Teach you to easily read the circuit diagram in the power supply circuit

When beginners are faced with circuit diagrams with dozens or even hundreds of components, and their connections crisscross and vary in form, they often feel at a loss. How can they easily understand them?

Don't be afraid. In fact, electronic circuits have strong regularity. No matter how complex the circuit is, after analysis, it can be found that it is composed of a few unit circuits. Just like the building blocks that children play with, although there are only a dozen or twenty or thirty kinds of blocks, they can be built into dozens or even hundreds of plane figures or three-dimensional models in the hands of children. Similarly, no matter how complex the circuit is, after analysis, it can be found that it is also composed of a few unit circuits. Therefore, as long as beginners are familiar with the commonly used basic unit circuits first, and then learn the ability to analyze and decompose circuits, they will get twice the result with half the effort.

Unit circuits can be divided into several categories according to their functions. Each category has many types, and there are probably hundreds of unit circuits in total. Below we will introduce the most commonly used basic unit circuits. Let's start with the power supply circuit.

1. Function and composition of power supply circuit

Every electronic device has a power supply circuit that supplies energy. There are three types of power supply circuits: rectifier power supply, inverter power supply and frequency converter. Most common household appliances use DC power supply. The simplest way to supply DC power is to use batteries. However, batteries have the disadvantages of high cost, large size and need to be replaced from time to time (storage batteries need to be charged frequently), so the most economical, reliable and convenient way is to use rectifier power supply.

The power supply in electronic circuits is generally low-voltage direct current. Therefore, if you want to convert 220V AC into direct current, you should first convert 220V AC into low-voltage AC, then use a rectifier circuit to convert it into pulsating direct current, and finally use a filter circuit to filter out the AC component in the pulsating direct current to get direct current. Some electronic equipment has very high requirements for the quality of the power supply, so sometimes it is necessary to add a voltage stabilization circuit. Therefore, the composition of the rectifier power supply generally consists of four parts, as shown in Figure 1. Among them, the voltage conversion circuit is actually an iron core transformer, and only the latter three unit circuits need to be introduced.

Figure 1 The four major components of the rectifier circuit

2. Rectification circuit

A rectifier circuit is a circuit that uses the unidirectional conductivity of a semiconductor diode to convert alternating current into unidirectional pulsating direct current.

(1) Half-wave rectification

The half-wave rectifier circuit only needs one diode, as shown in Figure 2 (a). VD is turned on during the positive half cycle of the AC power and turned off during the negative half cycle, and the load R receives pulsating DC power.

Figure 2 Rectifier power supply

(2) Full-wave rectification

Full-wave rectification requires two diodes and requires the transformer to have two secondary coils with the same number of turns and a center tap, as shown in Figure 2 (b). The load RL receives a pulsating full-wave rectified current, and the output voltage is higher than that of the half-wave rectifier circuit.

(3) Full-wave bridge rectifier

The bridge rectifier circuit composed of 4 diodes can use a transformer with only a single secondary coil, as shown in Figure 2 (c). The current waveform and output voltage value on the load are the same as those of the full-wave rectifier circuit.

(4) Voltage doubler rectification

A higher DC voltage can be obtained by using multiple diodes and capacitors. Figure 2 (d) is a voltage doubling rectifier circuit. When U2 is in the negative half cycle, VD1 is turned on, C1 is charged, and the highest voltage on C1 can be close to 1.4U2; when U2 is in the positive half cycle, VD2 is turned on, and the voltage on C1 and U2 are superimposed to charge C2, making the voltage on C2 close to 2.8U2, which is twice the voltage on C1, so it is called a voltage doubling rectifier circuit.

3. Filter Circuit

What is obtained after rectification is pulsating direct current. If a filter circuit is added to filter out the AC component in the pulsating direct current, smooth direct current can be obtained.

Figure 3 Filter circuit

(1) Capacitor filtering

By connecting the capacitor and the load in parallel, as shown in Figure 3 (a), the capacitor is charged during the positive half cycle and discharged during the negative half cycle, so that a smooth DC power can be obtained on the load.

(2) Inductor filtering

Connecting the inductor and the load in series, as shown in Figure 3 (b), can also filter out the AC component in the pulsating current.

(3) L, C filtering

The filter circuit composed of 1 inductor and 1 capacitor is called L type because it looks like an inverted letter "L", see Figure 3 (c). The filter circuit composed of 1 inductor and 2 capacitors is called π type because it looks like the letter "π", see Figure 3 (d), which is a circuit with better filtering effect.

(4) RC filtering

Inductors are expensive and bulky, so resistors are often used to replace inductors in electronic circuits with low currents to form RC filter circuits. Similarly, it also has L type, see Figure 3 (e); π type, see Figure 3 (f)

4. Voltage stabilization circuit

Fluctuations in the AC grid voltage and changes in the load current will cause the output voltage and current of the rectifier power supply to change accordingly, so electronic circuits with higher requirements must use a voltage-regulated power supply.

Figure 4 Voltage stabilization circuit

(1) Zener diode parallel voltage regulator

The simplest voltage stabilization circuit is a circuit that uses a voltage regulator tube in parallel with a load, as shown in Figure 4 (a). In the figure, R is a current limiting resistor. The output current of this circuit is very small, and its output voltage is equal to the stable voltage value VZ of the voltage regulator tube.

(2) Series voltage regulator circuit

The series voltage regulator circuit with amplification and negative feedback is the most commonly used voltage regulator circuit. Its circuit and block diagram are shown in Figure 4 (b) and (c). It detects the change of output voltage from the sampling circuit (R3, R4), compares it with the reference voltage (VZ), and adds it to the adjustment tube (VT1) after amplification by the amplifier (VT2), so that the voltage at both ends of the adjustment tube changes accordingly. If the output voltage drops, the voltage drop of the adjustment tube will also decrease, so the output voltage is increased; if the output voltage rises, the voltage drop of the adjustment tube will also increase, so the output voltage is depressed, resulting in the output voltage remaining basically unchanged. Based on this circuit, many variant circuits have been developed or some auxiliary circuits have been added, such as using a composite tube as an adjustment tube, a circuit with adjustable output voltage, a circuit using an operational amplifier as a comparison amplification, and adding auxiliary power supplies and overcurrent protection circuits.

(3) Switching type voltage regulator circuit

In recent years, the new type of voltage stabilizer widely used is the switching voltage stabilizer. Its adjustment tube works in the switching state, and its own power consumption is very small, so it has the advantages of high efficiency and small size, but the circuit is relatively complex.

There are many types of switching power supplies in principle. Its basic principle block diagram is shown in Figure 4 (d). In the figure, the inductor L and the capacitor C are energy storage and filtering elements, and the diode VD is a freewheeling diode that provides a current path for the L and C filters when the adjustment tube is in the off state. The switching frequency of the switching power supply is very high, generally several to tens of kilohertz, so the size of the inductor is not very large, and there are not many high-order harmonics in the output voltage.

Its basic working principle is: the sampling voltage is detected from the sampling circuit (R3, R4) and then compared and amplified to control a rectangular wave generator. The output pulse of the rectangular wave generator controls the on and off time of the adjustment tube (VT). If the output voltage U0 decreases due to changes in the grid voltage or load current, the output pulse of the rectangular wave generator will become wider, so the on time of the adjustment tube will increase, so that the L, C energy storage circuit can obtain more energy, and the output voltage U0 will be increased, achieving the purpose of stabilizing the output voltage.

(4) Integrated voltage regulator

In recent years, a large number of integrated voltage stabilizer products have been launched, with many varieties and different structures. Currently, the most commonly used are three-terminal integrated voltage stabilizers, such as the CW7800 series that outputs positive voltage and the CW7900 series that outputs negative voltage. The output current ranges from 0.1A to 3A, and the output voltages include 5V, 6V, 9V, 12V, 15V, 18V, 24V, etc.

This integrated voltage regulator has only three terminals, and all parts of the voltage regulator circuit, including high-power adjustment tubes and protection circuits, are integrated into the chip. When in use, just add a heat sink and connect it to the rectifier and filter circuit. There are few peripheral components, high voltage regulation accuracy, reliable operation, and generally no debugging is required.

Figure 4 (e) is a three-terminal voltage regulator circuit. In the figure, C is the main filter capacitor, C1 and C2 are capacitors for eliminating parasitic oscillation, and VD is a protection diode used to prevent the input short circuit from burning the integrated circuit.

5. Key points and examples of reading power circuit diagrams

The power supply circuit is a relatively simple circuit in electronic circuits, but it is the most widely used circuit. When you get a power supply circuit diagram, you should: ① First, decompose the entire power supply circuit in the order of "rectification-filtering-voltage regulation" and analyze it step by step. ② When analyzing step by step, you should distinguish between the main circuit and the auxiliary circuit, the main components and the secondary components, and understand their functions and parameter requirements. For example, in a switching voltage-stabilized power supply, the inductor, capacitor and freewheeling diode are its key components. ③ Because transistors are of two types, NPN and PNP, and some integrated circuits require dual power supply, a power supply circuit often includes different polarities, different voltage values ​​and several groups of outputs. When reading the diagram, you must distinguish the values ​​and polarities of each group of output voltages. When assembling and repairing, you must also carefully distinguish the polarities of transistors and electrolytic capacitors to prevent mistakes. ④ Be familiar with certain customary drawing methods and simplified drawing methods. ⑤ Finally, comprehensively connect the entire power supply circuit from front to back. This power supply circuit diagram is also understood.

Figure 5 is an electric blanket circuit. When the switch is in the "1" position, it is the low temperature gear. The 220V mains power is connected to the electric blanket through the diode. Because it is half-wave rectified, the electric blanket is supplied with about 100V pulsating direct current, which does not generate much heat, so it is in the insulation or low temperature state. When the switch is turned to the "2" position, the 220V mains power is directly connected to the electric blanket, so it is in the high temperature gear.

Figure 5 Electric blanket circuit

Figure 6 is a mosquito and fly killer that uses the voltage doubling rectification principle to obtain low-current DC high voltage electricity. After 220V AC is rectified by four times the voltage, the output voltage can reach 1100V. This DC high voltage is applied to the parallel metal wire mesh. Bait is placed under the mesh. When the fly stops on the mesh, it causes a short circuit. The high voltage on the capacitor discharges through the fly's body and kills the fly. After the fly's body falls, the capacitor is charged again and the grid resumes high voltage. This high-voltage grid has a very small current, so it is harmless to humans.

Figure 6 High voltage mosquito and fly killer circuit

Since insects are attracted to light at night, if a 3-watt fluorescent lamp or a small black light is placed behind the electric fence, it can attract and kill mosquitoes and harmful insects.

Figure 7 is a practical voltage-stabilized power supply. The output voltage is adjustable from 3 to 9 volts, and the maximum output current is 100 mA. This circuit is a series voltage-stabilized power supply circuit. It should be noted that: ① The drawing method of the rectifier bridge is different from that of Figure 2 (c). In fact, it is a bridge rectifier circuit. ② This circuit uses a PNP germanium tube, so the output is a negative voltage, and the positive pole is grounded.

Figure 7 Practical regulated power supply

③ Use two ordinary diodes to replace the voltage regulator. The forward voltage drop of any diode is basically constant, so a diode can be used to replace the voltage regulator. The forward voltage drop of a 2AP diode is about 0.3 volts, a 2CP diode is about 0.7 volts, and a 2CZ diode is about 1 volt. Two 2CZ diodes are used as the reference voltage in the figure. ④ The sampling resistor is a potentiometer, so the output voltage is adjustable.

A circuit that can amplify weak signals is called an amplifier circuit or amplifier. For example, the key component in a hearing aid is an amplifier.