Solar usb charger problem with 7805 voltage regulator

[iceman20]

Hi all!

I have been looking into building myself a usb charger for some time now. I am currently trying to simply hook up the solar panels I salvaged from a few garden lights to the voltage regulator and see if I can get the 5V that I need for the USB to work.

I got a UA 7805 voltage regulator (with the ST logo on it) and I looked up it's datasheet. It says the minimum voltage is 7V and max is 20V on the input for a stable 5V on the output.

I have stringed in series 8 solar panels and from my measurement they will never exceed the max of 20V on the regulator.

Now for my problem: Measurement on the stringed solar panels shows me I get about 14V, but when I connect them to the 7805 the voltage measurement drops to about 1.5V and also on the output pin I get about 0.4V.

I have tried all the combinations possible with the 3 pins on the 7805, to no avail.

Can you please help me? Am I doing something wrong? Do I need some other part , etc?

Tnx!

[fubar.gr]

With linear regulators like the 7805, the voltage has to be as close as possible to the minimum value, else you are simply wasting power.

So it would be better to connect the panels in parallel, or in a combination of series+parallel so you get an output as close to 7 Volts as possible.

Connecting the 7805 can't be any simpler. Pin 1 is input, pin 2 is ground and pin 3 is output.

You shouldn't be getting 1.5 Volts, something's wrong. Show us your circuit

[flynwill]

Most likely your solar cell string is unable to generate the ~4-8ma that the 7805 needs to operate.  Do you have them in full sun when you are testing?  I would suggest testing your cell sting powering resisters of several sizes (say 100, 1k, 10k, 100k), with the cells in full sun.  That information will tell you if you even have a chance of getting enough power.

[iceman20]

Hi guys!

Tnx for the rapid responses. Unfortunately I am at work atm, and I do not have the circuit with me. But I think since I am getting 14V and none of the cells individually gets more than 2.3V I think I have the cells wired correctly(also they came with color coded wires - red and blue)

I will be doing that asap fubar.gr, tnx for the advice. I think i'll get 2 strings of 4 in parallel, halving the max voltage to 8.5V. Btw, will I be needing a diode on each string?

Also I didn't think to measure the intensity of the current. I do have a couple of rezistors around, but as I said, I cannot do this now. Anyway if this is the case, fubar.gr's suggestion should  solve my problem, yes?

Tnx!

[BlueBill]

You don't want a linear regulator e.g. 7805, you do want a switching regulator e.g. LM2557 or you could try Roman Black's simple switcher. http://www.romanblack.com/smps/smps.htm

But...

It's likely to cost more to build than buy, you're going to need parts & tools plus unless you're careful you can damage your phone. Solar USB chargers aren't expensive and commonly available.
Cheap (questionable quality though) solar chargers can be had for $13 US
http://www.banggood.com/30000mAh-Solar-Charger-Battery-Power-Bank-For-iPhone6-Smartphone-p-919239.html

 iAnker makes some that are well reviewed.

 Fancy ones even have a speaker, I picked up a Rugged Rukus solar bluetooth speaker for $50 recently, handy.

[TimFox]

Solar cells are essentially constant-current devices (current proportional to irradiation) with voltage limited to a diode drop per cell.  If you test the circuit indoors, there is probably not enough photocurrent to operate the 7805.  However, with indoor lighting there is probably enough current to see rated voltage into only a voltmeter (high impedance load).

[iceman20]

Hi guys!

I have had time just to measure the circuit in sun light and even though I'm getting almost 20V, the intensity is very low ~2mA. Now the input voltage shows about 4.5V and the output is around 3V. I will be changing the circuit as you guys suggested asap and get back to you. Also, please let me know if i should add diodes on each set of solar cells.

Thanks for the link and suggestions BlueBill. Aside from ending up with a solar charger I'm doing this because I like creating stuff, so buying an already made solar charger not only will it be less satisfying but will make me pay alot more than I'm investing right now.

Ill be considering the LM2557 for my next solar circuit or if I decide to redo this one

Thank you all!

[Psi]

Keep in mind you will see full panel voltage with no load and minimal light hitting the cell.
But as soon as you draw even a tiny amount of current the voltage will fall like a stone until it reaches the point where it can supply the current you are trying to draw (based on light).   

To get the panel voltage at the rated current you need the panel to be hit by direct hot midday sun.

The solar panels you find in garden lights are likely really cheap china panels made to the lowest price possible.
There is a big difference between cheap panels and good ones. eg, 5% efficiency vs 22%