How to measure transistors with a multimeter
(1) Visual inspection method
(1) Identification of tube type
Generally, the tube type is NPN or PNP should be identified from the model number marked on the shell. According to the standard of the ministry, the second place of the triode model (letter), A, C indicates PNP tube, B, D indicates NPN tube, for example:
3AX is PNP type low-frequency low-power tube 3BX is NPN type low-frequency low-power tube.
3CG is a PNP high-frequency low-power tube 3DG is a NPN high-frequency low-power tube
3AD is a PNP low-frequency high power tube 3DD is a NPN 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 international popular 9011 ~ 9018 series of high-frequency low-power tube, in addition to 9012 and 9015 for the PNP tube, the rest are NPN-type tube.
② tube pole discrimination
Commonly used small and medium power transistors have metal round shell and plastic package (half-column type) and other appearance, Figure T305 introduces three typical appearance and tube pole arrangement.
(2) Use multimeter resistance to identify
There are two PN junctions inside the transistor, and the three poles of e, b and c can be distinguished by multimeter resistance. This method can also be used to identify the tube type when the model number label is ambiguous.
Determination of base pole
The base pole should be confirmed first when discerning the tube pole. For NPN tubes, with the black pen to the presumed base, with the red pen were in contact with the other two poles, if the measured resistance are small, about a few hundred ohms to a few thousand ohms; and the black and red pairs of pens, measured resistance are larger, more than a few hundred kilo ohms, this time, the black pen is connected to the base PNP tubes, the situation is reversed, measurement of the two PN nodes are positively biased in the case of red pens connected to the base.
In fact, the base of the small power tube is generally arranged in the middle of the three pins, available in the above method, respectively, the black and red pens connected to the base, not only to determine whether the two PN junction of the triode is intact (the same as the diode PN junction measurement), but also to confirm the tube type.
② Collector and emitter identification
Determine the base, assuming that one of the remaining pins for the collector c, the other for the emitter e, pinch the c-pole and b-pole with a finger (i.e., instead of a finger base resistor Rb). At the same time, the multimeter two pens were c, e contact, if the measured tube for the NPN, then the black pen contact c pole, with a red pen connected to the e pole (PNP tube on the contrary), observe the pointer deflection angle; and then set up another pin for the c pole, repeat the above process, compare the two measurements of the pointer's angle of deflection, the big one shows that the IC is big, tube is in the state of amplification, accordingly assumed that the c, e pole is correct.
2. Triode performance of simple measurements
(1) Use the multimeter resistance file to measure ICEO and β
Base open circuit, the black pen of the multimeter connected to the NPN tube collector c, red pen connected to the emitter e (PNP tube on the contrary), at this time, c, e between the resistance value of the ICEO is small, the resistance value of the ICEO is small, indicating that the ICEO is large.
With a finger instead of the base resistor Rb, with the above method to measure the resistance between c, e, if the resistance value is much smaller than the base open-circuit indicates that the β-value is large.
(2) Using a multimeter hFE file to measure β
Some multimeter hFE file, according to the table specified pole type inserted into the transistor can be measured current amplification coefficient β, if β is very small or zero, indicating that the transistor has been damaged, the available resistance file were measured two PN junctions, to confirm whether there is a breakdown or circuit breakage.
3. Selection of semiconductor transistor
Selection of transistors to meet the requirements of the equipment and circuit, the second to meet the principle of saving. According to the different uses, generally should consider the following factors: operating frequency, collector current, power dissipation, current amplification factor, reverse breakdown voltage, stability and saturation voltage drop. These factors have a relationship of mutual constraints, in the selection of the tube should seize the main conflict, taking into account the secondary factors.
Low-frequency tube's characteristic frequency fT is generally below 2.5MHz, while high-frequency tube's fT are from tens of megahertz to hundreds of megahertz or even higher. When selecting the tube, the fT should be 3 to 10 times the operating frequency. In principle, high-frequency tube can replace the low-frequency tube, but the power of the high-frequency tube is generally smaller, narrow dynamic range, in the replacement should pay attention to the power conditions.
Generally hope that the β selected larger, but not the larger the better. β is too high easily caused by self-excited oscillations, not to mention the general β high tube work more unstable, affected by the temperature. Usually β is selected between 40 to 100, but low noise and high β value of the tube (such as 1815, 9011 ~ 9015, etc.), β value of hundreds of temperature stability is still good. In addition, the whole circuit should also be selected from the cooperation of all levels of β. For example, the front stage with high β, the back stage can be used with β lower tubes; Conversely, the front stage with β lower, the back stage can be used with β higher tubes.
