Blog Two: Adding a UPS to the System

The Raspberry Pi Zero is currently powered by a five volt supply which plugs into mains power. If the mains power fails, power to the Pi would be lost. In this blog we will learn how to power the panel and Raspberry Pi Zero from a UPS, or uninterruptable power supply.

Figure 1 Battery UPS

In figure 1 above, a 12 volt battery is connected to J1 and a 13.5V AC to DC power supply is connected to J2. Switching power supply U1 is a five volt six amp voltage regulator and voltage regulator U2 is a 12 volt 5 amp voltage regulator. Switch SW1 is the power on / off switch which switches the regulators off by grounding the control pin of the regulators. This switch can be any toggle or rocker switch. Switch SW2 shorts out D1 so that the 13.5 volts can charge the battery. This switch should be capable of switching about 3 amps. To charge the battery, first turn off the regulators with SW1 and then turn on the charge switch SW2. Battery voltage should be checked under load (SW2 open) from time to time and after a power failure.

The circuit has two outputs. Connector J3 supplies twelve volt power to the system as well as a five volt auxiliary output. The remaining connector is a USB A female connector. This connector allows the Raspberry Pi to be connected to the UPS with a standard Raspberry Pi power cable.

The charging circuit is designed to charge a sealed lead acid battery only.