Author Options:

Is this all I need? Answered

My intention is to make a solar phone charger.
I cannot read a wiring diagram and there is a lot of lingo I do not understand.  So, I'm having a problem with some of the instructables.  
I was lucky enough earlier to be told I needed a voltage regulator........at least......well, what else?
solar cell 6V  50 mA
7805 voltage regulator
old phone charger
Is that it?
Thank you for your time and patience.

8 Replies

jeff-oBest Answer (author)2010-04-26

Your solar cell needs to be larger.  It must supply at least 7V, though 12V would be even better.  The first reason is that the 7805 voltage regulator has a "dropout" of about 2 volts (as Jack states below), which means that if you feed in 6V you'll only get 4 out.  So, at least 7.  But, also remember that the solar panel will only produce 7 volts in full brightness and when the current draw is below max, so you need some additional overhead to make up for that as well.

So, in order to get 5V at 100mA (the minimum I'd suggest for a battery charger), you should have four solar cells arranged in a series/parallel arrangement to get 12V at 100mA (again, in maximum sunlight).

Select as Best AnswerUndo Best Answer

jessejwk (author)2010-05-09

 You should also attach a heat sink to the regulator. These things convert extra voltage into heat, and it can get very hot. Just attach it to the metal part of the regulator.

Select as Best AnswerUndo Best Answer

onrust (author)jessejwk2010-05-09

The heat is not a problem.
My problem is I can keep my phone running but it does not charge. 
I'm looking for a person to do a collaboration on this project.  I'm thinking that if I add 2 AA batteries that may work....BUT, that means I'd have to add a diode and other junk I know nothing about.     Interested?  Anyone? message me thanks

Select as Best AnswerUndo Best Answer

gmxx (author)2010-04-25

 nope. you will need the lm7806 voltage regulator. you need to match voltage. the lm7805 might work, but it is less than optimal.

tie the grounds together, voltage in on pin 1, voltage out on pin 3. all the grounds connect on pin 2

Select as Best AnswerUndo Best Answer

Jack A Lopez (author)gmxx2010-04-26

Correct me if I'm wrong, but I think the goal here is to come up with a supply that resembles a USB port, i.e. something that looks to the phone to be a regulated 5 volt supply. 

One way to do this is to use a linear voltage regulator with a fixed output of 5 volts; e.g. the lm7805.  

The supply to the input terminal of the regulator should then be some voltage higher than Vout plus the dropout voltage. The dropout voltage is typically about 2V, so you want the solar panel to be able to supply some voltage greater than about 5+2=7V, at whatever quantity of current the phone wants to draw.  I don't know where that number 50 mA came from, but it sounds almost believable.

Anyway, I think that's your game plan, if you want to use a linear voltage regulator, like one of the 78xx.  

Select as Best AnswerUndo Best Answer

gmxx (author)Jack A Lopez2010-04-26

 the lm7805 would work for 5 volts. n my rapid reading tof the question, i understood those power requirements to be what the phone charger output.

Select as Best AnswerUndo Best Answer


Except that you really ought to be able to supply a USB minimum load unit, which is 100mA

Select as Best AnswerUndo Best Answer


Yeah.  That's a good point.  I mean if I'm aspiring to build this thing to something resembling the USB spec, then having it be capable of 100mA would be be neat.  I mean depending on what the cloud cover looks like that day.

I was looking for a web-example of a cheap solar panel to use with this.  I remember seeing a solar car-battery trickle charger, capable of, I think, something like100mA at 13V, or whatever the "charged" voltage for car battery is, um here:

1.5 Watt Solar Battery Charger (16 FUSD)

Then at the same site, I also saw this:

Universal USB 12 Volt Adapter (5 FUSD)

And that sort of made me think: Hack the solar panel off the first one and wire it into the second one.   The second one, the USB 12 volt adapter, that might even be a switching style regulator, which would be preferable to a linear style one like the 78xx, and able to squeeze more power out of the panel, if desired.

And then it occurred to me that for about the same amount of money, it might even be possible to buy a solar-USB charger, maybe even one that has its own battery, e.g.

Prolynkz SBC-10 Power Pak Solar Charger (25 FUSD)

Anyway, that's all food for thought, I guess.

Select as Best AnswerUndo Best Answer