Here's another power project, it's actually a copy of something I posted a while back. It's another diode OR UPS, this time meant to power a Raspberry Pi (or something else that takes 5 volts).

I found some cheap adjustable #buck converters on Amazon. The idea is to step down 12 V nominal, either from another battery, a power supply, or perhaps a solar panel, to about 7 V or so - this is labeled Vbuck in the schematic. From there current can flow into the battery, a 6 V gel cell lead acid in this case, via D1 and Rch. It can also flow through D3 into the 7805 regulator, which puts out 5 volts. If the input power to the buck converter is taken away, power flows from the battery to the 7805 through D2.

It's dirt simple, and my intent is to keep the 6 volt battery floated. Basically, pick the float voltage you want (say 6.6 V), and add the diode drop to it. This is about what you set the output of the buck converter to. The resistor Rch is just there to limit the current in case the battery gets significantly discharged - once it hits the float voltage you want, very little current will flow. Pick a low end voltage, maybe 5.5 V, and set the resistor such that the voltage drop gives you the current you want: (7-0.4-5.5)/Rch

I just used a 3.9 ohm resistor since I had it. Don't forget to make sure it can take the power, but in my case it should be around a quarter of a watt (it's a 2 watt resistor). You can also bump the voltage up of the buck converter to get a little more current if you want some more granularity.

The key is to have a buck converter you can adjust. The diodes here are big for what I'm using them for, but I had a tube of them from past dumpster diving. You could probably come up with a better circuit, utilizing a specially-designed switching supply, but this is quick and dirty and should work.

I checked voltages tonight, but may get around to trying it with an actual Pi tomorrow.

#power #electronics #UPS #backup #RaspberryPi #diodes #DiodeOr #5v