Fabrication of a dual-purpose variable DC voltage-stabilized power supply circuit

The working principle of the circuit is shown in the figure below. The biggest advantage of this circuit is that it uses two three-terminal adjustable voltage regulators LM317 and LM337. During maintenance use, when the switch K is turned to position "1", the AC voltage of 17.5V×2 is reduced from the secondary of the power transformer T, and is sent to the input terminals of LM317 and LM337 respectively after rectification by VD1~VD4, and then controlled by sampling resistors R1, R2 and output voltage adjustment potentiometers RPl, RP2, a continuously adjustable voltage of ±1.25~15V can be obtained at the output terminal. When the switch K position is turned to "2", the power transformer T group with double output groups is used as a single group, and is only sent to LM317 after rectification and filtering to obtain a voltage of +1.25~30V. Single power supply 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 across R1 and R2 of the sampling circuit; C7 and c8 are designed to prevent the self-excitation phenomenon that occurs when the output load is capacitive; VD5 and VD7 are used to prevent the discharge current of c7 and c8 from damaging the three-terminal voltage regulator when a short circuit occurs at the output/input terminal; VD6 and VD8 are used to prevent the discharge current of C5 and C6 from damaging the three-terminal voltage regulator when a short circuit occurs at the output terminal.

The pin functions of LM317 and LM337 are shown in Figure 2. All capacitors in this circuit are selected to have a withstand voltage greater than 50V. The power consumption of the 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 AC noise.