Solar phone charger, questions

johnnyfortner

I completed a solar phone charger and it is not working, please help me.

This is the plan I followed:


Only what I did different, was resistor one and two are both 1-kΩ 1/2-watt, and I did not use a blocking diode.
The panel is 1/2 watt and 6V max output.

Either there is not enough sun during the day anymore or I did something wrong with the resistors.

heres my blurry image of what the back of the board looks like:

BB.

Re: Solar phone charger, questions

johnnyfortner wrote: »

Only what I did different, was resistor one and two are both 1-kΩ 1/2-watt, and I did not use a blocking diode.
The panel is 1/2 watt and 6V max output.

Well--changing the ratios of the two resistors will change the regulated output voltage value--As you designed will now be ~2.50 VDC regulated output.

• V_out=V_ref*(1+R2/R1) + I_adj*R2

Generally, you would want R2/R1 ~ 4:1 ratio... and the second term does not really matter much unless you have really really high resistance value of R2.

And what kind of phone are you trying to charge? Older (some/all/none newer) Iphones would not charge with a simple solar charger--They needed some signal/pull up resistor or something before they would charge. (you may need pull up/pull down resistors on the USB data lines--I think--not sure).

-Bill

johnnyfortner

Re: Solar phone charger, questions

Yeah, I knew that was the problem. I just wanted to follow the plan exactly and the lady at radio shack was an electronic engineer...

Is this set up possible with one resistor 1kOhm resistor? I have 3 1kohm resistors left, or should I just go get like one 200 ohm and one 800 ohm?
its a new smart phone, 3.7V battery.

BB.

Re: Solar phone charger, questions

Just has to be a 4:1 ratio... Too low of resistor value--and they will pass a lot of current (possible overheat/waste energy). Too high of value, the "error" term will start to become significant.

Here is a chart of standard resistance values.... You should probably use 2% accuracy versions (E48) or at least measure the output voltage to ensure you are within USB spec. of 5.00 volts + 0.25 volts.

If you have 5x 1kOhm resistors--One 1kOhm for R2, and 4x 1kOhm in parallel for R1 will work. You should have a ~0.1 uF and ~1.0 uF input/output caps too (stabilize the device).

-Bill "been too many decades--probably blew the 4 parallel resistor problem" B.

niel

Re: Solar phone charger, questions

you probably should've gone with the values they presented in the circuit. i'm not sure what the desired output voltage should be as i don't own a cell phone, but the lm317 is able to be varied in its voltage (see potentiometer, or in layman's terms a variable resistor, in some of the schematics in the link) and usually has some capacitors to help stabilize it.

here are more circuits for the lm317. do note the ic needs to have a heat sink or poof, magic smoke.
http://datasheetreference.com/lm317-datasheet.html

BB.

Re: Solar phone charger, questions

I guess I should add that USB is rated up to 1 amp @ 5 volts maximum (5 watts). A 1/2 watt panel is going to take a LONG TIME to recharge (10x longer than a "full power" supply).

Also you need to check the specs. on the panel... Is it 6 volts Voc or 6 volts Vmp? Voc=6 volts is really not going to be high enough voltage.

-Bill

niel

Re: Solar phone charger, questions

now that you mention it bill, neither 6v voc or 6v vmp will be enough to power an ic regulator to achieve 5v. with the blocking diode in place that adds another .5v to .75v to be overcome on top of it all. a vmp of 8v or 9v would be better suited, but you won't find such an odd voltage panel. lowest may be the 32 cell pvs they use as car battery maintainers

btw they do make straight forward 5v ic regulators. example is lm7805. no resistors in that. you do need to also pay attention to the max input voltage to the ic as well. more blocking diodes can be used in series to drop more input voltage if it is close to the limit.

Photowhit

Re: Solar phone charger, questions

I built a simple 5V charger for my Nokia N800 with a 7805 voltage regulator and a 4-5 watt 6 volt panel, I did use a heat sink behind the voltage regulator but it's never gotten too warm. No resistors, no diode. The Nokia is pretty picky on incoming voltage, I had a few different cheap usb outlets that it wouldn't charge on.

PM me your address, if your in the US and I'll send you one of these 6V panels (likely next week unless I get called in today, Thursday is my "going to town" day.

The device, if designed to charge from a USB port, will limit the charge current, as USB ports have a minimum value of .5 amps and have been as much as 2 amps, currently they are shooting for 1 amp as a future minimum (I think)