Design and production of practical small regulated power supply

1. Circuit working principle

The regulated power supply circuit composed of the three-terminal adjustable positive voltage integrated voltage regulator LM317 is shown in the figure below. In the figure, the 220V alternating current is sent to the primary coil of the transformer through the fuse, and an AC voltage of about 9V is induced from the secondary coil and sent to a bridge rectifier composed of four diodes. The relatively stable DC power filtered by Cl is sent to the Vin terminal (pin 3) of the three-terminal voltage stabilizing integrated circuit LM317. After LM317 is provided with operating voltage by the Vin terminal, it can keep the voltage of its +Vout terminal (pin 2) 1.25V higher than the voltage of its ADJ terminal (pin 1). Therefore, we only need to use a very small current to adjust the voltage of the ADJ terminal, and then we can get a relatively large current output at the +Vout terminal, and the voltage is a constant 1.25V higher than the ADJ terminal. You can also change the output voltage by adjusting PR1. Anyway, LM317 will ensure that the voltage on the resistor connected to the ADJ terminal and +Vout terminal is 1.25V, so you can think of: when PR1 slides upward, the output voltage will increase.

The function of D5 in the figure is to prevent the LM317 from being damaged by excessive voltage from C3 when an unexpected situation causes the voltage of pin 3 of LM317 to be lower than the voltage of pin 2.

2. Component selection

The IC should be LM317 or other models with the same functions. The transformer can be a common 9N12V small transformer, and the diode can be 1N4001-1N4007. Cl can choose electrolytic capacitors with a withstand voltage greater than 16V and a capacity of 470~2200μF. C2 can use ordinary ceramic capacitors. The choice of C3 is similar to Cl. The resistor should be a small resistor of 1/8W.

3. Make printed circuit boards

Use ProtELDXP2004SP2 software to draw the schematic diagram, generate the printed circuit board diagram from the schematic diagram, and use the thermal transfer method to produce the printed circuit board. Use a single-sided copper-clad board with a size of 100×50mm. The reference picture is as shown below (omitted).

4. Installation and debugging

When assembling, pay attention to the polarity of diodes and electrolytic capacitors.

Because the operating current of LM317 is small, it does not need to add a heat sink. After installation, check again and turn on the power if everything is correct. At this time, use a multimeter to measure the voltage at both ends of Cl. There should be a voltage of about 11V. Then measure the voltage at both ends of C3. There should be a voltage of 2-7V. Then adjust PR1, the voltage across C3 should be able to change, just adjust it to the voltage you need. The output end can be connected to a cross plug wire to connect it to electrical appliances such as a Walkman.

5. Extended application The output voltage of LM317 can be continuously adjusted from 1.25V to 37V. Its output voltage can be calculated by the following formula:

Output voltage = 1.25×(1+resistance from ADJ terminal to ground/resistance from ADJ terminal to +Vout terminal).

If you need other voltage values, you can choose to change the resistance of the relevant resistor to get it. It is worth noting that LM317T has a minimum load current problem, that is, it can only stabilize the voltage when the load current exceeds a certain value. This current varies from 3 to 8 mA depending on the manufacturer of the device. This can be solved by connecting a suitable resistor to the load terminal.