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24V to 5V

Discussion in 'Electronic Basics' started by John Popelish, Apr 11, 2007.

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  1. The methods used in switching power supplies is to turn the
    full 24 volts on for 5/24ths of the time at some high rate,
    and low pass filter the resulting pulse stream with a low
    loss LC filter so that the 5 volt average appears at the output.

    You can buy all this prepackaged, or roll your own.

    http://www.interq.or.jp/japan/se-inoue/e_ckt22_2.htm
     
  2. Guest

    I need to convert a 24V source to 5V / 1.0 A max output. I tried using
    a 7805 but it generates alot of heat. Is there a way of gettinh 5V
    without the excessive heat generation? Thx!
     
  3. Meat Plow

    Meat Plow Guest

    The drop in voltage is going to generate heat no matter what. Maybe you
    could experiment and spread the drop across 2 devices 24-12-5 and use a
    larger heatsink or two. Are you afraid the heat is going to shorten the
    life of the 7805 or is it just undesirable in a certain environment?


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  4. Add a fan and heat sink?

    Use additional regulators to sucessively drop the voltage down(say a 20V to
    15 to 12 to 8 to 5). This essentially just spreads out the heat over the
    different regulators). This difference here is that you can use the outputs
    for power sources too(Although you have to be careful about the total power
    used).

    Your best bet is simply to get a transformer that actually drops down to the
    voltage you need. Your problem is that your wasting about 16W(max) in the
    regulator for no reason. 16W is not much if you get a better regulator but I
    think its way to much for the 7805.

    I would suggest you run down to radio sluts and get a 78M05 or even the
    LM317. These are better regulators that are essentially drop in replacements
    for what you want. You could also use an external transistor to handle the
    current(and can achive much larger currents if you wish).

    Either get a new regulator, use some method do distribute the heat across
    different elements, use a heat sink and/or fan, or get a transformer or
    wall-wart that gets you around 10V instead of 24V.

    Jon
     
  5. Switch mode PS?
     
  6. John Fields

    John Fields Guest

     
  7. John Fields

    John Fields Guest

    ---
    Sorry, no cigar! Not even a lollipop.

    The correct answer is "Use a buck regulator".

    The reason is that, doing it that way, power will be transferred
    from the source to the load reactively, with a very small penalty
    paid for the transfer.

    If he needs 5 watts out, max, and a 90% efficient regulator, the
    regulator will have to dissipate 1/2 watt, which is decent.

    The other way, if he goes totally linear, no matter how he splits
    things up he'll have to dissipate:


    Pd = (Ein - Eout) * Iout = (24V - 5V) * 1A = 19 watts

    in the regulator for 5 watts into the load.

    Totally indecent.
     
  8. John Fields

    John Fields Guest

    ---
    neglecting the quiescent current in the regulator, for 5V across a 5
    ohm load (1A):

    Pd = (Ein - Eout) * Iout = (24V - 5V) * 1A = 19 watts
    ---
    ---
    Not true. The LM317 requires a two resistor voltage divider to set
    the output voltage.
    ---
     
  9. Meat Plow

    Meat Plow Guest

    Well the point is moot since a self contained smps is the way to go.
    I'm more of an experimenter and trial and error learner than book.
    And I have much more experience with linear devices than reactive learning
    in an era when everything was filled with tubes. Built my first 50 watt
    guitar amp from scratch 33 years ago and not from a kit but from a xerox
    copy of a Traynor amp.

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    #1 Offishul Ruiner of Usenet, March 2007
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    #1 Bartlo Pset, March 13-24 2007
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  10. jasen

    jasen Guest

    switched mode power conversion - a buck converter.
     
  11. Meat Plow

    Meat Plow Guest

    They cost more than a buck.

    --
    #1 Offishul Ruiner of Usenet, March 2007
    #1 Usenet Asshole, March 2007
    #1 Bartlo Pset, March 13-24 2007
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    COOSN-266-06-25794
     
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