LM317 regulated power supply

This voltage-stabilized power supply can generate 3V-12V DC voltage for laboratory use.

The 220V AC voltage is stepped down and rectified into a 21V pulse DC voltage, which is filtered by a 470μF capacitor C1, and C2 is used to bypass the high frequency.

The chip LM317T (IC1) is a series three-terminal adjustable positive voltage regulator, which can provide a current of not less than 1.5A in the range of 1.2V-37V. However, in this circuit, only the 5-speed switch S2 is used to generate the 3V, 5V, 6V, 9V and 12V voltages. This can be achieved by using different resistance values ​​between pin 1 of IC1 and ground.

When R2 between pins ① and ② of IC1 is 220Ω, the output voltage can be calculated as follows: Vo=1.25(1+Rx/R2), where Rx is the resistance between pin ① of IC1 and ground. When S2 is in the "12V" (disconnected) position, the equivalent series resistance Rx=R3+R4=1900Ω. In other positions, Rx is equal to the parallel value of 1900Ω and the resistance of the selected position of S2. The attached table lists the Rx values ​​of S2 in various voltage positions.

The voltage regulator chip LM317T should be equipped with a heat sink. Assuming the output voltage is 3V and the load current is 1.5A, the voltage drop across IC1 is close to 10V. At this time, the power consumption of IC1 is 10×1.5=15W. To dissipate so much heat, you must use a heat sink of 4cmx10cm. Keep the minimum voltage difference between IC1 input and output at 3-4V. Basically, the output voltage can be stabilized.

Since the LM317T has short-circuit protection, its output does not require a fuse.