DIY: Maintenance and charging dual-purpose voltage-stabilized power supply circuit diagram

　　For home appliance maintenance personnel, a maintenance power supply is essential. However, a professional maintenance power supply is quite expensive. If the power supply is properly improved, a stable power supply that is inexpensive, easy to use, and reliable in quality for maintenance and charging can be made. This stable power supply is introduced to readers for reference.

　　For convenience, the power supply is designed into two independent low-voltage and high-voltage outputs, as shown in the figure. Among them, the low-voltage output is dedicated to the maintenance of low-voltage electrical appliances (such as tape recorders, walkmans, semiconductor radios, etc.) and the charging of batteries and rechargeable batteries; the high-voltage output is dedicated to the maintenance of color TVs. Take the low-voltage output as an example to explain its working principle. The 32V voltage of the secondary side of the transformer is rectified by VD1~VD4 and filtered by C1 to obtain a DC voltage of about 45V. This voltage is passed through the integrated three-terminal voltage regulator LM338K to obtain a stable output voltage. By adjusting the potentiometer RP1, a stable DC voltage of 1.25V~37V can be obtained at the output end. In the figure, LED1 is the indicator light of the voltage-stabilized power supply, R1 is the current-limiting resistor of LED1, and C2 is used to filter out the high-frequency harmonic components of the power supply and eliminate parasitic oscillations. VD5 and VD6 are designed to prevent short circuits at the input and output ends and protect LM338K.

　　According to the characteristics of LM317T, the input voltage should be increased and the adjustment terminal resistance should be appropriately increased so that the maximum voltage difference between the input and output terminals does not exceed 40V to avoid damaging LM317T. According to the power supply voltage and component values ​​in the figure, a DC output of 90V to 130V and a current of not less than 1A can be obtained from the output terminal. VD13 is designed to prevent the reverse charging of the capacitor in the color TV from damaging LM317T. The circuit composed of VD14, LED2 and R5 is both an indicator light of the high-voltage power supply and a circuit designed to maintain the minimum working current of LM317T.

　　When selecting components, LM338K must be genuine. The maximum output current of the voltage regulator can reach 5A. It is best to wind the power transformer yourself, with a power of not less than 180W. The low-voltage secondary current is greater than 5A, and the high-voltage secondary current is not less than 1A. The rectifier diode can use 2CZ57D or an imported 200V/6A rectifier full bridge. The DC current meter head uses the 85C1 type 5A meter head, the low-voltage meter head uses the 85C1 type 50V meter head, and the high-voltage meter head uses the 85C1 type 150V meter head. In addition, LM338K and LM317T should be installed with a sufficiently large heat sink. RP2 and S2 in the low-voltage power supply are set for charging No. 5 and No. 7 batteries. RP2 can use a multi-turn rotary potentiometer. In actual use, in order to prevent long-term high-current charging from overheating and damage to the transformer and voltage regulator, a DC instrument fan can be connected and placed in an empty space inside the machine to dissipate heat.