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12V Uninterruptible Power Supply

A simple and fully automatic, 12V uninterruptible power supply system for small/medium power appliances.

This is a simple and fully automatic, 12V uninterruptible power supply system for small/medium power appliances. This power supply specifically designs to drive routes and modems to provide uninterrupted internet and telephone services in areas affected by frequent power failures.

This 12V uninterruptible power supply initially designs to drive my fiber optic modem/router. The key reason to build this power supply is to get continuous internet and phone connection during power failures. Core components of this power supply are a constant voltage charger, 12V DC power supply, AC line monitoring unit, and 12V high capacity sealed lead-acid battery. The entire system designs using locally available components.

The charging circuit of this system builds around using the popular LM350 voltage regulator. This regulator calibrates to provide both 14.4V fast charging and 13.6V trickle charging. Based on the condition of the battery, the MCU will determine the appropriate charging mode.

At the online state, the LM2576-12 switching regulator provides 12V output to the driving system. (in my arrangement, a fiber optic router).

The PIC16F688 MCU monitors the AC line and battery to controls the output voltage of the LM350 regulator and the output relay. The firmware of this MCU written using the Microchip XC8 compiler, and all the source code and configuration files are available to download at the project repository at GitHub.

The sealed 12V lead-acid battery is the most crucial component of this unit. 12V, 9Ah battery is the most recommended for this system. At the time of this writing, this battery cost around Rs. 3500 (US$ 17.60), and I bought it from a local online store.

In online mode, this system provides 12V output. In offline mode, if it has a fully charged battery, it delivers an output of 12.5V.

This uninterruptible power supply is suitable for appliances that need 12V - 13V input with a maximum of 2A current. With the configuration described above, this system continued to power my Huawei HG8245H5 fiber optic router for more than 7 hours.

This power supply unit is an open hardware project. All the schematic, PCB design files, firmware source codes, and compiled binaries are available to download at the project repository at GitHub.”

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