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[Help!] Having all kinds of issues with a simple project.

Discussion in 'General Electronics Discussion' started by noahfarrington, Jan 31, 2013.

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  1. noahfarrington


    Jan 31, 2013
    Hey all! I've been having tons of trouble with a really simple project I've been working on. The idea was for a solar phone charger. Pretty easy, right? Just a 5 volt regulator and a usb out? Wrong.

    Firstly, I've never been able to get any device I've used to charge with the solar panel. I've tried numerous devices. I have 2 solar panels: One rated for 6 volts at 250 mA and one for 9 Volts and 168 mA. With both panels, my phone tells me it's charging, but the battery discharges instead. I've tried using both LM7805's and LM2940CT (Low dropout regulator), and I've also tried using voltage dividers to give the voltages to D+ and D- that some phones need for charging. None of this has worked with the panels. I can plug a battery pack into the charger and everything works fine, although the phone charges pretty slowly. Also, when the input voltage is high at all (above 7-ish volts) the voltage regulator gets hot. I was using it today with a 12-volt lead-acid battery, and even with a heatsink the voltage regulator heated up to over 100 Fahrenheit within 3 minutes. I'm really confused about all of this, as this is supposed to be pretty simple stuff. Any help would be appreciated, plus if you need any other information I can provide it.

    PS. One thing I forgot, is that while using the 9-volt panel, I measured only 2 volts coming out of the regulator, even though I checked the voltage before the regulator and it was right around 9. The regulator I'm using right now is the LM2904, which is a low-dropout regulator.

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    At the current demanded by the phone, the voltage droops significantly.

    You may find it better to place a 5.1V zener diode in parallel with the solar panel.

    This will limit the open circuit voltage that can be delivered, but not limit the power that can be provided (so you'll get the most from your panel)
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    As Steve said, the voltage from the solar cell will drop under load.

    The cell that's rated for 6V and 250 mA is probably only capable of supplying 250 mA in direct sunlight, and perhaps not even then. Typical cellphone chargers are rated to supply 1A. So you need a much bigger solar panel.

    When the phone tries to draw current from the solar panel to charge the battery, the panel voltage sags and the phone cannot draw much current out of it. The phone may think the battery is charging, because there is still some voltage on the charger input, but it will not charge.

    This all fits with the explanation Steve and I have given you.

    Right, the regulator will dissipate power according to P = I V where P is power dissipated (in watts), I is the current flowing through the regulator from input to output, and V is the voltage dropped ACROSS the regulator (from input to output). So when you provide a good solid 12V supply into the regulator, two things will happen. First, the regulator will be able to supply the current drawn by the phone as it charges the battery (probably somewhere between 0.5A and 1A) and the voltage across the regulator will be significant.

    Assuming the regulator is dropping 7V (12V - 5V) and passing 750 mA, it will dissipate about 5 watts. With no heatsinking, a small regulator will easily exceed 100 degrees Celsius after a short time at 5W dissipation.

    Steve's suggestion of using a 5.1V zener across the solar panel might help (but don't connect it across a 12V battery!) but the zener will dissipate significant power and may need to be heatsinked as well. Personally I would stick with a low-dropout regulator rated for at least 1A and add heatsinking as required.

    Please be careful with your numbers! An LM2904 is not a regulator; it's a dual op-amp! I guess you mean LM2940CT-5.0.

    I guess when you measured 9V coming out of the 9V panel when the phone wasn't connected to the regulator output.

    When you connect the phone (load) to the regulator output, the phone tries to draw a lot of current from the regulator, which in turn tries to draw it from the solar panel. This is what makes all the voltages sag. So if you measure the solar panel output with the regulator and phone connected, you'll probably find it's only a little more than the 2V that you measure at the phone.
    Last edited: Feb 1, 2013
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