power sup heat

Discussion in 'General Electronics Chat' started by BobW55, Feb 22, 2014.

  1. BobW55

    Thread Starter Active Member

    Jul 22, 2008
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    I built a simple 24 VAC to 5 VDC power supply using the following:

    24 VAC 10VA wall wart.
    583-RS101 1 amp 50v Bridge rectifier
    L78S05CV 2 amp 5 volt regulator.
    I have a 100uF 35 volt electrolytic cap across the output of the rectifier, properly polarized.
    I have a 22uF 35 volt cap across the output of the regulator.
    A single led with a 150 ohm resistor on the output of the regulator to indicate 5 volts ON.
    I did put a small heat sink on the regulator, although it should not need it as of yet.
    I let the thing sit ON over night and noticed that the regulator was really warm-hot. Not too hot to touch, but warmer than I thought it should be to just light an led. Are my caps the wrong size maybe?

    Bob
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,367
    The problem is the wall wart voltage is too high.
    A 24VAC wall wart is going to output 34VDC.
    The 5V regulator has to dissipate 29V @ 20mA = 0.58W.

    At higher currents the 5V regulator will get very hot.

    You really need a 7.5 to 9VDC wall wart.
     
  3. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,887
    1,017
    First get a smaller supply. There are 5V cheap wallwarts out there. I have a whole box of them from old devices.

    Then look at building a switching regulator. With some of the new chips today they aren't that much more difficult that a 7805.
     
  4. BobW55

    Thread Starter Active Member

    Jul 22, 2008
    43
    0
    Unfortunately I need to use 24vac, the motor that the rest of the circuit will be controlling runs off 24VAC. Have to see if I have another bridge rectifier that may have a higher voltage drop on it.
    I was afraid the high input voltage might be pushing it.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    In that case what you need is a buck converter to take the voltage down to 5V.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Or if the load current drawn off the 5V output is fairly constant, you can use the old Mickey Mouse trick of adding a series power resistor to burn up some of the power. But using a switcher is better.
     
  7. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,887
    1,017
    Or add a second small transformer to bring the voltage down to something reasonable.

    But I would go with the buck convertor, they are really easy to work with today.

    Hint: Those USB car chargers have a buck convertor. You would need to check out the datasheet to see is they could handle that input voltage. Typically they use the MC34063.
     
  8. BobW55

    Thread Starter Active Member

    Jul 22, 2008
    43
    0
    Darn,
    Forgot about the RMS vs Peak of the AC.
    Not much room on the perf board to add more to it. Decided to use a 9 VDC wall wart and just run an extra wire for the switched AC leg. It is just powering a little AC syncro motor from a humidifier that refuses to run off DC.
    Thanks for the input.
    Some times I should be smacked for what I forget to consider.
     
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