Let's talk about the detailed explanation of 7 application circuits of diodes[Copy link]
Many beginners are very "familiar" with diodes. When talking about the characteristics of diodes, they can blurt out its unidirectional conductive characteristics. When talking about its application in circuits, the first reaction is rectification. They don't know much about other characteristics and applications of diodes. They also think that if they master the unidirectional conductive characteristics of diodes, they can analyze various circuits in which diodes participate. In fact, this idea is wrong and to some extent, it harms themselves, because this directional thinking affects the analysis of the working principles of various diode circuits. Many diode circuits cannot use unidirectional conductive characteristics to explain their working principles. In addition to the unidirectional conductive characteristics, diodes have many other characteristics. In many circuits, the working principles of the circuits composed of diodes cannot be analyzed by using the unidirectional conductive characteristics. Instead, more characteristics of diodes need to be mastered to correctly analyze these circuits, such as the simple DC voltage regulator circuit composed of diodes, the temperature compensation circuit composed of diodes, etc. 9.4.1 Simple DC voltage regulator circuit with diode and troubleshooting Simple voltage regulator circuit with diode is mainly used in some local DC voltage supply circuits. It is widely used because of its simple circuit and low cost. The simple voltage regulator circuit with diode mainly utilizes the characteristic that the tube voltage drop of diode is basically unchanged. The tube voltage drop characteristic of diode: after the diode is turned on, its tube voltage drop is basically unchanged. For silicon diode, this tube voltage drop is about 0.6V, and for germanium diode, it is about 0.2V. As shown in Figure 9-40, it is a simple DC voltage regulator circuit composed of three ordinary diodes. VD1, VD2 and VD3 in the circuit are ordinary diodes. They are connected in series to form a simple DC voltage regulator circuit.
1. Circuit analysis ideas It is difficult to analyze the working principle of a circuit that you have never seen before, and it is even more difficult for beginners who do not have comprehensive basic knowledge. The main analysis ideas of this circuit are as follows. (1) It can be seen from the circuit that three diodes are connected in series. According to the characteristics of the series circuit, if these three diodes are turned on, they will be turned on at the same time, and if they are turned off, they will be turned off at the same time. (2) According to the judgment principle of whether the diode is turned on, the positive pole of the diode is connected to a voltage that is much higher than the negative pole. Whether it is a DC or AC voltage, the diode is in the on state at this time. It can be seen from the circuit that the positive electrode of VD1 is connected to the DC working voltage +V in the circuit through resistor R1, and the negative electrode of VD3 is grounded, so that a sufficiently large forward DC voltage is applied to the three series diodes. From this analysis, it can be seen that the three diodes VD1, VD2 and VD3 are turned on under the action of the DC working voltage +V. (3) It can also be seen from the circuit that no AC signal voltage is added to the three diodes, because a large-capacity capacitor C1 is connected between the positive electrode of VD1, that is, point A in the circuit, and the ground, bypassing any AC voltage at point A to the ground. 2. Explanation of the principle that diodes can stabilize DC voltage In the circuit, the three diodes are turned on under the forward bias of the DC working voltage. After being turned on, the effect on this circuit is to stabilize the DC voltage at point A in the circuit. As we all know, the structure of a diode is a PN junction. In addition to the unidirectional conductive property, the PN junction has many other properties. One of them is that the voltage drop of the diode remains basically unchanged after it is turned on. For the commonly used silicon diode, the voltage drop between the positive and negative electrodes after it is turned on is 0.6V. Based on this characteristic of the diode, it is very convenient to analyze the working principle of a simple DC voltage regulator circuit composed of ordinary diodes. After the three diodes are turned on, the voltage drop of each diode is 0.6V, so the DC voltage drop after the three diodes are connected in series is 0.6×3=1.8V. 3. Fault detection method The most effective way to detect the three diodes in this circuit is to measure the DC voltage on the diodes. Figure 9-41 shows the wiring diagram for measurement. If the measured DC voltage result is about 1.8V, it means that the three diodes are working properly; if the measured DC voltage result is 0V, it is necessary to measure whether the DC working voltage +V is normal and whether the resistor R1 is open circuit. It has nothing to do with the three diodes because the possibility of the three diodes breaking down at the same time is small; if the measured DC voltage result is greater than 1.8V, check whether one of the three diodes has an open circuit fault.
5. Detailed description of circuit analysis The detailed description of the analysis of the simple DC voltage regulator circuit with diodes is as follows. (1) In circuit analysis, the unidirectional conductivity of diodes can be used to know that the diodes are in the on state, but it cannot explain what specific effect these diodes have on the circuit after they are turned on. Therefore, it is not possible to correctly analyze the working principle of the circuit using only the unidirectional conductivity characteristics. (2) Among the many characteristics of diodes, only the characteristic that the voltage drop of the tube remains basically unchanged after being turned on can most reasonably explain the function of this circuit. Therefore, based on this point, it can be determined that this circuit is to stabilize the DC working voltage at point A in the circuit. (3) When there are multiple components in the circuit, you must try to figure out the main components that realize the function of the circuit, and then conduct analysis around it. Use the main characteristics of the component in the analysis to make a reasonable explanation.