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adding some diodes to the input of a 3 terminal regulator

Discussion in 'Electronic Design' started by [email protected], Aug 4, 2013.

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  1. Guest

    I have a project that's using an LM2940-5V regulator with a source voltage of 9-12v. I'm pulling around 500ma through it, which is generating a lot of heat on the regulator.

    I'm wondering if I can just add a few IN400x diodes to the input of the regulator to drop the voltage and reduce the heat dissipation. Good idea? Bad idea?
     
  2. you can, it will just move some of the dissipation from the regulator to the diodes

    -Lasse
     
  3. Guest

    How come you can't use a heat sink?
     
  4. Phil Allison

    Phil Allison Guest

    ** To avoid heat, you could put a few NiCd or NiMH cells in series with the
    supply and reg IC and charge them.



    ..... Phil
     
  5. Guest

    How come you can't use a heat sink?

    I was trying to find a clever way to cheat. It sounds like the heat has to be dissipated, if not in the 3 terminal regulator than in the diodes (so I might as well stick with just the regulator). Next time I'll design a switching regulator into the board!
     
  6. amdx

    amdx Guest

    Can you get to the AC input? Probably not, but if...

    Use a nonpolarized capacitor in series between the secondary and the
    bridge.
    Calculation is beyond my ability, but experiment until you have enough
    overhead to feel comfortable. Speaker crossover caps work fine, and can
    be paralleled for low ESR and proper value. My starting point; I'd start
    with 40 uf and see which way to go.
    Mikek
     
  7. rickman

    rickman Guest

    Maybe I missed something. How would adding diodes to the input of the
    regulator ruin line regulation? A resistor might make some impact since
    it would produce a change in voltage with a change in current and some
    of that would appear at the output, although not so much really. But a
    diode is essentially a constant voltage device once biased on with very
    small changes in voltage drop with large changes in current. So how
    would this mess up line regulation as long as you keep the voltage above
    the dropout requirement?
     
  8. rickman

    rickman Guest

    If you are using a TO-220 type device with leads, you can replace it
    with a switching regulator in a similar package. CUI makes them, sold
    by Digikey, for most output voltages including +5v of course. I use
    them in a test fixture and they seem to work well, even with a lower
    dropout voltage than the spec requires. I'm running 5 and 12 volt
    outputs from a 15 volt input, saves a lot of power.
     
  9. whit3rd

    whit3rd Guest

    Somewhat, this depends on the source: is that a regulated, or just a filtered 9-12V?
    To shed heat outside the regulator, you could either put a resistor between the
    source and regulator input, or a resistor/capacitor parallel pair, if surges are expected.
    A 1W diode and its mounting/heatsink is more expensive than a 1W resistor and its mounting.
    The voltage drop on the resistor has a higher dV/dI than an equivalent string of
    diodes, though.

    Thinking outside the box, you could go to a lower-current regulator and a PNP transistor,
    with some current-sharing resistors, and move most of the heat dissipation into the
    transistor (assuming the transistor can take higher temperatures than the IC regulator).
    There's also the possibility of adding a choke to the AC into your source rectifiers, which
    will drop V without dissipating power.
     
  10. Guest

    Some three terminal regulators are quite sensitive to any input
    resistors, since they start to oscillate. To eliminate this, a big
    capacitor directly at regulator input to ground is required.

    Put the resistor/diode/choke between the rectifier bridge and the main
    reservoir capacity, so the capacitor is next to the regulator input.
    Choke input power supplies (or C-L-C) were popular in the tube
    rectifier era, since tube rectifier could not handle huge peak
    currents into a simple big capacitor. The choke in L-C or C-L-C
    configuration extended the conduction angle and the average DC voltage
    was close to the secondary RMS voltage.

    Unfortunately the choke at 100/120 Hz needed to be big and heavy and
    needed to also handle DC current (air gap), so it is understandable
    that chokes are seldom used these days.
     
  11. Robert Baer

    Robert Baer Guest

    Total heat dissipation will be the same; all that would do is
    distribute the heat sources.
    Want more efficiency and less heat generation?
    Use a switcher.
     
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