Ideas on solarCharge v.1

First of all, thanks to Chris Connor for posting v.1 on the Make: Blog!

Now, some of the considerations I've come up with for v.2:

The lack of a good ferrite core may prove the undoing of this design. As it is, the ones that I have access to seem a bit large (over 1" long and .5" wide), and the more ferrite, the higher the flux density needed to achieve saturation, and therefore the higher the current needed to drive the dc-dc upconverter. This is also why I don't want to use a regular transformer. However, a small enough isolation transformer, like those used in VCRs, TVs, stereos, etc., might just do the trick.

Next time I'm building on a breadboard. Period.

The capacitor has to be limited to below 300 µF, since the upconverter runs at about 50 kHz. If you're wondering why this is, check out the concept of reactance.

Similarly, the original Joule Thief implementation of this upconverter ran a white LED. This means that they were using a half-wave rectifier on the circuit. If I manage to pump the voltage up to about 5.6 Vrms out (with a +2.8 V offset, meaning that the AC signal is all above the zero line), I'll be able to put a regular low-wattage diode in to trim some of the oscillations, and increase the size of the next capacitor downstream. However, I could make this an LED if I can get the design up to 6.7 Vrms.

Do I really need an LED in there, though? Somebody weigh in on this in the comments. I know that the higher the voltage (short of probably 8-9 Vrms) the better, so I'll build for this anyway, but it does seem to me that the LED might just be a waste of wattage, as might the rectifier itself.

Anyway, enough about v.2. I'd better get building.

solarCharge

After having seen this article on the Make:Blog, I said this:

How about connecting a joule-thief circuit (DC-DC upconverter) to a cap on one side and a small solar cell (like the ones in calculators) on the other? You pull off of the cap to the cell phone directly. If I remember correctly, most cell phones have internal Vregs between the battery and the rest of the circuitry, so that takes care of your OV protection. I'll see if I can build this, and then get back to you.

I then went and built the solarCharge, v.1. This is essentially the Joule Thief with a small solar cell instead of a battery, and a large (50 V, 680 µF) capacitor across the Vout leads. I'm working with the materials that might be readily available to someone who's been able to scrounge through something like an old calculator junk pile, so I don't have a ferrite core, which is probably limiting me significantly. I think what I'll do for v.2 is build without limits to what the parts are, but with a minimum quantity of parts, and track the expense.