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can you parallel voltage regulators to increase amprage?

Discussion in 'Electronic Basics' started by izzi4, Oct 8, 2004.

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  1. izzi4

    izzi4 Guest

    Hello, i'm looking to put together a 12v 3amp power supply. Can i parallel
    multiple regulators to add up to 3 amps or will this not work? Any other
    simple suggestions to get this working?
     
  2. It is difficult to get the regulators to share the output current
    equally. One will tend ot pass most of the current till the load
    causes it to current limit and the next one comes on, etc. So one
    tends to overheat and shut itself down.

    Better to buy a higher current device like an LM323 and modify it to
    provide 12 volts.
    http://cache.national.com/ds/LM/LM123.pdf
     
  3. izzi4

    izzi4 Guest

    any idea of where to get these inexpensivly? I always get stuck paying
    double for shipping and for only one component it seems dumb.
     
  4. Yeah. Digikey socks you with a $5 surcharge for an under $25 order,
    though they do ship via first class to save some on shipping a very
    small order.

    Mouser has no minimum order surcharge but charges a bit for shipping.
    The LM350K (diamond shaped TO-3 case for $5.70 and the power tab
    to-220 LM350T for $1.53 (harder to heat sink as well). You will have
    to go through the order process to find out what shipping they
    charge. I never buy one of anything. I always get at least one
    spare.
    http://www.mouser.com/
     
  5. To let a power-transistor take most of the load:


    .--------- --------------.
    | \ v |
    | --- |
    | | |
    | | ____ |
    V_in | ___ | | | | V_out
    o-----o-|___|----o----|7805|-----o--------o
    |____|
    |
    |
    |
    ===
    GND
    (created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
     
  6. Byron A Jeff

    Byron A Jeff Guest

    -
    -"izzi4" <> skrev i melding
    --> Hello, i'm looking to put together a 12v 3amp power supply. Can i
    -parallel
    -> multiple regulators to add up to 3 amps or will this not work? Any other
    -> simple suggestions to get this working?
    ->
    -
    -To let a power-transistor take most of the load:

    That's one way to go. You size R1 so that the transistor
    starts to conduct (Vbe of 0.6V) when the current across the small regulator
    rises to a set point (say 200ma for a standard 1A 78XX part).
    So using Ohm's Law:

    R = V/I = .600V/.200A = 3 ohms

    As the current demands of the load increases the small regulator will
    cause more current to be drawn across the pass transistor.

    Also note in the modifird verson below, there is a 7812 instead of a 7805.

    BAJ

    -
    -
    - .--------- --------------.
    - | \ v |
    - | --- |
    - | | |
    - | | ____ |
    - V_in | ___ | | | | V_out
    - o-----o-|R1_|----o----|7812|-----o--------o
    - |____|
    - |
    - |
    - |
    - ===
    - GND
    -(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
    -
    -
    -
     
  7. Clarence

    Clarence Guest

    Maybe it is just the schematic is hard to read, but the transistor should be a
    pnp power type with the emitter connected to V_in. R1 is only a return and
    about 470 ohms to 1K is recommended. The regulator will draw as much current
    as needed to raise the voltage on it's output pin, and by driving the base
    current it is (I_out / Beta)
    A MPS2955 would be a good choice for the transistor.
    NOTE: minimum over head for the regulator is increased by the V_be drop.
     
  8. Jamie

    Jamie Guest

    i wouldn't do that.
    because the chance you will get them to share the load equally is
    1:1000 and one them will end up doing most of the work and the 2
    others just loafing along.
     
  9. me

    me Guest

    It works if you use equilizing resistors as if you were paralleling BJTs,
    but an external pass transistor is a better way to go.
     
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