Need help making a solar Iphone Charger

Discussion in 'The Projects Forum' started by strm47, May 2, 2012.

  1. strm47

    Thread Starter New Member

    May 2, 2012
    4
    0
    Hi! I am new here and new to circuit building (I do electronics but havent done much component level and it's been a while since I graduated tech school. So, I have a 6V 1.5W solar panel. I also have a DC to USB board I pulled from a car charger. What I would like to do it connect a battery between the solar panel and usb charger so that I could use it to charge the phone etc in cases of night-time charging or cloudy days (I camp a lot and plan on using it then). Any ideas how I can regulate the circuit to put out between 200-500 mA and 5V? Thanks!
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,782
    939
    1.5 Watts at 6 Volts is 250 milliamps of output current from your solar panel. I assume the 1.5 Watt rating is also the MAX continuous output in full noonday sun.

    Do you have a volt meter? You need to measure the voltage and be sure of what it really is. Very possible that the output voltage is closer to 8 volts in anticipation of consumers wanting to charge a 6 volt deep cycle lead acid battery, which takes more than 6 volts to accomplish.

    May be that this will be as simple as adding a battery and a diode.
     
  3. strm47

    Thread Starter New Member

    May 2, 2012
    4
    0
    I do have a meter. I have metered it out and it does peak around 6V. Ive got a 1 amp diode to keep the panel from draining the battery and also a 5v regulating transistor (7805 i think?) So I have the battery going through the diode to the transistor, connected to the input of the transistor with the + of a 9v battery. I meter out the negative leg (battery and solar panel connected) and the output of the transistor but am getting 5v and about 1.5A, when the solar panel is only putting out about 2.5v (away from light). Im not sure exactly how to go about getting my current where I need it to be since the voltage coming out of the solar panel will fluctuate, and if a 9v battery is the best battery to use in this circuit.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    You can not use the 7805 regulator. You need to get a low dropout regulator. They are quite common at major component dealers
     
  5. evilclem

    Member

    Dec 20, 2011
    118
    16
    The solar panel voltage will vary with sunlight and load. Do not use a 9V battery with a 6V solar panel, the 9V battery will never charge.

    If you wish to continue using the 6V panel then you need to obtain 5 NiMH AA batteries in series. We charge such a battery pack with a diode and 10 ohm resistor.
     
  6. strm47

    Thread Starter New Member

    May 2, 2012
    4
    0
    So, just to make sure I have the right idea, I should run the panel through a diode, to a 10 ohm resistor, out to 5 aa series connected batteries, then out to a dropout regulator like a Lm2937 then out to the USB circuit?
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    I don't think you need the 10Ω resistor, IMHO. It's there to limit the charging current, which is a valid concern, but I don't think your panel is big enough to be a concern. The rating is probably optimistic. Use your meter to confirm.

    You said you already have a USB board from a car charger. The regulator in that device normally drops voltage from the system down to 5V. It might work for you as-is.
     
  8. strm47

    Thread Starter New Member

    May 2, 2012
    4
    0
    Thanks for all the input guys. I appreciate it! My last question I guess is how can I keep a constant current of about 200-500 mA getting to the USB circuit with the varying voltage output from the solar Panel and the batteries?
     
  9. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    The USB specification call for a voltage of 4.40V and up to a maximum voltage of 5.25V. Your Iphone should charge inside this voltage range. Do not wory about the current. That will the charger chip inside your phone take care of.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    An iPhone won't be happy with just any USB charger. It's going to look for specific voltages on the data pins and may not initiate charging if it doesn't see those. It's not hard to fix, but it's sure a pain to discover the hard way.
     
Loading...