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solar (direct) usb port

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user325153:
Hi  8)
I recently bought 5w mono crystalline solar panels and i was wondering if i could charge usb devices with it. I tested the panels in sunlight and they seem to actually be able to work at 5w  :). I know there are nice lithium packs that can charge from solar panels but I'd like to avoid any battery's. I created a little circuit on a breadboard with a 7805 in order to get 5v output. If you don't mind, a few newbie questions.


* Can i just put a random current up to 500mA on the usb devices or should i stick to 100mA (which it can do in cloudy conditions)?
* Will lithium chargers in usb devices work or will they be fooled by variable current?
* In ultra low light conditions the voltage goes down so much that it's no longer within the usb voltage spec. Should I worry about this as there is obviously not much current then? Is there a simple solution?

Jimmy:
I wouldn't worry about the current so much as the thing you are charging will normally take care of that however over current protection will be good if there is a short. I would put under voltage protection to cut out at 4.75v

Have you tested how much current you can get out of the solar cells with a dummy load ?

ejeffrey:
You will have to look into the USB charging specs to convince a device to draw more than 100 milliamp.

I don't know what happens to USB charging circuits if they try to draw more current than the source can supply.  In an ideal world, they would continue charging at a reduced rate, but they may simply shut down, or charge extremely slowly.  If you experiment on this, please report back :)

Zero999:
There should be no need for overcurrent protection for solar cells, unless the current rating of the cells is high enough to damage the wiring. The LM7805 is a bad idea because it looses 2V to 3V, you need to use a low drop-out regulator.

user325153:
Hey guys, thanks for your response. I decided to just connect everything up to my phone and test it. My phone is a Samsung android device and apparently it will just accept 5v on the power lines of usb and then detects it as "USB connected". There is also an AC cord which puts 0.7A into the phone.
It looks like i am still going to have to buy some sort of battery pack because the phone will only really charge in direct sun. When in shadow the phone produces a worrying kHz sound and doesn't really discharge or charge. When the current increases the frequency drops. I could still try to connect a second 5w panel in parallel, but then it wouldn't be very mobile any more.

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