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Quick question - voltage regulators in parallel?

Discussion in 'Electronic Design' started by lain, Dec 6, 2006.

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  1. lain

    lain Guest

    Hi, just a quick question, I'm working on a power supply for a project
    and I need a +5v @ 2amp supply. I've got some LM7805's handy, but
    they're rated at 1amp each. This question probably sounds stupid, but I
    don't know the inner workings of these regulators so I'm not sure if
    it's safe: can I hook them up to the same supply and connect the output
    pins together to get my +5V @ 2A output?
    Thanks!

    -Eric A.
     
  2. Joerg

    Joerg Guest

    Nope. I would definitely not do that. They never track each other exactly.
     
  3. lain

    lain Guest

    Alright, thanks Joerg. Do you know of any 2A-rated regulators like the
    trusty 7805 series?
     
  4. Eeyore

    Eeyore Guest

    No. They'll probably end up having an argument over who's boss.

    Graham
     
  5. Henry Kiefer

    Henry Kiefer Guest

    There is a version up to 3A but I don't know the part number.

    - Henry
     
  6. Joerg

    Joerg Guest

    There used to be 5A versions but they might have been obsoleted. There
    is no big market for 5V anymore and today everyone uses switchers. When
    I designed big 5V supplies in the old days I did it in discretes since
    the higher power regulators were hard to get and quite expensive.
     
  7. They might argue, but try putting a small R, say 0R2 in series with
    the o/p of each regulator, it might even the balance, maybe, possibly,
    if they are from the same batch.

    Of course this will affect the Vout, it depends on your requirements

    But check that they have the same o/p voltage at the same load first


    martin
     
  8. Joerg

    Joerg Guest

    The 78H05 was the 5A version but AFAIK it's long since obsolete. For a
    one-off hobby project it may be possible to find one somewhere.
     
  9. Henry Kiefer

    Henry Kiefer Guest

    There is even an option to use a 7805 with a PNP transistor and the 7805
    does regulation where the PNP powers.
    If the OP goes behind the 78xx part number there is surely enough equivalent
    parts around with higher current spec.

    And there is a 7805-like switcher chip made by TI.


    - Henry
     
  10. lain

    lain Guest

    Unfortunately if this works I'll be making a lot of them, so the
    one-off solution won't work.. however after digging around the
    datasheet for this 7805 I see it says it's fine to go over 1A so long
    as you provide adequate heatsinking... so I'll just slap a nice
    heatsink on it & some thermal paste and hope for the best...

    - Eric A.
     
  11. Eeyore

    Eeyore Guest

    Most 'standard' 7805s are 1.5A rated these days but you may of course have old
    ones.

    Graham
     
  12. Well, not a great idea.

    What's going to happen is one of them will have a setpoint a few
    millivolts higher than the other one. The one with the higher setpoint
    will try to pull the voltage up to its desired level. Since it's
    unlikely to put out 2 amps, it will go into current limiting and
    effectively become just a say 1.25 amp current source.

    Then the one with the lower setpoint will become the voltage regulator,
    putting out 0.75 amps at it's setpoint voltage.

    This might be a stable situation, for small values of the word
    "stable".

    All of course in theory. In reality they might oscillate, or the
    higher one might go into a full thermal shutdown. Some of them have a
    designed in thermal hysteresis-- so it might go into wild on-off cycles.
     
  13. Chris Jones

    Chris Jones Guest

    There is an adjustable one, I think it's the LM338T
    http://www.national.com/pf/LM/LM338.html

    You will need an extra two resistors, and I suggest metal film, 1% or
    better.

    Chris
     
  14. Guest

    Why not look into the PTH series from TI? Switching regulator modules
    in a variety of packages and input voltages. Plus they don't heat up
    that much.
     
  15. lain

    lain Guest

    Awesome, looks like that would be my best option, as I will also be
    producing a few other voltages.. now for another quick question, I'll
    have 12volts coming in to the circuit, but it's going to run from a car
    so I'm expecting a LOT of noise on the line. Would, say, five 6800uF
    Electrolytic capacitors in parallel with the incoming 12V (about 0.034
    Farad) work well to filter it? I've been playing with the idea of using
    "super capacitors", but they're pretty pricey and because most of them
    are rated around 2.5volts that I can find, I'd have to use them in
    series and though the overall capacitance would be higher by a decimal
    place or so, I understand putting caps in series should be avoided..
    Any thoughts from you experts out there? Thank again!!

    -Eric A.
     
  16. I remember the PNP trick, but couldnt remember which data sheet it was
    is, prolly Natsemi, a suitable place to retire old 2N3055s' maybe


    martin
     
  17. lain

    lain Guest

    Ohhh neat, never seen these before. But! - I'm on an extremely tight
    budget and these linear regulators are only about $1 each, whereas the
    PTH-series seems to run about $11 to $30 each (mouser.com being my
    source), but I've got 'em bookmarked for future projects now, thanks!

    -Eric A.
     
  18. Eeyore

    Eeyore Guest

    About as well as trying to stop an elephant with ummmm.... anything not very
    big.

    Have you heard of these things called LC filters ?


    Graham
     
  19. redbelly

    redbelly Guest

    You can use the extra series resistors for a quick test / prototype,
    but for longer term you might as well get some 2A 5V regulators:

    http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=497&M=L78S05CV

    Also possible: use a high-current bypass transistor as shown in Figure
    14 here:
    http://www.fairchildsemi.com/ds/LM/LM7805.pdf
    You will need a pnp transistor that can handle the extra 1 Amp.

    Mark
     
  20. Eeyore

    Eeyore Guest

    Why not calculate the heatsinking required ?

    What's the input voltage ?

    Graham
     
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