Wednesday, September 5, 2012

Cheap Solar Power - Hacking the Explorer2, Part 2

The situation is that I have a solar panel that's not particularly useful for it's intended purpose.  Actually, it's two panels.  They don't quite meet expectations.  So what do I do? 

I'm not going to say I have the world's monopoly on good ideas or good sense, but here's how I think about it.  The panels will deliver 5.5V at 750 mA.  If you're keeping score, 5.5 V times 0.75 A is 4.125W and two panels gives me 8.25W.  If I connect them in series, that's 11V at 750 mA (check that 11 V times 0.75 A is 8.25W).  If I connect them in parallel, I get 5.5 V at 1500 mA.  Either way of directly connecting them is the most efficient thing I can do and the closest I'll get to that 8.25W.  Anything else I do will consume some fraction of a watt and lead to recovering less than that 8.25W.  (In the real world, there are imperfections, such as the two panels never delivering exactly the same voltage, one will try to force current into the other, and so on.  We design to optimize the variables as best as we can).  If I'm lucky, I can get around 90% of that 8.25W or 7.5 Watts. 

Let's say I want to charge a 12V gel cell.  I need 13.6V to charge that, so it's more than I can get by putting the two in series.  A DC to DC converting switching power supply can convert to either a higher or lower voltage, so I'm going to choose to parallel the two panels into a 5.5V source at 1.5 A.  I can make almost any output voltage easily with the power supply so the starting voltage doesn't matter much.   

"Not so fast, Graybeard", you reasonably say, "I hardly even know what a switching power supply is, and that's only because you talked about it once in that series you promised to write for us but keep avoiding.  How am I supposed to know how to design and build one?!?"  To be honest, I don't know how to design one, either.  Never have.  But it's electronics, so it's more understandable to me than a lot of other things in life, and building it is more a matter of having the tools and knowing how to solder.  As it turns out, the companies that make switching power supply controller circuits really want to sell them (well, duh!), and they're very helpful.  Texas Instruments, the company that brings you Digital Light Projection in movies, is a giant in this field and has free online tools that will design several circuits to your specifications so that you can choose the approach you want.  (Linear Technology is another giant in the field).  I don't know if hobbyists can use TI's "Web Bench" feature.  You will need to register with their website but I don't know if or how often they'll try to call you for sales.  Certainly if you were trying to set up a company to sell solar power systems, they'd be happy to talk with you and help you get your design perfected. 
I'm not done with this, (or, as I like to say, I'm not "done" done) because I'm not sure what this solar charger system is going to be: is it going to be for big lead acid batteries, or just an improvement for the few USB charged things I own.  I have that solar panel system I talked about here for charging a moderately-sized battery.  This one just won't add much to that show.  It's probably best for those USB thingies.  

More later.


  1. Ah, yes, good old "LT".
    I always liked their logo, and have built a lot of stuff using "samples" from them.
    Maxim is also pretty generous with "samples", if you get the right person on the phone.

    1. For sure. I've seen in some amateur publications that Analog Devices honors all sample orders from hobbyists, too. Just register on the site and order what you need. Their lower end DDS chips are popular in some QRP radios and guys are just buying them directly from ADI.