Small Solar Array question

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HarveyH42

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I've been working on an outdoor lighting project for a few months (off and on, so many other things), and getting very close to completion.

I'm looking to wire 4 (maybe more, if needed) solar cells in series/parallel to get an output of 12v @ 100mA. Two configurations come to mind, and I'd think the output would be the same. I have no experience to draw from, so figured I'd ask and see if it even matters. Did a quick diagram, attached.

I etched and drilled a controller board, haven't stuffed the parts yet. Basically, it will charge 8 AA NiMH cells in series (9.6 volts), through a diode. I think they are 1100 mAH, so overcharging is unlikely. The controller will switch on the load at night time. The load, being an RGB LED fader. Which is built and functioning nicely.

Anyway, I've got about a dozen of these solar cells, so can add more if needed, but want to keep it minimal, as after the first is working, I'd like to build a second one.
 

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I think you will find that diagram A results in a series circuit, so your current would be 100mA, whereas diagram B is in parallel, so you end up with a max of 200mA.

the general rule with solar panels is to series connect the fewest panels needed to get the voltage you want, then parallel up sets to increase the current output.

Batteries are the same, in series, Amp/Hours is the same, parallel it adds up.

You should place a diode in series with the output of the solar array also. This prevents the batteries from dumping into the panels when its dark, heating the panels and possibly damaging them.
 
In circuit A, I have 2 sets of 2 parallel wired solar cells, these wired in series.
In circuit B, I have 2 sets of 2 series wired solar cells, then wired in parallel.

Both are a combination of series and parallel.

So, in the first, 6v@100mA + 6v@100mA =12v@100mA
Second is 12v@50mA + 12v@50mA = 12v@100mA

There are 4 cells in the array, each cell is 6 volts (no load) @ 50 mA.
I want 12 volts @ 100mA output from the array.
 
My bad, was squinting at the diagram, should have opened it. Anyways, B is the better way to do it. Partial shading has a greater effect on some panel designs than on others, with B, you would maintain your terminal voltage more often and have less panel heating. I realize these are small panels, but I think its still the way to go, and is the way Shell, Siemens, and US Solar suggest I should do it for my applications at work.

My point about series and parallel was that the general " style" of the array was either series or parallel. In circuit A, shading some panels has a far more drastic effect than shading some panel in circuit B.
 
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Thank you, this is what I needed to know. Also, are any blocking diodes need to seperate cells, or just between the array and battery. Wouldn't a shaded cell take away from the active cells?

I know that the batteries will discharge through the panel, without a blocking diode, seems like inactive cells would draw power.
 
In practice, you really only need to separate the "branches", that is the finished series sets of panels from each other. That is the problem in circuit A, there is no place for the blocking diode, except at the output, so you get an "all or nothing" array. So in circuit B, you could have a diode at the end of each series section, and no more battery back-feed, and no panel back-feed either.

On a side bar, you may be surprized at the actual terminal voltage of the panel under load, it is drastiacaly different than O/C voltage. Typical 12 volta panels ( 50 - 60 Watt jobs ) will often be 19 volts or more O/C, 14V at rated load.
 
Yeah, the batteries pack is 9.6 volts, hoping the difference isn't quite so drastic.

Any diode suggestions. I have several different choices, a lot of 1n4001 and 1n914 figured either would work. Have a small box of unsorted, so could find most anything.
 
Of the 2, I would use the 1N4001, given the higher current rating, the 1N914 would likely pack up, its about a 75ma device. A schottky would be nice, but I wouldn't bother probably, use the 1N4001.
 
i think both are the same circuits.in figute A if we interchange cell2 and cell3 we will get the figure B.so according to me both circuits have same characteristics
 
Hello sriman..............................
Perhaps this will help you see the differences a little more clearly. It was a bit confusing for me also until I redrew them
 

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zevon8 said:
Of the 2, I would use the 1N4001, given the higher current rating, the 1N914 would likely pack up, its about a 75ma device. A schottky would be nice, but I wouldn't bother probably, use the 1N4001.
Click to expand...
Why Schottky?
 
The choice of a Schottky is to get the lower forward voltage drop. Inserting the diode will create a loss, so the smaller the loss the better. Most commercially made solar regulators with reverse blocking will use MosFets or similar, so that very low insertion losses are realized.
 
I've got quite a collection of mix diodes (save most everything). Any common suggested part numbers? Also, any physical charactoristics which might help in sorting through them? The numbers are kind of small, tough to read. There is glass, metal, plastic, ceramic, black, white, blue casings. Various sizes and shapes.
 
Older part numbers that were pretty common would be 1N5817, 18, 19 or MBR115, 20, 30. These are in typical 1N4001 cases. Industry standard numbers were in the 1N58XX for alot of schottky devices.

Quickest way would be to grab a DVM and do the diode test on the devices, compared to a known, like a 1N4001. You will see a much lower forward drop on schottky diodes. Once you sort them this way, figure the 1N4001 size cases are 1 Amp devices, bigger generally more current.
 
I looked through the box for a while, but didn't find even one, but there must be hundreds of them. Kind of wanted to get this thing working, so went with the 1n4001. I did sit down and assemble the PCB I etched last weekend, also soldered together the batteries. The ones I went with were in groups of three, so went with nine instead of eight. Also decided to go with a 6 cell panel, instead of 4. Will follow the #2 scheme, and omit seperate series diodes, just the single at the battery (already had it included on the PCB). I can add them later, or on the second one, if this one works as hoped.

I've got it all hooked up to a 12 v panel (car trickle charger), the batteries should discharge overnight, and hopefully recharge tomorrow. Should know about noon if all is good.
 
The circuits are working great. I have it sitting in a window, and the RGB fader is still going good at 1:30 AM, about 3 hours or so.

The attached circuit is what I used to charge the batteries and switch on at night. Going to replace the diode with a schotsky (sp) when they get here. Haven't been up to the hardware store yet to get what I need to weatherproof and mount it outside yet. Still a few details to workout in that department.

The switching circuit works pretty good. I had a simpler design, but it slowly came on as it got dark, which wasted power. Not to mention to early.
 

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