Its output voltage is:, where: VREF= 1.2v 1, component selection ①: lm3 17 integrated block, one radiator and one screw and nut; ②: A transformer from 220 volts to 12 volts and a power cord; ③: ceramic chip capacitor ③:0. 1uf, electrolytic capacitor 470uf or 1000uf, electrolytic capacitor 220uf, electrolytic capacitor 10uf, and the capacitor withstand voltage can be16v; ④: diode d1~ D6 as in 4007; ⑤: The potentiometer with resistance R 1 less than 220Ω, 200Ω and RW of 5.1kΩ can meet the output voltage adjustment range; R2 is 510Ω, which can ensure that the LED current is between 20-30mA; ⑥: A green or red LED. 2. Precautions: ① The resistance of the input stage and the output stage of the transformer can be measured by a multimeter, with the input stage having high resistance and the output stage having low resistance; The red line of a general transformer is the input stage; ②: ②: The pins of lm 3 1 7 are1(adjustment terminal), 2 (output terminal) and 3 (input terminal) from left to right respectively;

1. structural block diagram: according to the design index requirements, the regulated power supply consists of transformer, rectifier circuit, filter circuit, regulated circuit and indicator lamp circuit. The principle block diagram is shown in Figure 3- 1 below.

Figure 1 DC stabilized power supply block diagram ① Transformer: The function of transformer is to convert 220V alternating current into low-voltage alternating current required by rectifier circuit. (2) Rectifier circuit: The rectifier circuit converts the secondary voltage of the transformer into unidirectional pulsating DC by using the unidirectional conductivity of the diode. ③ Filter circuit: The function of filter circuit is to smooth waveform and convert pulsating DC into smoother DC. ④ Voltage stabilizing circuit: the output voltage of the filter circuit still fluctuates. For electronic equipment with high requirements, a voltage stabilizing circuit is needed, and the voltage output by the voltage stabilizing circuit is almost constant. Two. Integrated voltage stabilizing block voltage stabilizing circuit: most integrated voltage regulators adopt series voltage stabilizing circuits, and the composition block diagram is shown in Figure 3-3. In addition to the basic voltage regulator circuit, various protection circuits are often connected to protect the integrated voltage regulator from damage when it is overloaded.

Fig. 2 Block diagram of three-terminal integrated regulator circuit Because of its superior performance, convenient installation and debugging and wide application, it meets the requirements of this design index, so the adjustable integrated regulator circuit LM3 17 is adopted in this design. Third, the bridge rectifier circuit: (1) The structure principle of the rectifier circuit is to convert alternating current into direct current, and its basic principle is to use the unilateral conductivity of the crystal diode. The input and output waveforms of bridge rectifier circuit and signal are shown in Figure 4- 1 below. The output DC voltage is:

Figure 2 Bridge rectifier circuit and output waveform (2) Selection of main components and calculation of main parameters of bridge rectifier circuit: ① Selection of transformer According to the design index, the highest output voltage of regulated power supply is 12V, and the lowest output voltage of filter circuit is 15V. When Uo= 1.2U2, the minimum value of U2 is 12.5V, the rated output current is 300mA, and the output power of transformer is 3.75w. Considering the allowable variation range of power supply voltage is 10%, in order to keep U2= 12.5V at the lowest voltage, and The transformer can be 220 V/15 V/5 ... ② The choice of rectifier diode in the filter circuit, and the maximum average rectifier current if in the diode is usually 2 ~ 3 times larger than the load current. IF=3×300mA=900 mA, the maximum reverse voltage of diode UBR =1.414×15v = 21.21v. Considering a certain margin, a rectifier diode with current of 1.5A and withstand voltage of 50V can be selected, such as IN4007. Fourth, the filter circuit: (1) The form of the filter circuit Although the output voltage of the rectifier circuit contains a certain DC component, the pulse is large and it needs filtering to get a smoother DC voltage. Commonly used filters are C type, P type and G type. This task only studies C-type and (RC) p-type filters. Figure 4- 2 shows the waveform and output voltage of C-type filter circuit of bridge rectifier.

Figure 4- 2 The output voltage of the bridge rectifier C-type filter circuit and its waveform filter circuit is related to the filter capacitance, which is generally taken as UO =(0.9 ~ 1.4)Ui(2) Selection of filter capacitance. In order to obtain better filtering effect, the filter capacitance should meet the condition of RLC = (3 ~ 5) t/2. At this time, uo ≈1.2u2. Since the minimum output voltage of the filter circuit is 15V and the rated load current is 300mA, RL = 15/0.3A = 50ω. Let c1= 4× t/2rl = (4× 0.02s)/(2× 50ω) = 800uf, and let the nominal value of c = 1000 uF.