Power supply design questions.

Discussion in 'The Projects Forum' started by dswartz2, May 19, 2009.

  1. dswartz2

    Thread Starter Active Member

    May 19, 2009
    31
    0
    Hi,

    I am in the process of trying to design a power supply that can take 230V down to 24Vdc.

    I'm using a full wave bridge rectifier with a transformer that has a turns ratio of 0.1.
    I am using a 7824 regulator to produce the constant 24Vdc output.

    I've calculated the circuit and got the Vo(peak) to equal 31.1V.

    I tried to solve for Cf, the first capacitor between the rectifier and the regulator, and got 2.03mF.

    My question is, what values should I use for C1 and C2 (the caps on the input and ouput of the regulator)?

    And does this overall design sound correct to anyone? I want to make sure i'm on the right track.

    Thanks for any help
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your current draw will be no more than an amp with the 7824, so figure on something on the order of 1000 - 2200 uF for the filter capacitor. Use a 50 volt rated capacitor at least. Capacitors are cheap, so it won't hurt to have more capacity than you really need.

    Use the ones suggested by the data sheet up next to the regulator.
     
  3. dswartz2

    Thread Starter Active Member

    May 19, 2009
    31
    0
    Thanks for the info.

    I wired this up, and I used a 3,300 uF capacitor for the filter. I didn't put a cap on the input of the regulator, but then I used a 4.4 uF cap on the output.

    The result is about 22.5 Vdc output on the load, with about 200mV ripple.

    When I measure the voltage across the 3,300 uF filter, I get 24 Vdc with about the same ripple (200mV).

    Why am I getting these values at the spots that I am getting them? and what should I do to fix it?
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    What is the voltage on the buffer capacitor without the 7824?

    Greetings,
    Bertus
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    What is the output (AC) of the transformer? The DC on the filter should be close to 1.404 times the AC output. For a 24 volt output, you're going to need about 27 volts on the filter.
     
  6. dswartz2

    Thread Starter Active Member

    May 19, 2009
    31
    0
    The AC voltage coming out of the transformer is about 22.5V.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    With 22.5 Volts AC you wouls expect to have 31.5 Volts DC on your buffer capacitor.
    Take a look at the attached PDF for some explanation.

    Greetings,
    Bertus
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Good material in the link.

    From the title "ETEC 1120" it seems to be course material used in teaching. As such, accuracy is of prime importance.

    It seriously need to be proof read by someone else as flipping a few pages I already pick up several errors.

    [​IMG]
     
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