Re: Simple (but long) solar charging question
- From: Lars Torben Wilson <larstorben@xxxxxxxxx>
- Date: Fri, 02 Jun 2006 07:07:26 GMT
Chris wrote:
Hi, Torben. Quick summary:
* There's no way you'll get the circuit above to charge the NiCd
battery from the solar panel you have. Even if you use a low forward
drop schottky diode, the panel voltage will have to exceed 1.4V before
the diode becomes forward biased enough to conduct anything into
charging the battery. And you'll need more than that to start getting
a good charge. Battery voltage rises as it's being charged. Sorry.
Hi Chris,
Thanks for the thorough answer; it helps a lot.
I forgot to mention that the above circuit isn't expected to work with
that panel; I was just used it as a prototype to see it working. I use
a light bulb as the "sun" when trying it at my desk. I do actually
have another panel which is on the one I was trying outside, but it's
too ungainly for my purposes.
* Your idea B) of "convering some of that amperage into voltage" is
commonly called a step-up switching power supply. There are ICs you
can use which will control a step-up switcher to boost your 1V into 3V
or more. If you really wanted to pursue this, you would look at the
Linear Technology line of micropower step-up and flyback regulators.
You can actually have the solar panel voltage drop well below 1V, and
still get a regulated output. Of course, since TANSAAFL (There Ain't
No Such Thing As A Free Lunch), your power into the regulator can't
exceed your power out. Since you can only supply 400mW (1V * 400mA),
your 3V output can't exceed 133mA.. Since switching regulators are
typically 80% to 90% efficient, you realistically won't expect more
than 100mA or so at 3V. However, that might be enough for your NiCd
battery, especially if you can get more inductance into the circuit to
slow down the oscillator.
And the oscillator that's in there would probably be even less
efficient, right? I stuck a pot in for the 1K resistor in the
oscillator and found a rather bizarre relationship between its value
and the output from the LED--but I did get the LED brighter than
it had been.
Three big difficulties here. The first is, putting together one of
these will certainly cost quite a bit more than just swapping out the
solar panel. Second, once you've developed the step up switcher,
you'll need another circuit to limit current into the NiCd battery (a
simple resistor won't cut it here). Third, switchers are inherently
tricky, and if your big gun is an Eico 460 with a non-functioning
trigger circuit, you're not going to get very far if something goes
wrong (and with switchers, something almost always does, especially for
those who don't carry the scars of battle). From a newbie perspective,
this tells us to slowly put down the scope probe and step away from the
concept.
Don't worry, I do spend most of my time trying to learn the easier stuff.
I have already pretty much done what I've wanted to with my idea anyway--
I got the answer you wrote, which explains a bunch of what I was after.
Fodder for the notebook.
It might be worthwhile for you to look at a Linear datasheet to see how
it's done. Go to the Linear website:
http://www.linear.com/index.jsp
and punch in LTC3400 under search. Look at the product page and the
data sheet, and see what you think. But remember, even though the
typical apps circuit looks simple, there are big problems lurking under
the surface. At a minimum, you'll need a well thought out etched
circuit board with a ground plane to make this happen. And a good
scope.
I agree on the scope thing, and as for hidden gotchas, Art of
Electronics has been revealing piles of them I wasn't aware of. Not
trying to say I grasp them all yet, but I'm getting a better idea of
what I'm getting myself into with this stuff.
You're not "mad", you've asked a couple of really intelligent newbie
questions. If the fate of civilization depended on it, it could be
done. Unfortunately, A) looks like the only realistic newbie option.
And I'd go with a larger 6V solar panel capable of supplying 200mA
minimum, so I could just use a series resistor to safely and reliably
charge the battery.
Good stuff. Still finding out what kinds of panels are out there.
My final system might end up using an old yacht setup or some such.
Vancouver is one of the great places to be in early summer -- get out
and enjoy the day.
Earlier this evening my gf and I were outside on the porch, in awe at
the downpour. :) But yeah, it's usually pretty idyllic.
Good luck
Chris
Thanks for the answer!
Torben
.
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