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Dc Power supply !!!

Discussion in 'Power Electronics' started by fuzedmass, Jan 7, 2010.

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  1. fuzedmass


    Jan 7, 2010
    Iam going to make a DC power supply....

    I have a 220V (rms) to 20V (rms ) A.C step down transformer with 3.5A rating.

    My power supply will have following features:-

    1. +5v (D.C) (LM 7805 IC)
    2. +0 -- +15V [variable] (D.C) (LM 317 IC)
    3. +0-- -15V [variable] (D.C) (LM 337 IC)
    5. 18.5V (D.C) ???

    Now my question is that I want 18.5 DC to be used for my Laptop charger...
    On my laptop charger (DELL D600) there is written:
    "Output 18.5 DC ; 3.3A"

    But my 1st problem is that MY 18V IC (LM 7818) have maximum current rating of 1.5A and 2nd it that is not 18.5V....

    How can I overcome this problem...:confused:

    P.S : I will make circuit on PCB...

  2. JasonB


    Jan 7, 2010
    Well, you need to find yourself an adjustable voltage regulator. Some can do 2A or so, you may be able to put them in parallel but that's just a guess. I have build a 24v supply using a number of adjustable regulators, it will work just fine. Good luck, I'm sure there is someone on here that can help you out further...
  3. Resqueline


    Jul 31, 2009
    You'll get 26.5V & 2.5A DC from that transformer.
    So the transformer may be overloaded (at times) when using the laptop.
    And the 18.5V regulator will have to be capable of dissipating at least 20W.
    Increasing the voltage 0.5V is just a matter of placing a diode in the negative lead of the regulator.
    Increasing the current is best done with a 0.33-0.47 Ohm resistor and a PNP-transistor in the input wire of the regulator.
    There are usually many such application examples in the datasheets for those regulator.
  4. davenn

    davenn Moderator

    Sep 5, 2009
    The LM338 is an excellent adj regulator with a 5 amp rating and out be ideal for your situation.
    My only worry is that you are usin g a transformer with a 3.5 amp rating to supply a device that requires 3.3 amps
    As I have just stated in another reply in this forum ... you have no headroom with your current supply and the
    transformer is gonna get hot working to its limit !!! you really need a transformer capable of ~ 5 amps so its not working so hard.

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