Solar powered RPi?

[tozian]

I have a project I'm thinking of and I want it to be solar powered. I think I heard that the RPi had a 5v voltage regulator on it, but I'm wondering if it's safe to connect one of Adafruit's 6v solar panels to it, or is it neccessary to set up some kind of external voltage regulator. If so, how would you do that?

[adafruit_support_mike]

The RasPi does not have an onboard regulator. That's one of the challenges of working with it. There's a 6v @ 1.1A polyfuse intended to keep the worst voltage spikes and current surges from killing the CPU, but as a general rule you want your 5v and GND rails to be smooth and well regulated before you connect them to a RasPi.

Be aware that the RasPi officially wants 700mA @ 5v, or 3.5 Watts. You'll need at least that much power to make sure it can keep running at peak load, and will probably want a bit more to compensate for losses when you convert from 6v to 5v.

Since solar isn't always reliable, you'll probably want to use a battery to buffer the power. That makes life interesting because the best batteries for the job are lithium polymer cells, but LiPos have an operating voltage of 3.7v.. exactly wrong for working with 5v supply rails.

The safe, sane, and non-explodey way to make it all work is explained in this tutorial: http://learn.adafruit.com/multi-cell-li ... g/overview

[tozian]

Okay, I'm not sure I understand entirely how this works. I'm just beginning with electronics and this tutorial is just a wee bit over my head. Anyhow, I think that the project does this:

The solar-powered charger charges two Li-Po batteries equally. The RPi then draws current from the batteries (through a 5v regulator) so that it's able to run on a smooth current, because the power from the solar panel tends to fluctuate.

Is this correct? Sorry if I sound a bit... stupid.

One more thing, where in the world do I get a 3-pole double-throw switch? You don't seem to carry them.

[tastewar]

Boy, I don't want to confuse things more, but a different choice would be to use a single LiPo battery, the solar LiPo charger (http://www.adafruit.com/products/390), and a MintyBoost (http://www.adafruit.com/products/14) to boost the battery voltage to USB level. With that arrangement, the charger and load and battery can all be connected at the same time -- no manual switching required, and the solar charger is very intelligent about powering the load directly, charging the battery, etc.

As to where to get things Adafruit don't sell, Mouser, Newark and DigiKey are all big distributors that will stock just about anything you can dream of. For very small orders, I believe DigiKey has the edge in that they offer fairly inexpensive 1st class postage. ($2.99, I believe)

[tozian]

Yes, I did think of using a mintyboost. I just wanted the cheapest solution possible. And I thought of looking up the part in Newark, but I never even got around to it.

Thanks for the help!

[adafruit_support_mike]

Is this correct? Sorry if I sound a bit... stupid.

Asking for help when you don't understand something is the opposite of stupid. It's honesty.. and if there's one thing Makers respect, it's honesty. Nobody knows everything, everyone's a noob at something, and you can't bluff the laws of physics. Those of us who know a path through the trees have usually met half the forest face-first.

A MintyBoost doesn't supply quite enough current to make a RasPi work. The RasPi wants 700mA, while the MintyBoost provides 500mA.

Besides..

The solar-powered charger charges two Li-Po batteries equally. The RPi then draws current from the batteries (through a 5v regulator) so that it's able to run on a smooth current, because the power from the solar panel tends to fluctuate.

That's absolutely correct.

[tastewar]

while the MintyBoost provides 500mA.

Thanks for the correction!

[tozian]

One more thing, does this thing power the pi and charge the batteries at the same time, or does the switch reguate that?

[adafruit_support_mike]

It's an either/or thing. The circuit is arranged so the batteries charge in parallel, but are then put in series to create a 7.4v rail that can be stepped down to 5v for the RasPi. Flipping the switch moves the circuit from CHARGE mode to RUN mode, and vice versa.

You could probably build a continuous charge-and-run system by putting two battery charger circuits in series.. use a separate charger and solar panel for each battery. I haven't actually tested it or dug far enough into the documentation to see whether that's permissible or A Very Bad Idea, so I'd suggest doing some serious reading before you try it.

[tastewar]

Which is why the MintyBoost *would* be nice.

Maybe it's time for a v4 of the MintyBoost, with more things looking for up to 1A of current out of USB...