How to measure triode with multimeter
Discrimination of Triode Electrode and Tube Type
(1) Visual inspection
① Discrimination of cast
Generally, whether the tube type is NPN or PNP should be identified from the model marked on the tube shell. According to the standard issued by the ministry, the second digit (letter) of the triode model, A, C represent PNP tubes, B, D represent NPN tubes, for example:
3AX is PNP type low frequency low power tube 3BX is NPN type low frequency low power tube
3CG is PNP type high frequency low power tube 3DG is NPN type high frequency low power tube
3AD is PNP type low frequency high power tube 3DD is NPN type low frequency high power tube
3CA is PNP type high frequency high power tube 3DA is NPN type high frequency high power tube
In addition, there are 9011-9018 series of internationally popular high-frequency low-power tubes. Except for 9012 and 9015, which are PNP tubes, the rest are NPN tubes.
② Identification of tube poles
Commonly used small and medium-power triodes have metal round shells and plastic packages (semi-cylindrical) and other shapes. Figure T305 introduces three typical shapes and tube arrangements.
(2) Use the resistance file of the multimeter to judge
There are two PN junctions inside the triode, and the three poles e, b, and c can be distinguished by the resistance file of the multimeter. In the case of ambiguous model marking, this method can also be used to determine the cast type.
① Identification of the base
When judging the tube, the base should be confirmed first. For the NPN tube, use the black test lead to connect the hypothetical base, and the red test lead to touch the other two poles respectively. If the measured resistance is small, it is about several hundred ohms to several thousand ohms; The resulting resistance is relatively large, above several hundred thousand ohms. At this time, the black test lead is connected to the base. For PNP tubes, the situation is just the opposite. When both PN junctions are positively biased during measurement, the red test lead is connected to the base.
In fact, the base of the low-power tube is generally arranged in the middle of the three pins. The above method can be used to connect the black and red test leads to the base respectively, and it can be determined whether the two PN junctions of the triode are intact (connected to the PN junction of the diode). The measurement method is the same), and the tube type can be confirmed.
② Discrimination between collector and emitter
After determining the base, assuming that one of the remaining pins is the collector c and the other is the emitter e, pinch the c pole and the b pole with your fingers (that is, replace the base resistor Rb with your fingers). At the same time, touch the two test leads of the multimeter to c and e respectively. If the tube under test is NPN, touch the c pole with the black test pen and connect the e pole with the red test pen (the PNP tube is opposite), and observe the deflection angle of the pointer; then set another The pin is the c pole, repeat the above process, compare the deflection angles of the two measurement pointers, the larger one indicates that the IC is large, the tube is in an enlarged state, and the corresponding hypothetical c and e poles are correct.
2. Simple Measurement of Transistor Performance
(1) Measure ICEO and β with a multimeter resistance file
The base is open, the multimeter’s black test lead is connected to the collector c of the NPN tube, and the red test lead is connected to the emitter e (the PNP tube is opposite). At this time, a large resistance value between c and e indicates that the ICEO is small, and a small resistance value indicates that the ICEO is large.
Replace the base resistance Rb with your finger, and use the above method to measure the resistance between c and e. If the resistance value is much smaller than when the base is open, it indicates that the β value is large.
(2) Measure β with multimeter hFE file
Some multimeters have hFE files, and the current amplification factor β can be measured by inserting the triode according to the pole type specified on the table. If β is very small or zero, it indicates that the triode has been damaged. You can use the resistance file to measure the two PN junctions respectively to confirm whether There is breakdown or open circuit.
3. Selection of semiconductor triode
The selection of transistors must first meet the requirements of equipment and circuits, and second, it must conform to the principle of saving. Depending on the application, the following factors should generally be considered: operating frequency, collector current, power dissipation, current amplification factor, reverse breakdown voltage, stability, and saturation voltage drop. These factors also have a mutual restrictive relationship. When selecting management, we should grasp the main contradiction and take into account the 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 megahertz to hundreds of megahertz or even higher. When selecting tubes, fT should be 3 to 10 times the operating frequency. In principle, the high-frequency tube can replace the low-frequency tube, but the power of the high-frequency tube is generally relatively small, and the dynamic range is narrow, so attention should be paid to the power condition when replacing.
It is generally hoped that the β selection will be larger, but the bigger the better. If β is too high, it will easily cause self-excited oscillation, not to mention that tubes with high β generally work more unstable and are greatly affected by temperature. Usually β is between 40 and 100, but for tubes with low noise and high β value (such as 1815, 9011~9015, etc.), the temperature stability is still good when the β value reaches hundreds. In addition, for the entire circuit, β should also be selected from the coordination of all levels. For example, if the front stage uses a high β tube, the subsequent stage can use a tube with a low β; on the contrary, if the previous stage uses a low β tube, the subsequent stage can use a tube with a high β.
The collector-emitter reverse breakdown voltage UCEO should be selected greater than the power supply voltage. The smaller the penetration current, the better the temperature stability. 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 will affect the performance of the circuit in some circuits. It should be selected according to the specific conditions of the circuit. When dissipating power, a certain margin should be left according to the requirements of different circuits.
For transistors used in circuits such as high-frequency amplification, intermediate frequency amplification, and oscillators, transistors with high characteristic frequency fT and small inter-electrode capacitance should be selected to ensure high power gain and stability at high frequencies.
