Saturday, March 14, 2015

DIY mini-UPS for charging phones or running wifi router or Raspberry Pi

Currently Pakistan is going through an energy crisis. Consequently I am faced with frequent hour long power outages. I have a largish UPS for running an assortment of electrical appliances but I am always vary of using it to power DC electronics.

This is where this DIY mini-UPS comes in handy. It runs off a 12V motorcycle battery and is charged using a standard 12V DC power adapter. It is capable of powering either a standard wireless router (that requires 12V DC), a Raspberry Pi or charging a phone.

The switching from line voltage to battery voltage is fast enough that the wireless router doesn't stutter and the Raspberry Pi keeps running without a hitch.

I followed the instructions from this excellent tutorial which also gives a wonderful explanation for how the circuit works. I did make a few modifications of my own, simplifying the circuit to fit my needs. In particular I connected the DC output directly to the input because the voltage drop across both the 1N4007 diode and the TIP 127 was unacceptable when running off of the battery. The modified circuit is:

(the circuit diagram was drawn using circuitikz package in LaTeX. Here is the how-to)

And here is the end-product, shifted to a perf-board after being tested on a bread-board:
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