Current Limit Of Charger

Discussion in 'The Projects Forum' started by Munyaneza Ghislain, Dec 29, 2015.

  1. Munyaneza Ghislain

    Thread Starter New Member

    Dec 29, 2015
    Hello everyone,
    It's my first post on AAC. I am a Belgian student in electronics engineer and i work also part time . I am in charge of finding a better way to charge tablets used in restaurants. But I have issue with the power supply. The system we use is very simple and i am probably missing a lot of things that's why i though of asking in order to improve it .



    The system consist of a power supply that gives current to 2 PCB in order to have a contact charging for tablets instead of using wires. When the tablet is in contact with the pcb charger’s pins it closes the circuit and light the led corresponding We use 3 Power supply ( 2 output V+ and COM ) for 60 tablets ( 10 per PCB ).


    Chinese High current swiching power supply Model S 100W AC Input: 110V/220V +/- 15% DC Output: 5V 20A ( 2 output )
    Aliexpress 5V 20A power supply


    The Charger PCB with the loaded spring is used to charge the tablet whenever there is a contact. The middle and the right row are connected in series with respectively V+ and COM output of the Power supply . The left row of pins is used to close the LED circuit so the user can know when the tablet is charging . The connection between the 2 pcbs is made with a “wire-to-board”. Resistors are 1.5K and these are white LED .


    The center of the connector is the positive and the 2 others are connected together as Negative . When one tablet is connected one of the negative pins is used to to light the corresponding LED. The connector is soldered on the DC input of the tablet.


    Voltage Drop : the power supply tend to have a voltage drop from 5V to 3.5V-2.5V when i put the maximum tablet and most of the time it comes with a sizzling sound.
    Heating : During the charge the tablet tend to overheat.


    When i measure the current ( 5% charge ) for one tablet is aproximately 1.5A but when i put the maximum for one output ( 10 tablets ) it never goes over 9.5A . Plus , since the tablets are stacked it tend to overheat also , like really hot 60-70°C
    I was thinking of adding a resistor to limit the current as i think overcurrent is the problem .
    I don't mind limiting the charging rate as it would charge all night .
    So my question was would it work to just add a resistor and if so what value should i use for 500ma ?

    Appreciate your time !
    Last edited: Dec 29, 2015
  2. Picbuster

    Well-Known Member

    Dec 2, 2013
    This will not work. Reason the voltage will drop.
    What you need is a current source at a constant voltage.
    At the other hand tablets do have their own build-in battery loader and should care about this.
    Or are you connecting the batteries directly to your device? ( please do not it could make the batteries to blow up (explode).)
  3. tcmtech

    Distinguished Member

    Nov 4, 2013
    If your power supply is a dual output unit and you can only get ~9 amps from a single output I would try putting both outputs together ans see what i does.

    If it still cant get past ~9 amps without having a major voltage drop the odds are is a highly over rated unit and you need a bigger better quality one to carry the load you want.

    Also given the low working voltage and moderately high currents the system could be carrying I would recomend using heavy gauge wire like 8 gauge multistrand copper for all the high current wiring plus id the power supply has a output voltage fine tune voltage cheat it up about to something like 5.5 - 5.8 volts open circuit.
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    Obviously the 20Amp PSU is not what it is crack up to be.
    Or your wiring is not thick enough.

    You should route each tablet connection directly from PSU terminals. One for each