2805 Regulator Issue for USB/iPhone Charger

Discussion in 'The Projects Forum' started by Pelmore, Jun 13, 2013.

  1. Pelmore

    Thread Starter New Member

    Jun 13, 2013
    I am trying to add a USB charger to a motorcycle for charging an iPhone 4. I followed the schematic from this instructable but am having an issue with the voltage regulator output fluctuating.

    I'm using a 12v motorcycle battery for the source coupled with a LED and switch so I can turn it off and not drain my battery. Initially I started out using a 2805 voltage regulator to reduce 12v down to 5 volts. The 5 volts were then connected to the USB, but when that failed to charge the iPhone. More google searching showed that pins 2 and 3 on the USB need voltage as well. A plugged in iPhone wall charger and multimeter displayed the necessary voltages for the two pins as 2.95v (white D-) and 2.28v (green D+).

    In order to maintain a stable 5 volts, I added two electrolytic capacitors to the circuit. I used some capacitors I had on hand, one 47 μf capacitor at the voltage in and a 22 μf capacitor at the voltage out.

    I hooked up the Vout to the red USB wire and connected the black USB wire to the ground. To get the necessary voltage for the 2 and 3 pins on the USB I used two voltage dividers:
    [470/(560+470)] x 5v = 2.28v (green) D+
    [330/(220+330)] x 5v = 3v (white) D-

    Once everything was hooked up, my multimeter displayed 4.94 volts at the Vout. When I connected the iPhone 4, the charging icon flashed on. The 4.94 volts started to drop to 4.82 volts and would then fluctuate anywhere from 4.7 volts to 4.88. Once it got low enough, the iPhone would display the message of "Charging not supported with this accessory."

    I don't know why my Vout is fluctuating. I thought the capacitors were supposed to help eliminate that. Perhaps I have hooked them up incorrectly? I've included a schematic and picture of the breadboard. If they are hooked up correctly, am I not triggering the right amount of amps to start the charging circuit in the iPhone? If I remember correctly the iPhone looks for 1 Amp but can charge on 500mA...

    Any help would be appreciated!

  2. wayneh


    Sep 9, 2010
    It's not the capacitors. My guess is your 7805 is overheating due to the fairly large current the iPhone wants, plus the ~7V drop across it. When it overheats, it starts to shut down to protect itself. A heat sink would help. Have you tried touching it? Be careful if you do.

    You could place a low ohms power resistor in series with the input to the regulator. At 1A draw by the iPhone, a 4Ω resistor would drop 4 volts and take some of the heat off of the regulator. It needs to be rated for 10W or more to avoid overheating.

    The voltage will drop under load even within specifications. You may be able to keep the phone happy by just using a slightly different resistor divider on those data pins.
  3. wayneh


    Sep 9, 2010
    Oh, wait, did you really mean 2805? I think that's a low drop out regulator (nice) but it can only handle current below 300mA (not nice, for a phone). You need a 7805.