solarPower first prototype!

Hi, I had two 9V/1W solar panels laying around at home and I was wondering what to do with them for a long time. Hopefully I’ve put them to good use finally! I also happened to have around some MC34063 DC/DC switching converters, wich I would like to use too, so this is basically what came out of them!

I bought my solar panels at mouser with a stock of components I needed for an old project, you can find them with this product code: 750-00031. They are made by parallax. The MC34063 are generic jellybean chips, as Dave Jones says (thanks a lot for the video too if you’ll ever read this, check all his videos and site, there’s a lot of great stuff over there), wich you can find everywhere, I’ve got mine in a SOIC-8 package at in an electronic components store in Mestre, but you can find them online for a few cents too.

Basically what I wanted to design was a DC/DC converter wich converts the power form the solar panels down to 5 volts  to use with any usb chargeable device you would like to charge/power, unluckily the power from the panels wouldn’t be enough to charge at a decent rate (let’s say 500mA) anything so I decided to use some AA NiMh batteries to store the charge and get the nice side effect that it works also with no sun at all.

What I came up with was basically this, much from the data sheet, I’ve just tweaked the values calculating them with the formulas also from the data sheet.
After calculating them by hand I’ve found out this nice page which contains a nice calculator ready for you to use..

Main schematics
Main schematics

I designed a board (which you can find at the bitbucket repository in the link at the end of the article) and toner transferred it to a copper clad board and etched with H2O2 + HCl in proportion of 2:1 as I found on this instructables.
It didn’t turn out too bad, I was a little worried for the little package of the DC/DC converter, but  turned out that wasn’t much of a problem at all.

I’ve built it up and tried it out, of course the first time I’ve soldered it upside down getting a nice short circuit -_- the nice thing is that it survived pretty well and after getting it correctly soldered up it worked like it was supposed to! I’ve measured out how it worked. I checked it with a simple NPN 2N222 transistor hooked up in a common emitter configuration with I controlled with a 20K potentiometer on the base and remaining down to 200/250mA  just not to blow it up. The ripple was under 100 mV  and it didn’t drop the voltage more than 50mV, so I was quite happy with it.

The battery pack I made provides 7,2 volts with 2300 mAh nominal current (Energizer). I know it’s not the best way to do it, you can do it better and with nicer materials, but that’s how I made it:

I used aluminum kitchen foil, a lot of black electrician tape and plastic to keep them tight together. I folded pieces of aluminum foil on themselves to create a thicker stripe with I cut and shaped like in the picture.

Battery connections
Battery connections

The bigger part it’s used to connect to the – terminal of the battery and the tape it’s used to insulate a bit one from the other
I put them in series like in the following picture.

battery pack
battery pack

I placed the pieces of plastic over the contacts and taped it all up after trying it out ending up with this:

finished battery pack
finished battery pack

After connecting the 2 “leads” to some wires scavenged form an old pc fan I proceeded to put 2 diodes in series with each solar panel and connecting them together to get to the input connector, connected the battery and measured some more with actual components instead of my bench suppl. I created an USB board to connect a cable to it and proceeded with some real life test under the sun!
I noticed that with full sun the system it’s working properly but when it’s darker it has some drop from the battery as long as they get emptier and close to their end life as I would expect.

Some pictures of the test.

Inner connections
Inner connections
Closed "box"
Closed "box"

My android phone was happy with it and so was I!

ScreenShot
ScreenShot

The trick to behave like a line adapter was to short out the 2 data line together, it will just say charging (USB) instead and draw 300 mA instead of a nice 500 mA like it’s doing this way. There is a strange way of charging an apple ipod touch/iphone/ipad/iwhatever using very big capacity batteries, as Ladyada found out here (check her site too, it’s really useful!).
I’ve made a board to set the 2 data lines voltage also to 2 volts just switching some jumpers if you need that.

I’ve learned quite a lot of things from this and there’s still much room for improvement, like getting an output status, monitoring the battery over-charge and a bunch of other things I’m working on, but for a first prototype I’m quite happy with it!
I’ve learned pretty much how to use the kicad EDA and learned some things about DC/DC switching regulators. I would like to add an AVR uC (Just because I’m used to them) to keep track of the output voltage and battery pack charge, maybe controlling a low power display or something like that.

If you want you can download everything I worked in from here: Bitbucket – solarPower there’s also a download for a little release with the schematics in pdf and the gerber board files.

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