Shulou(Shulou.com)06/01 Report--

Transistors are used in a small amount in our digital and analog circuits, and multiple transistors are also used on our pioneer board. In the part of the LED lamp on our board, there is the use of this transistor. Q16 in the LED circuit of figure 3-5 is a PNP transistor.

Figure 3-5 LED circuit

Preliminary views on Triode

Transistor is a very commonly used master and drive device, the commonly used transistor is divided into silicon tube and germanium tube according to the data, on the contrary, the voltage drop is slightly different, silicon tube is widely used, while germanium tube is used less. This course will use the parameters of the silicon tube to stop the explanation. There are 2 varieties of Triode, which can be distinguished as PNP type and NPN type. Let's take a look at it first, as shown in figure 3-6.

Figure 3-6 Triode representation diagram

The Triode has three poles. From figure 3-6, the pin on the left side of the transistor is called base. There is an arrow on both ends, one end connects to the base, the other end is connected to the emitter e (emitter), and the remaining pin is the collector c (collector). This is the content that must be remembered, can be integrated, later gradually used more and more, each time integrated once, many times in the future will deepen the mind.

The truth of Triode

The transistor has three task forms: cut-off, reduction and saturation. The reduced form is mainly used in the imitation circuit, and the usage and calculation method are relatively complex, so we can't use it temporarily. The digital circuit mainly uses the switching characteristics of the transistor, only using the two forms of cut-off and saturation, so we only explain these two uses. The type and usage of transistor I summed up a formula for everyone, everyone should remember this formula: arrow inward PNP, turn-on voltage along the arrow, voltage on, current control.

Let's analyze the formula one by one. Everyone can look at figure 3-6, there are 2 varieties of transistors, the arrow facing inside is PNP, and the arrow facing outward is naturally NPN. In practice, according to the needs of the practical circuit to choose which type to use, everyone will use more than a few times, very complicated.

The crux of the usage of the transistor lies in the voltage condition between the b pole (base) and the e level (emitter). For PNP, the e pole voltage only needs to be more than 0.7V higher than the b level, and the transistor can be connected smoothly between the e level and the c stage. In other words, the master end is between b and e, and the master end is between e and c. Similarly, the turn-on voltage of the NPN Triode is 0.7 V higher than that of the e pole. In short, if the beginning of the arrow is 0.7 V higher than the end, the e pole and c pole of the transistor can be connected. This is the explanation of "turn-on voltage over the arrow, voltage on". Let's take a look at figure 3-7.

Figure 3-7 usage of Triode

Let's take figure 3-7 as an example. The transistor base is connected to an IO port of the single chip microcomputer through a 10K resistor. Assuming P1.0, the emitter is directly connected to the 5V power supply, the collector is connected to a LED lamp, and a 1K current limiting resistor is connected in series to the negative GND of the power supply.

If P1.0 by our program to a high level 1, then the base b and emitter e is mostly 5V, that is to say, e to b will not have a voltage drop of 0.7V, at this time, the emitter and collector will not be on, then vertically look at the circuit in the Triode is disconnected, no current smooth, LED2 lights will not be lit. If the program gives P1.0 a low level 0, then the e pole is still 5V, so there is a voltage difference between e and b, and there is a voltage drop of about 0.7V between the Triode e and b, and the voltage of (5-0.7) V will be on the resistor R47. At this time, there will be a connection between e and c, so the LED lamp itself has a voltage drop of 2V, and the transistor itself has a voltage drop of about 0.2V between e and c, which we ignore. Then there will be a voltage drop of about 3V on the R41, and it can be calculated that the current in this wrong circuit is about 3mA, which can successfully light up the LED.

The first concept, current mastery. As mentioned earlier, the Triode has three forms: cutoff, shrinking, and saturation, needless to say, as long as there is no connection between e and b. If we want to make this transistor in a saturated state, which is what we call a switch feature, we must be satisfied with a premise. All transistors have a reduction multiple β. In order to be saturated, the current of b pole must be greater than the current value between e and c divided by β. This beta, about the commonly used transistor, can be thought of as about 100. Then we must calculate the resistance of R47 above.

We have just calculated that the current between e and c is 3mA, so the minimum b-pole current is 3mA divided by 100equals 30uA, and about 4.3V voltage will fall on the base resistance, then the maximum base resistance is 4.3V/30uA = 143K. Resistance value only needs to be smaller than this value, of course, it can not be too small, too small will lead to single-chip microcomputer IO port current over-burnt Triode or single-chip microcomputer, STC89C52 IO port output current maximum actual value is 25mA, I recommend not to exceed 6mA, we use voltage and current calculation, we can calculate the minimum resistance value, we take the experience value in figure 3-7.

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