(note) Understanding of the Basic circuit unit of Circuit design (15)

A circuit diagram usually has dozens of or even hundreds of, their lines cross, the form is changeable, beginners often do not know where to start, how to read it. In fact, the electronic circuit itself has a strong regularity, no matter how complex the circuit, after analysis can be found, it is composed of a few unit circuit. As if the children play the building blocks, although only a dozen or twenty or thirty kinds of blocks, but in the hands of children can be made of dozens of or even hundreds of kinds of graphic or stereoscopic model. The same reason, the complex circuit, after analysis can be found, it is also composed of a few unit circuit. So as long as beginners first familiar with the basic unit circuit, and then learn to analyze and decompose the circuit's ability to understand the general circuit diagram should be not difficult.

According to the function of the Unit circuit can divide them into several classes, each class has many kinds, all the unit circuit probably always have hundreds of kinds. Below we select the most commonly used basic unit circuit to introduce. Let's start with the power circuit.

First, the function and composition of power circuit

Each electronic device has a power supply circuit that supplies energy. Power supply circuit has rectifier power supply, inverter power and frequency converter three kinds. Most of the common household appliances use DC power. The simplest way to power a DC supply is to use a battery. However, the battery has a high cost, large size, need to change from time (battery is often charged) shortcomings, so the most economical and reliable and convenient is the use of rectifier power.

The power supply in the electronic circuit is generally low-voltage direct current, so in order to convert from 220 volts to DC power, we should first turn the 220 vac into a low voltage alternating current, then use the rectifier circuit to become pulsating DC power, and finally filter out the pulsating DC power in the AC components to get direct current. Some electronic equipment on the quality of the power requirements are very high, so sometimes need to add a voltage regulator circuit. Therefore, the composition of the rectifier power supply generally has four parts, see Figure 1. wherein the transformer circuit is actually an iron core transformer, need to introduce is only the back three kinds of unit circuit.

Second, rectifier circuit

A rectifier circuit is a circuit that uses the unidirectional conductivity of a semiconductor diode to turn alternating current into a unidirectional pulsating DC power.

(1) Half-wave rectification

A half-wave rectifier circuit requires only one diode, as shown in Figure 2 (a). In alternating current half postmenstrual VD conduction, negative half postmenstrual VD cutoff, the load R is the pulsating DC power

(2) Full-wave rectification

Full-wave rectification requires two diodes, and the transformer is required to have two laps of the same secondary coil with a center tap, as shown in Figure 2 (b). The load R L is obtained is the pulsating full-wave rectifier current, the output voltage is higher than the half-wave rectifier circuit.

(3) Full-wave bridge rectifier

A bridge rectifier circuit consisting 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 the full-wave rectifier circuit.

(4) Voltage doubling rectifier

A higher DC voltage can be obtained by using multiple diodes and capacitors. Figure 2 (d) is a twice times voltage rectifier circuit. When the U2 is negative semi-postmenstrual VD1 conduction, C1 is charged, the maximum voltage on the C1 can be close to 1.4u2; When U2 is half postmenstrual VD2 conduction, C1 voltage and U2 are stacked together to charge the C2, so that the C2 voltage is close to 2.8U2, which is twice times the voltage on the C1, To call the voltage Doubler rectifier circuit.

Third, filter circuit

After rectification, a pulsating direct current is obtained, and if a filter circuit is used to filter out the alternating current in the pulsating DC, a smooth DC power can be obtained.

(1) Capacitor filter

The capacitor and the load in parallel, 3 (a), the positive half-week when the capacitance is charged, the negative half-week capacitor discharge, you can make the load smooth DC power.

(2) Inductance filter

The inductor and the load are connected in series, 3 (b), can also filter out the AC components in the ripple.

(3) L, C filter

A filter circuit consisting of 1 inductors and a capacitor is called an L-shape because it resembles an inverted letter "L", as shown in Figure 3 (c). A filter circuit with 1 inductors and 2 capacitors because the letter "π" is called the π type, see Figure 3 (D), which is a better filter circuit.

(4) RC filter

The cost of the inductor is high, the volume is large, so in the current is not too large electronic circuit commonly used resistors replace the inductor and composed of RC filter circuit. Similarly, it also has an L-type, see Figure 3 (E); π type, see Figure 3 (F).

Four, voltage regulator circuit

The fluctuation of AC power grid voltage and the change of load current will change the output voltage and current of the rectifier power supply, so the high demand of the electronic circuit must use the regulated power supply.

(1) Voltage-stabilized tube shunt voltage regulator Circuit

A voltage regulator and load parallel circuit is the simplest voltage regulator circuit, see Figure 4 (a). In the diagram, R is the current limiting resistor. The output current of this circuit is very small, and its output voltage is equal to the stable voltage value V Z of the regulated tube.

(2) Series-Type voltage regulator circuit

A series-type voltage regulator with amplification and negative feedback is the most commonly used voltage regulator circuit. Its circuit and block diagram are shown in Figure 4 (b), (c). It is detected from the sampling circuit (R3, R4) of the output voltage changes, compared with the reference voltage (V Z) and amplified by the amplifier (VT2) added to the adjustment tube (VT1), so that the voltage at both ends of the adjustment tube changes. If the output voltage drops, so that the adjustment tube pressure drop is also reduced, so the output voltage is raised, if the output voltage rises, so that the adjustment tube pressure drop also rise, so the output voltage is depressed, the result is the output voltage basically unchanged. In this circuit based on the development of a number of variants of the circuit or to add some auxiliary circuit, such as the use of composite pipe as adjustment tube, output voltage adjustable circuit, with op amp for the comparison of amplified circuit, as well as the addition of auxiliary power and overcurrent protection circuit.

(3) switch-type voltage regulator circuit

In recent years, the new regulated power supply is widely used. Its adjustment tube work in the switching state, its own power consumption is very small, so there are high efficiency, small size and other advantages, but the circuit is more complex.

There are many kinds of switching regulator power supply from the principle. A schematic diagram of its rationale is shown in Figure 4 (d). The inductance L and capacitor C in the figure are energy storage and filter elements, and the diode VD is a freewheeling diode that provides a current path for the L and C filters when the tube is in the off state. Switching power supply switching frequency are very high, generally for a few ~ tens kilometers khz, so the volume of the inductor is not very large, the output voltage of the high harmonic is not much.

Its basic working principle is: from the sampling circuit (R3, R4) to detect the sampling voltage by comparison to control a rectangular wave generator. The output pulse of the rectangular wave generator is the conduction and cutoff time of the control Adjustment tube (VT). If the output voltage U 0 due to the fluctuation of the grid voltage or load current, the output pulse of the rectangular wave generator will become wider, so the adjustment tube on-time increase, so that the L, C energy storage circuit to obtain more energy, the result is to make the output voltage U 0 is promoted to achieve a stable output voltage.

(4) Integrated voltage regulator circuit

In recent years, there have been a large number of integrated voltage regulator products, many varieties, the structure is also different. At present, there are more than three-terminal integrated voltage regulator, the output positive voltage of the CW7800 series and output negative voltage of the CW7900 series and other products. Output current from 0.1A to 3 A, the output voltage is 5V, 6V, 9V, 12V, 15V, 18V, 24V and many other.

This integrated voltage regulator has only three terminals, and all parts of the regulator circuit, including high-power adjustment tubes and protection circuitry, are integrated in the chip. When used, just add the heat sink and then receive the rectifier filter circuit behind the line. Less peripheral components, high voltage regulation accuracy, reliable work, generally do not need to debug.

Figure 4 (e) is a three-terminal regulator circuit. The figure of C is the main filter capacitance, C1, C2 is to eliminate the parasitic oscillation of the capacitance, VD is to prevent the input short-circuit to burn the integrated block and the use of protective diodes.

Five, power circuit reading the main points and examples

The power circuit is the most widely used circuit in the electronic circuit, but it is very simple. Get a power circuit diagram, should: ① first Press "rectification-filter-regulator" sequence of the entire power circuit decomposition, step by step analysis. ② the main circuit and auxiliary circuit, the main component and the secondary component in the stepwise analysis, the function and parameter requirements are clarified. For example, in switched-regulated power supplies, inductance and freewheeling diodes are key components. ③ because transistors have both NPN and PNP types, some ICS require dual power supply, so a power circuit often includes different polarity voltages and several sets of outputs. The values and polarity of the output voltages of each group must be distinguished when reading the graph. The polarity of transistors and electrolytic capacitors must be carefully distinguished during assembly and maintenance to prevent errors. ④ is familiar with certain customary and simplified representations. ⑤ finally integrated the entire power supply circuit from front to back comprehensively. This power circuit is also understood.

Example 1 electric blanket temperature control circuit

Figure 5 is a electric blanket circuit. The switch in the "1" position is a cryogenic file. 220 kv mains through the diode after the electric blanket, because it is half-wave rectification, the electric blanket at both ends of the add is about 100 volts of pulsating dc power, heat is not high, so is the insulation or low temperature status. Switch to the "2" position, 220 kv mains directly to the electric blanket, so it is a high temperature file.

Example 2 high-voltage electronic mosquito-killing device

Figure 6 is the use of the principle of double-voltage rectifier to obtain low-current high-voltage power of the mosquito-killing device. 220 volts ac after four times times the voltage rectifier output voltages up to 1100 volts, the DC high voltage added to the parallel wire online. NET down bait, when the flies stop on the net caused short-circuit, the high pressure on the capacitor through the fly body discharge to kill the flies. When the body of the fly falls, the capacitor is recharged and the grid is back high. This high voltage grid current is very small and therefore harmless to people.

Since insects have phototaxis at night, it is possible to trap mosquitoes and harmful insects by placing a 3-watt fluorescent or small black light on the back of the grid.

Example 3 practical regulated power supply

Figure 7 is a practical regulated power supply. Output voltage 3 to 9 volts adjustable, output current Max 100 ma. This circuit is a series-regulated power supply circuit. Note that: The ① Rectifier Bridge and Figure 2 (c) is different, in fact, it is a bridge-type rectifier circuit. ② This circuit uses the PNP type germanium tube, so the output is negative pressure, positive grounding. The ③ uses two ordinary diodes instead of the regulator tube. The forward pressure drop of any diode is essentially constant, so a diode can be used instead of a regulator. The forward pressure drop of the 2AP diode is about 0.3 volts, the 2CP type is about 0.7 volts, and the 2CZ type is about 1 volts. Two 2CZ diodes are used as reference voltages in the figure. The ④ sampling resistor is a potentiometer, so the output voltage is adjustable.

(note) circuit design (15) Basic Circuit Unit understanding