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Homemade Solar Charger Oscillating and Not Charging

Discussion in 'General Electronics Discussion' started by abingenui, Jul 20, 2013.

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  1. abingenui

    abingenui

    3
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    Jul 20, 2013
    Hello,

    I am a recent addition to these forums and am seeking some advice or troubleshooting help regarding a buck converter I built to charge my smartphone.

    So my design utilizes the lm2574 5.0 switching regulator with two 1.5W, 6V, 250mah solar cells connected in series. According to the datasheet(it's the 8 lead DIP), I needed a 470uH inductor which I have.

    Now I have everything else connected as it should be and have double checked a multitude of times as to the circuit's correctness. I test the output voltage and it stays constant at 5V DC when the cells are under full sun. When I plug in my phone, the charging light comes on. It does not last forever and my phone goes into cycles of "charging" and "not charging". Using the multimeter, I find that the voltage cycles between 3.9-5.0 V. I know smartphones nowadays are really picky with what they are charged with.

    I'm taking a wild guess (I am not a electrical engineer, I just fiddle about these things as a hobby), but I think that every time the phone goes to draw power, the voltage drops and then the phone's stops drawing power when it realizes the charger can not keep up. This just goes on, and on, and on.

    So my question is, does anybody have any idea how to remedy this?

    Here is the url for the lm2574 chip. The inductor selection graphs are located about 2/3rds the way down.
    http://www.ti.com/lit/ds/symlink/lm2574hv.pdf

    *Also, I think this thread is in the wrong forum. I thought I had selected circuit help. Guess not.
     
    Last edited: Jul 20, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    You're almost certainly guessing right. You phone probably wants around 1A from a panel unable to supply it.

    The answer is to get a larger panel.

    Or a larger sun.
     
  3. abingenui

    abingenui

    3
    0
    Jul 20, 2013
    The thing is I've charged my phone on chargers with less than one amp. It charges fine on any USB port from my desktop/laptop/etc which is anywhere from 100-500ma. I'm guessing the phone requests the full 500ma. A quick google search makes me believe that there is no limit to the lowest amperage the phone will accept, though there doesn't seem to be a solid answer on the matter. To perhaps increase the amperage, I have some other solar cells that are 6V but with assorted lesser current ratings. Could I try putting those in parallel with my current array? My phone is the HTC Evo 4g lte, for reference.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    OK, but at best, you'll get 250mA from this panel.

    Depending on the phone it may detect it's not connected to actual USB and assumes it can draw full current (for some devices this could well exceed 1A.

    Also, it's doubtful your panel is delivering the full 1.5W

    Also, it's not an mppt device, so as it won't find the point at which it's getting the most power.
     
  5. abingenui

    abingenui

    3
    0
    Jul 20, 2013
    So with some fiddling and adding of panels, I was able to get 5.09 volts at 475 ma. Now when I plug in the phone it shuts itself off, probably due to a protective charging circuit. So now I have to find why the circuit is bugging out with the charger attached. Typically it'd be because of voltage or current spikes, right? The datasheet talks about adding a capacitor and inductor to the output of the circuit to decrease ripples. Have no idea if I am making sense, just kinda talking out of my butt to see if anybody agrees before I go off and buy more components,
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    When you say you get 5.09V at 475mA, how did you measure that?

    If you placed a load that drew 475mA on it, and measured the voltage and current under the same conditions, then those figures are valid, but the voltage was probably far higher without load (and that would have caused the phone some grief)

    If, instead, those figures were obtained by placing first the voltmeter across the panel, then the ammeter, the results simply indicate the maximum voltage and current. (I would expect perhaps 4V @ 350mA as being a more typical maximum power load for this arrangement)
     
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