Simple USB UPS

This is an very simple circuit to act as a Uninterruptible Power Supply for a usb powered device. I'm using it to prevent reset of Raspberry Pi's during short power flickers caused by high motor current draws on a battery powered system. The circuit I'm using is able to maintain full power to a Raspberry Pi with usb peripherals (a laser mouse and a keyboard) for about 80 seconds after main power loss.

Step 1: Build It!

Parts Required:
- micro USB power cable

- Three (3) 2.3V rated 50 Farad Supercapacitors (this design omits balancing resistors as it relies upon capacitor leakage. If low leakage caps are used, balancing resistors are advised)

- One (1) 10kOhm resistor

- One (1) 4.7V Super Bright LED

- Soldering Iron

- Vinyl Tape

The three supercapacitors were connected in series effectively creating a 6.9V rated 16.33F capacitor and then the LED in series with the resistor were connected in parallel with that. Be mindful of polarity of the LED and the supercaps. Reverse voltage applied to the supercaps and quickly destroy them.

Next a normal usb power cable is cut in the center and the overall jacket and/or shield is striped back two inches on both ends revealing four (4) individual insulated conductors. The red insulated conductor is +Vdc and the black insulated conductor is -Vdc. The blue and white conductors are signal wire. Solder the striped ends of the two red conductors (one from each half of the usb cable) to the positive (long lead side) of the supercap circuit and the two black ends to the negative (short lead side) of the circuit. Tape or reconnect the blue and white conductors.

Solder each connection then wrap each bare conductor and the whole assembly in electrical tape except leaving only the LED exposed. Then the device is complete.

Connect it as you would any normal usb power cable (this is not recommended for devices that use both the signal and the power conductors). Note that there can be quite a lag between the time the cable is connected and it is able to power a device, it may take up to a minute or so for the capacitors to charge up to 5V, the LED will provide indication of charge on the capacitors. Another note: If the fully charged cable is disconnected from power (i.e. unplugged) the LED will remain on until the stored potential drops below its shutoff threshold, this could be quite some time, but is nothing to be concerned about.