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LM 7805

Discussion in 'Beginner Electronics' started by Gunner, Dec 24, 2003.

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  1. Gunner

    Gunner Guest

    Can you put two lm7805's in parallel
    to gain grator current. or will one
    interfear with the other
     
  2. UncleWobbly

    UncleWobbly Guest

    generally not... there will always be tiny variances in the output voltage
    and one will tend to pull current from the other.. these currents can be
    considereable if there is no resistance between the outputs... ohms law will
    tell you that even 20mV divided by 0ohms is infinite current... not actually
    infinite but they will get hot and it will be a very inefficient way of
    getting a greater current source. What you could do is to use one of the
    5Amp versions (can't remember the number off the top of my head, LM309K or
    sunnik like that?) which comes in a power transistor can (TO5?) or if that
    is not enough, use a 78L05 with a 1N4001 in series with it's ground
    connection, as the base voltage for an emitter follower regulator formed
    from power transistors... the current capacity in this form of PSU design is
    virtually limitless with the right tranny. The diode in series with ground
    will actually make the output voltage about 5.6V which will counter the
    voltage drop of the power transistor. Bags of designs for this sort of high
    current PSU design exist from the days of neat bipolar TTL, before the
    benefits of CMOS took over with 74L & 74C devices etc.

    have fun.
     
  3. The datasheet for the 78XX series regulators gives application hints for
    raising the output current capacity:
    http://www.national.com/ds/LM/LM78XX.pdf (page13).
    There's an LM323 (3A) if you want to keep it simple. They produce an LM338
    (adjustable) rated at 5A. The LM309 is rated at 1A.
    Bryan
     
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