Solar Cell Phone Charger

Discussion in 'The Projects Forum' started by RodneyB, Jul 18, 2014.

  1. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
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    I am trying to make a little cell phone charger using a USB socket. I have built it as per the attached diagram.

    I get a regulated 5 Volt output but the phone does not seem to be charging the battery monitor shows that its plugged in and charging but the percentage charge seems to go down.

    I tried it on my ipod and nothing happened.

    Have a left something critical out? Does it need a blocking diode.

    Any advice is greatly appreciated.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    Keep in mind that the 7805 needs an input voltage that is about 3 volts higher as the output.
    Also the regulator needs some current by itself to regulate.
    This quiescent current is about 6 mA.

    Bertus
     
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  3. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    So If I understand correctly this panel is two small for what I am hoping to achieve. Is there a more efficient way of regulating the voltage.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Last edited: Jul 18, 2014
  5. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    You could probably connect the PV cell directly to the battery and let the battery and PV cell self regulate. That will work as long as the PV cell output current is not too high so as to over charge the battery. Also a diode in series with the output will prevent the PV cell from discharging the battery under low light conditions.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,094
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    Most i-devices need specific voltages applied to the USB data pins, in order to charge properly. Look up your specific device and see what it needs, then set up a voltage divider (2 resistors) to produce that voltage for each data pin.

    But before you bother, make sure you have 5 volts on the power pins. It seems unlikely based on the specs you posted.
     
  7. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    Thank you for all the advice and help. What I don't understand is when the charger is plugged into the phone, it indicates that it is charging. However the percentage of charge goes down.

    I turned the phone of and again it indicates it is charging. When I turn the phone again the charge has gone down.

    I don't have a diode in series with the output, could this be a problem
     
  8. shakilabanu

    Member

    Jul 8, 2014
    44
    2
    Almost all digital devices these days have a battery management controller inside them, that looks not just for voltage, but also a certain amount of current - which varies from device to device. It is possible that the charging indication that you see when you plug in is because of the momentary current delivered by the charge in the capacitors... hence the device shows it is charging, but then since there isnt any current charge there after the actual charging itself doesnot happen.
     
  9. RodneyB

    Thread Starter Active Member

    Apr 28, 2012
    633
    13
    thank you for all the advice. I have removed the circuit with the 7805, put a diode in series with the + of the usb port from the solar panel and a 100Uf capacitor across the + and - and its charging.
     
  10. shakilabanu

    Member

    Jul 8, 2014
    44
    2
    As long as the voltage of the panel is 6V or so...that shouldn't be a problem...however this is certainly not recommended if the voltage of the panel were to go higher than that... watch it.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    Cool that you got it working. This tells you where the problem was.

    I'd eliminate the capacitor. It is not contributing anything and could potentially be damaging. It would charge to the open-circuit voltage while the phone is disconnected and then supply a surge at that voltage when the phone is plugged in.

    As suggested, I'd also check the voltage at the phone while the thing is connected in full sunlight. I'd get nervous about anything over, say, 6V. The phone expects 5V. If the full-sun voltage is too high, just add another diode. It will drop about 0.7V across itself and protect your phone from over-voltage.
     
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