Question about solar panel circuit

I'm putting together a solar powered cell phone charger and I need some advice from someone who knows a thing or two about photovoltaics.

I've spent the better part of my free time the past three days or so absorbing as much information about electrical charging over USB and solar cell output as possible. I've seen a lot of helpful DIYs, and they would be great if I was following their method exactly. Unfortunately my project has serious space and power constraints. It's not for trickle charging an additional battery, it's for providing direct power to a USB phone.

Here are my specifications. I need 5V and preferably ~500mA. Anything over 400mA should work fine. I *think* I've come up with a way to get that within my space requirements but I'm not positive it will work because I'm mixing two different size solar cells in one circuit. I can't seem to find a definitive answer as to whether or not this is OK, and what the results will be.

I've attached a diagram of my plan. Two 5.5V/70mA solar cells wired in parallel with seven 1.5V/50mA cells. If I'm assuming correctly, this will give me an output of 5.5V (before the regulator) and 490mA.

I don't know how or if the lower voltages of the smaller cells will effect the output. Can I use a combination of high voltage and high amperage cells in one circuit to get the output I need?

I've talked with people in the Science and Tech forums and I've gotten a long way on the electrical circuit aspects and USB issues, but I'm hoping someone here will know a thing or two more about solar cells to tell me if my plan is viable as it is or if I will have to modify it.

Thanks for any help.

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solar panel can be used together with LED lighting technology,more energy-saving and environment friendly
prenato5 years ago
If the cells are 1.5V each as shown in the first diagram, you could wire four in series (4x 1.5 = 6v) and add a Schottky diode in series 6-0.5= 5.5 v. Now you have a more balanced voltage and the diode prevents the larger cells from discharging through the smaller cells. However, you will only get the same 50 mA from the four cells in series. Think in terms of power. The power delivered at the end must be the same. You can only get P= V I = 1.5 *50 mA from each cell no matter how you hook them. You have a total of 1.5*0.050 *7 + 5.5 *0.070 *2 = 1.295W available to you. Seems you need 5V * 500 mA in your project which amounts to 2.5 W so this won't work no matter how you arrange the cells. You don't have enough power unless you can gather more of those 1.5V cells...
mcl895 years ago
Hello, I'm not very experienced in this but I have taken several courses about this in school. You can't have an array with panels of different outputs because they all will adopt the output of the smaller panel. In your case the the bigger panels (5.5V/70mA) will give you an output of 1.5V/50mA(smaller panels). I have some information the teacher gave us when I took the class so if you want it msg me and I will share it with you.
mcl89 mcl895 years ago
you can also go to this website to look for a panel that fits your needs
lemonie7 years ago
It's not wired correctly, in darkness you'll only get 1.5V from the cells and nothing out of the regulator.
In sunlight you'll be pushing current through the cells (5.5V against 1.5V) which would probably not be good for them past being fully charged.
The regulator GND also needs to be connected to -ve.

You want a solar-charger, have a look for one here perhaps?

HittingSmoke (author)  lemonie7 years ago
I've looked at all of those and none of them meet my space and voltage/current requirements. The only way I can see to get the precise output I need is to mix and match solar cell sizes, I just don't know how to wire them properly.

This is another diagram I brainstormed last night. Am I correct that this would output 11V and 1A, or 5V/1A with the 5V regulator?

Also, would you mine elaborating on what I need to hook the ground up to please? (I know almost nothing about electricity, can't find any references to -ve on google) This is the voltage regulator that I have.

I have the positive from the array hooked into the input, the output going to the USB and the ground going to the USB. I know the metal side with the screw hole is supposed to be grounded as well but I don't know any way to do that on a device that I'm going to be carrying around on my person.

Thanks a lot for the tips. I'm hoping the new layout will keep extra voltage from being pushed through the lower voltage cells.
This design looks like it has a serious reverse bias path- aren't the two 5.5V cells putting 10.5V backwards across the 0.5V cells? (Assuming they are actually 0.5V, not mislabeled). I thought solar cells were usually meant to be used in strings in series, and only put in parallel when you have identical cells or strings in parallel. Mixing and matching like this will always be complicated and almost inevitably make some sub-optimal use of the cells. Bear in mind that voltage regulators exist to get a specific voltage out of a non-specific one, so your panel only needs to exceed the minimum voltage/current for your desired usage. You shouldn't have to juggle your existing cells to get a specific output voltage- just over 7 or whatever the threshold for your regulator is.
I'm not in a good state of mind right now (beer), but I'll go this far:
GND needs to be electrically-connected to the -ve such that you have +V and 0V (GND).

In your main diagram it's the GND from the regulator to -ve.

HittingSmoke (author)  lemonie7 years ago
I still can't find any reference to what -ve is so I'm not quite sure what you mean.
In your original diagram / schematic, the "Gnd" needs to connect to the "-" "To USB" L
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