LM317 adjustable DC stabilized dual power supply circuit

The dual-purpose variable DC stabilized power supply is an essential maintenance instrument for radio enthusiasts. Here we introduce an output of ±1.25V~15V or +1.25V~30V, with an output current of about 1.5~2A. It's easy to make, convenient and hassle-free to use.

The working principle of the circuit is shown in the figure below. The biggest advantage of this circuit is the use of two three-terminal adjustable voltage regulator blocks LM317 and LM337. During maintenance and use, when the switch K is turned to position "1", the secondary of the power transformer T is reduced to an AC voltage of 17.5V×2, which is rectified by VD1~VD4 and sent to the input terminals of LM317 and LM337 respectively. Through the control of the sampling resistors R1 and R2 and the output voltage adjustment potentiometers RP1l and RP2, a continuously adjustable voltage of ±1.25~15V can be obtained at the output end. When the selector switch K is set to "2", the dual-output power transformer group T is used as a single group. After rectification and filtering, it is only sent to the LM317 to obtain a voltage of +1.25~30V. Single power output, so it can be easily used in maintenance circuits that require a single power supply or a slightly higher voltage.

R1 and R2 in the circuit are used to protect the load current of not less than 5mA; C5 and C6 are bypass capacitors designed to reduce the ripple voltage at both ends of R1 and R2 of the sampling circuit; C7 and C8 are designed to prevent The self-excitation phenomenon occurs when the load at the output terminal is capacitive; VD5 and VD7 are used to prevent the discharge current of c7 and c8 from damaging the three-terminal voltage regulator block when the output/input terminal is short-circuited; VD6 and VD8 are used to prevent the output terminal from being damaged by the discharge current. When a short circuit occurs, the discharge current of C5 and C6 damages the three-terminal voltage regulator block.

The pin functions of LM317 and LM337 are shown in Figure 2. All capacitors in this circuit are components with a withstand voltage greater than 50V. The power consumption of two three-terminal adjustable voltage regulator blocks is about 15W.

Remember to add enough heat sinks, and the dotted line of the transformer center tap must be firmly grounded to prevent interference from hum.