A mnemonic for measuring the base of a transistor with a multimeter
The determination of transistor type and pin discrimination belongs to the electronic foundation. Generally, beginners in electrical technology will come into contact with the inversion of the transistor and find the base; PN junction, fixed tube type; Along the arrow, there is a large deviation; Uncertain measurement, move your mouth. Perhaps it will be much easier, but of course, memorizing the mnemonic without knowing its meaning is also useless.
The first sentence of the mnemonic: Three reversals, find the base pole
As we all know, a transistor is a semiconductor device that contains two PN junctions (not separate PN junctions in series). According to the different connection methods of the two PN junctions, the transistor can be divided into NPN type and PNP type.
The testing of the transistor is carried out using the ohmmeter's Rx100 or Rx1k range. The pointer type multimeter has a red pen indicating the negative pole of the multimeter and a black pen indicating the positive pole of the multimeter battery. When testing the transistor type, the first step is to determine the base position of the pins. Measurement method: Randomly select two electrodes (such as 1 and 2), first measure the forward and reverse resistance, and observe the deflection angle of the watch needle; Next, take the 1 and 3 electrodes and
2. Measure the forward and reverse resistance of three electrodes upside down, and observe the deflection angle of the watch needle. In these three inverted measurements, there must be two measurements with similar results: that is, in the inverted measurement, the pointer deviates more once and less once; The remaining measurement must be reversed, and the deviation angle of the pointer before and after measurement is very small. The unmeasured pin this time is the base we are looking for
The second sentence of the mnemonic: PN junction, find the tube type
After conducting an appeal measurement, we have determined the pin base and can then determine the tube type based on the PN junction direction between the base and the other two electrodes. Measurement method: Place the black probe of the multimeter on the base electrode and the red probe on any other electrode. If the pointer of the multimeter deviates significantly, it indicates that the tested transistor is an NPN type transistor; If the deviation angle of the pointer on the meter head is very small, then the tested tube is PNP type.
The third sentence of the mnemonic: clockwise arrow, large deviation
And after identifying the tube type through the base b, we can then determine the collector c, so which one is the emitter e?
NPN type transistor structure and current flow direction
NPN type transistor: According to the principle of the flow direction of the penetration current of NPN type transistors, the black and red probes of a multimeter are used to measure the forward and reverse resistances Rce and Rec between the two poles in reverse. Two measurement results are obtained, and it is found that the pointer deflection of the meter head is small. After careful observation, it will be found that there is a slightly larger deflection angle once, that is, the current flow direction is: black pen → c pole → b pole → e pole → red pen, which coincides with the flow direction of the symbol arrow of the transistor. Therefore, at this time, the black probe must be connected to the collector electrode c, and the red probe must be connected to the emitter electrode e.
NP type transistor structure and current flow direction
PNP type transistor: The principle is equivalent to measuring NPN type. After testing, the results are obtained as follows: black pen → e-pole → b-pole → c-pole → red pen. Similarly, the current flow direction at this time matches the arrow direction of the transistor symbol. Therefore, the conclusion can be drawn that "at this point, the black pen is located at the emitter e, and the red pen is located on the collector c.".
The fourth sentence of the mnemonic: Watch well, move your mouth
In the measurement process of the third mnemonic, if it is difficult to distinguish which time the deflection pointer is larger, then it is necessary to move the mouth. So how should we move our mouths? When conducting the third mnemonic measurement, hold the junction between the red and black pen and the pin with both hands, then hold the base electrode b with your mouth (or touch it with your tongue), and compare the results of the two tests. Due to the human body acting as a DC bias resistor, the two comparisons will become more pronounced.
