What's wrong with this circuit? 12-0-12 1 Ampere Transformer Mobile Phone Charger?

Discussion in 'General Electronics Chat' started by amit1972, Jan 25, 2016.

  1. amit1972

    Thread Starter New Member

    Sep 15, 2013
    I have already build up a circuit with 12-0-12 1 Amp. Step Down Transformer against AC 220-240~. Connected bridge rectifier with 4 pcs. of IN4007
    Diode 470 micro farad 50 volt electrolytic capacitor supplying 15.49 DC going to breadboard having 7805 Power Regulator IC with 470 micro farad in the output with 5.1volt Zener Diode. After that connecting USB jack to my Android phone. My phone is taking charges but as the time goes, voltage drops up to 4.42 Dc & taking long time for the ampere to fluctuate continuously e.g. .32mA max to 00 mA lowest?

    Where is the fault actually ? Why it doesn't give constant output of 5 /5.01 volt dc with a constant Ampere during charging of my Android Phone ?

    Awaiting for your valuable suggestion ?


  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    It would be very helpful if you posted a schematic first. Even a hand drawing would come in handy.
  3. #12


    Nov 30, 2010
    I'm thinking the power dissipated by the 7805 chip might be overheating it so it goes into safety mode. How hot is it?
    Did you install the capacitors required to avoid the chip going into oscillation?

    You can not expect that regulator chip to be accurate to 5.01 volts (2/10 of a %) but you can expect it to be stable at whatever voltage it actually produces. Also, the voltage supply is not in charge of forcing a constant current into your battery, the phone is in charge of that.

    Annnd..he logged out in 2 minutes. Nothing to see here. Move along.
    Last edited: Jan 25, 2016
    cmartinez and Lestraveled like this.
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    I agree with #12. The 7805 is dissipating too much heat and is shutting down.

    Post a schematic.
  5. SLK001

    Senior Member

    Nov 29, 2011
    What is the 5.1 Zener diode doing in your circuit?
  6. hp1729

    Well-Known Member

    Nov 23, 2015
    Hmmm ... 15 V in, 5 V out means 10 v dropped across the regulator, at how much current? Gives a total of how much power? From your description I agree, getting too hot. Heat sink time.
  7. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    cmartinez likes this.