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Simple (but long) solar charging question

Hi all,

(Just reread and grasped the length of this--sorry. Not looking for specific answers, just insights into things I am perhaps misunderstanding--any pointers to good reading help. :) )

Another problem from an experimenting noob.

I have a schematic for a simple solar-recharging garden light. This thing uses a very simple blocking oscillator to power a bright white LED (3.9V) from a 1.25V AA NiCad cell, and works wonderfully.

1N4004

.--------->|---------o--------------------. | | | | | | o-----' _'_ | | | .-------UUU-----o-----. | | | | | | | | | | 20-T-20 | | | /+\\ | .-. | | /+\\ ( ) | | | | | ( ) \\-/ | | | 1K | | \\-/ 1V PV | | '-' | | | 1.25V NiCad | | | |/ | | | | '-------------| V -> | | | | |> - | | | |/ Q1 2N4401 | | 3.9V | | '---| | | | | |> | | | | Q2 2N4401 | | | | | | | | | '-----------o---------------o-----o-------' (created by AACircuit v1.28.6 beta 04/19/05

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Short description: Q1, the 1k resistor, and the inductor form a blocking oscillator which pulses into the ~1.25V from the battery into fairly high frequency ~3.5-4V pulses which are sufficient to power a bright white LED. Q2 disables the oscillator when the solar panel hits enough voltage. The 1N4004 prevents the battery from discharging though the solar panel.

Full explanation from someone wiser than I:

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As I said, it works great--at least, it powers the LED quite nicely from the battery.

Here's the problem--as you see from the schematic, I only have a

1V/400mA solar panel. Sources I have read seem to indicate that you want more voltage from the charging source than the battery can provide in order to charge the battery. Obviously the above circuit falls short of that goal.

Other things I have read indicate that amperage is also important, as far as charging rate goes--charge at C/10 for a fairly safe long-term charge etc. But it seems to me that the amperage won't mean much if the voltage is insufficient to push it into the battery. Am I way off track here? Can a good charge into a 1.25V NiCad be had from a 1V

400mA panel (in reasonable light)? Or should I look into getting a A) higher-voltage, lower-current panel, or B) converting some of that amperage into voltage?

That was the first question, in a very roundabout format. :)

The second question involves me having chosen B) above to work on. I reasoned that I might be able to have the solar panel, instead of simply disabling the oscillator when it hits a high enough voltage to do so, reverse the connections and instead pump its output through the oscillator into the battery.

Now, one problem has to do with my switching topology, but my question about *that* will have to wait until I'm a little more familiar with AACircuit. The real question I have is: will that even work for charging a battery? As I understand it, the output from a blocking oscillator is a series of very fast, very short pulses. However, it looks fine (solid DC line) on my scope. My scope may well be too slow to show it, though (EICO 460 (pawn shop special)). But my DVM shows around 4V when the solar panel has enough source light to power the LED.

Am I mad? Yes, I am testing this but I have a shady yard in Vancouver--which is pretty cloudy these days. It seems 4V should be plenty to charge the battery, if I can work out the switching--if the pulsing doesn't affect the charging.

So am I barking up the wrong tree here? Will the tiny pulses from the oscillator (which I have read about but not personally observed) not provide enough average power to charge the battery? Or is the DVM--which I think gives an average DC voltage reading--showing me what the battery would be seeing--enough to charge it well?

Thanks for any insight,

Torben

Reply to
Lars Torben Wilson
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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.

  • 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.

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.

It might be worthwhile for you to look at a Linear datasheet to see how it's done. Go to the Linear website:

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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.

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.

Vancouver is one of the great places to be in early summer -- get out and enjoy the day.

Good luck Chris

Reply to
Chris

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.

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.

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.

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.

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.

Earlier this evening my gf and I were outside on the porch, in awe at the downpour. :) But yeah, it's usually pretty idyllic.

Thanks for the answer!

Torben

Reply to
Lars Torben Wilson

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