Switching diodes-conduction and cutoff conditions

Transistors have a large number of applications in our digital and analog circuits, and many transistors are used on our development boards. In our board LED light part, there is the application of this transistor, figure 3-5 led circuit in the Q16 is a PNP type transistor.

Figure 3-5 LED Circuit
Preliminary understanding of transistor

Transistor is a very common control and drive devices, commonly used transistor according to the material divided into silicon tube and germanium tube Two, the principle of the same, pressure drop slightly different, silicon tube is more common, and the use of germanium tubes less, this course with the parameters of the silicon tube to explain. There are 2 types of transistors, namely PNP and NPN. First come to know, as shown in Figure 3-6.

Figure 3-6 schematic diagram of transistors

Transistor a total of 3 poles, from figure 3-6, the left side of the pin is called the base (base), an arrow in the middle, a connection to the base, the other end is connected to the emitter e (emitter), the remaining pin is the Collector C (collector). This is the content that must be remembered, rote memorization can, behind slowly use more, each time rote memorization once, many times later will deep in mind.
The principle of transistor

Transistor has cut-off, amplification, saturation of three working states. Amplification state is mainly used in analog circuits, and the usage and calculation methods are more complex, we can not use for the time being. The main use of digital circuit is the transistor switch characteristics, only use the cutoff and saturation of the two states, so we also only to explain these two usages. Transistor type and usage I have summed up a formula for everyone, we have to remember this formula: arrow facing the PNP, on-off voltage along the arrows, voltage conduction, current control.

Here we have a sentence to parse the formula. We can look at Figure 3-6, the transistor has 2 types, the arrow facing is PNP, that the arrow outward nature is NPN, in the actual application, according to the actual circuit needs to choose exactly which type, we use a few times will be, very simple.

Transistor usage characteristics, the key point is the B-pole (base) and E-Class (emitter) between the voltage situation, for the PNP, the e-pole voltage as long as the higher than the B-class 0.7V, the transistor E-Class and C-class can be smoothly conduction. That is, the control ends between B and E, and the controlled end is between E and C. Similarly, the on-off voltage of the NPN transistor is a B-pole ratio e very high 0.7V, in short, the arrowhead of the beginning of the end of the high 0.7V can be conductive transistor e-pole and C-pole. This is about the "on-voltage straight arrow over, voltage conduction" explanation, we take a look at Figure 3-7.

Figure 3-7 Usage of transistors

Let's take a look at figure 3-7 for an example. Transistor base through a 10K resistor to a single-chip IO port, assuming that the P1.0, the emitter directly to the 5V power supply, the collector connected to a small LED light, and in Series a 1K current-limiting resistor finally received the power supply negative GND.

If P1.0 by our program to a high Level 1, then the base B and emitter e are 5V, that is, E to B will not produce a 0.7V pressure drop, this time, the emitter and collector will not be conduction, then the vertical look at the transistor is disconnected, no current through, LED2 small lights Also will not be lit. If the program gives P1.0 a low level of 0, then E is still 5V, so there is a pressure difference between E and B, between the transistor E and B is also conductive, between the transistor E and B there is probably a 0.7V pressure drop, and then (5-0.7) V voltage will be on the resistor R47. This time, between E and C will also be conductive, then LED small lamp itself has 2V pressure drop, transistor itself between E and C about 0.2V pressure drop, we ignore. Then on the R41 will have about 3V pressure drop, can be calculated, this branch of the current is about 3mA, you can successfully light the LED.

The last concept, current control. Said before, transistor has cut-off, amplification, saturation of three states, the cutoff is needless to say, as long as between E and B is not conductive. We want this transistor to be saturated, which is what we call a switching feature that must satisfy a condition. Transistors have a magnification β, in order to be saturated, the B-pole current must be greater than the current value between E and C divided by β. This beta, for commonly used transistors, is probably considered to be 100. Then we have to calculate the resistance of the R47.

Just now we have counted, the electric current between E and C is 3mA, then the minimum B-pole current is 3mA divided by 100 equals 30uA, probably 4.3V voltage will fall on the base resistor, then the maximum base resistance is 4.3v/30ua = 143K. Resistance value as long as this value is small, of course, it can not be too small, too small will lead to the single-chip IO port current is too large burned transistor or single-chip microcomputer, STC89C52 IO port input Current maximum theoretical value is 25mA, I recommend not more than 6mA, we use voltage and current calculation, we can calculate the minimum resistance value , we take the empirical value in Figure 3-7.