Matteo_T
Posts: 2
Joined: Mon Jun 09, 2014 11:31 am
Location: Italy

Multiple power sources

Mon Jun 09, 2014 10:15 pm

Hello!

I own a Raspberry Pi and I'd like to use the OpenSprinkler Pi board to control my sprinkler irrigation.
OpenSprinkler Pi provides 5V to the RPi via GPIO pins as power source.

I know that failure to properly switch off the Raspberry Pi may corrupt the SD card and it will be necessary to re-image it.

That's why I'd like to use a DC step-down power supply module (like this http://www.ebay.it/itm/NEW-DC-12V-24V-4 ... 3f37183e63) + a truck battery (12V 120Ah 700A) as UPS just for the RPi.
The DC converter would be connect to the RPi via micro-USB.

I wonder what happens when you have 2 different power sources.

Is it a bad idea? Could it damage the RPi? Which is the best solution to keep the RPi always running, despite power outages, please?

Thanks for any advice!

Bye!

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Burngate
Posts: 6152
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK Tralfamadore
Contact: Website

Re: Multiple power sources

Tue Jun 10, 2014 10:01 am

It's not generally considered good practice to connect two power supplies together, but it (probably) won't damage the Pi.

Both supplies will have a voltage-sensing circuit connected to their output, and will drive current into it to keep that output at a constant set voltage.
It's impossible for them to have exactly the same set voltage
If you connect the two outputs together, one will sense the voltage is too low and drive more current, while the other will sense the voltage is too high and drive less current.

What happens when the voltage is too high even with no current out is in the lap af the gods - or the designer of the supply.
It's possible that the designer thought of that scenario, and took pains to allow for it - but don't bank on it.
It may just give up and let the output float, or it may try to sink current to bring the voltage down. Or it may go up in flames.

Matteo_T
Posts: 2
Joined: Mon Jun 09, 2014 11:31 am
Location: Italy

Re: Multiple power sources

Tue Jun 10, 2014 11:35 am

Thank you indeed Burngate for the answer.

Since it's not generally considered good practice to connect two power supplies together, I should use only the micro-USB that is connected to the DC step-down power supply module + lead-acid battery in order to guarantee uninterrupted energy to the RPi.
By the way, the lead-acid battery is always kept charged by a transformer.

The RPi is joined upside down with the OSPi v1.4 (please, see pictures attached) and the only way to interrupt the power supply via GPIO is to cut pins number 2 , 4 and 6 on the RPi, I guess.
Am I right?

Any other idea to avoid ruining the RPi's GPIO?

Bye!
Attachments
OSPi v1.4 and RPi.JPG
How to assemble the RPi on the OpenSprinkler Pi (OSPi) v1.4 board
OSPi v1.4 and RPi.JPG (45.7 KiB) Viewed 2316 times
OpenSprinkler Pi (OSPi) v1.4.JPG
The OpenSprinkler Pi (OSPi) v1.4 board
OpenSprinkler Pi (OSPi) v1.4.JPG (39.86 KiB) Viewed 2316 times

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Gert van Loo
Posts: 2486
Joined: Tue Aug 02, 2011 7:27 am
Contact: Website

Re: Multiple power sources

Tue Jun 10, 2014 12:07 pm

It's not generally considered good practice to connect two power supplies together, but it (probably) won't damage the Pi.
Probably not but the danger is not with the Pi but with the supplies. If any of the two supplies is not build for, let me call it, 'back pressure' you can damage the supply ultimately causing a fire.
It is the reason why in the end I had to remove the 5V, 2A supply from the Gertduino design.

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