marinahanna1
Posts: 3
Joined: Wed Oct 21, 2015 3:21 am

Switching between two DC power supplies to power RPi

Sat Oct 31, 2015 1:32 am

I am using a raspberry pi for a school project and I am looking for a method to switch between two different power supplies. We have a solar panel that is connected to a charge regulator and the charge regulator is connected to a battery. In order to make it as efficient as possible, my group wanted to alternate between a 5V output from the charge regulator and a 5V output from a rechargeable battery. Essentially, I want the charge regulator to be the primary source of power so that it will supply the 5V all the while recharging the battery. When the solar panels aren't providing sufficient power, I want the battery to be the main power supply. Therefore, I am looking for a circuit that can monitor when the 5V from the charge regulator begins to fall and then switches to the battery.

I found this http://www.linear.com/product/LTC4353 and was wondering if this would work with what I'm trying to do.

Thanks!

GordonTGopher
Posts: 70
Joined: Sat Dec 01, 2012 11:25 am
Location: Letchworth Garden City

Re: Switching between two DC power supplies to power RPi

Sat Oct 31, 2015 10:52 am

Depends if you are looking to build something or buy something, the MoPi (https://pi.gate.ac.uk/pages/mopi.html) does what you are looking for, as a ready built solution.

Regards,

Gordon

JimmyN
Posts: 1109
Joined: Wed Mar 18, 2015 7:05 pm
Location: Virginia, USA

Re: Switching between two DC power supplies to power RPi

Sat Oct 31, 2015 3:07 pm

You may be able to use a Schottky Barrier Rectifier, it's also referred to as a Switch-mode Power Rectifier. I have used them as a fail safe for radio controlled planes so it would automatically switch to a backup battery/BEC if voltage on the main battery dropped too low or the BEC failed. Since it uses Schottky diodes it switches very fast, I can unplug the main battery pack while it's running, and it will instantly switch to the backup without the radio receiver restarting or glitching. Should work for a RPi without it rebooting.

Basically it has two inputs, and one output, it's an "OR"ing setup. The output will always be supplied by the input with the highest voltage. It is a diode so it will drop some voltage across it, but Schotty's are in the .15V to .45V range rather than the .6V+ of a conventional diode. But at low current such as the RPi you're probably looking at the low end of the range. They're cheap, you shouldn't have trouble finding them for 50 cents or less, so not much is lost if it doesn't work.

You'll need to look around and select one with the lowest voltage drop that you can find. Or if you can't find a Schottky Barrier Rectifier IC with suitable ratings you could look for an individual Schottky diode that will work and use two of them together to form your own switch mode rectifier. It's pretty simple, each of the anodes would be an input, the cathodes are tied together and become the output, but they do have to be identical Schottky diodes.

marinahanna1
Posts: 3
Joined: Wed Oct 21, 2015 3:21 am

Re: Switching between two DC power supplies to power RPi

Sat Oct 31, 2015 4:22 pm

GordonTGopher wrote:Depends if you are looking to build something or buy something, the MoPi (https://pi.gate.ac.uk/pages/mopi.html) does what you are looking for, as a ready built solution.

Regards,

Gordon
Is there any way to purchase these in the US? This seems to be exactly what we need, but it seems like they only sell it in the UK or Canada.

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kusti8
Posts: 3439
Joined: Sat Dec 21, 2013 5:29 pm
Location: USA

Re: Switching between two DC power supplies to power RPi

Sat Oct 31, 2015 5:24 pm

I'm pretty sure Pimoroni will ship to the US. There is no US distributor, but I've bought something from a similar company in the US, and overseas shipping was fine. I would check with them.
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