measure ICEO and β
The base electrode is open, and the black lead of the multimeter is connected to the collector c of the NPN tube, while the red lead is connected to the emitter e (opposite to the PNP tube). At this time, a high resistance value between c and e indicates a low ICEO, while a low resistance value indicates a high ICEO.
Replace the base resistance Rb with your finger and measure the resistance between c and e using the above method. If the resistance value is much smaller than when the base is open, it indicates that β High value.
(2) Use a multimeter to measure the hFE range β
Some multimeters have an hFE range, and the current amplification factor can be measured by inserting a transistor according to the specified polarity on the meter β， if β If it is very small or zero, it indicates that the transistor has been damaged. Two PN junctions can be measured using a resistance range to confirm whether there is a breakdown or open circuit.
3. Selection of semiconductor triodes
The selection of transistors should first meet the requirements of equipment and circuits, and secondly comply with the principle of conservation. According to different purposes, the following factors should generally be considered: operating frequency, collector current, dissipated power, current amplification coefficient, reverse breakdown voltage, stability, and saturation voltage drop. These factors have a mutually constraining relationship, and when selecting management, the main contradiction should be grasped while considering secondary factors.
The characteristic frequency fT of low-frequency tubes is generally below 2.5MHz, while the fT of high-frequency tubes ranges from tens of MHz to hundreds of MHz or even higher. When selecting pipes, fT should be 3-10 times the working frequency. In principle, high-frequency tubes can replace low-frequency tubes, but the power of high-frequency tubes is generally relatively small and the dynamic range is narrow. When replacing, attention should be paid to power conditions.
General hope β Choose a larger size, but it's not necessarily better. β Too high can easily cause self-excited oscillation, let alone average β The operation of high pipes is often unstable and greatly affected by temperature. usually β Multiple choices between 40 and 100, but with low noise and high noise β Value pipes (such as 1815, 9011-9015, etc.), β The temperature stability is still good when the value reaches several hundred. In addition, for the entire circuit, the selection should also be based on the coordination of all levels β。 For example, for the previous stage β High, the latter level can be used β Lower pipes; On the contrary, the previous level uses β Lower level can be used for later stages β Higher pipes.
The reverse breakdown voltage UCEO of the collector emitter should be selected to be greater than the power supply voltage. The smaller the penetration current, the better the stability of temperature. The stability of ordinary silicon tubes is much better than that of germanium tubes, but the saturation voltage drop of ordinary silicon tubes is larger than that of germanium tubes, which can affect the performance of certain circuits. It should be selected according to the specific situation of the circuit. When selecting the dissipative power of transistors, a certain margin should be left according to the requirements of different circuits.
For transistors used in high-frequency amplification, intermediate frequency amplification, oscillators, and other circuits, transistors with high characteristic frequency fT and small interpole capacitance should be selected to ensure high power gain and stability even at high frequencies.
