Electrolytic voltage sizing power supply

Discussion in 'The Projects Forum' started by Hamlet, Nov 2, 2015.

  1. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I'm working and learning about power supplies.
    How does one size the voltage of an electrolytic cap after
    the diode bridge? At say 24vAC from transformer,
    then after diode bridge I read about 21v, but when
    the cap is installed, it voltage reads closer to 35v.

    Do I need a smoothing capacitor rated for 25v, or 35v?
    Maybe a little extra on top?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
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  3. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Thanks! 50v cap should do it.
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    Rules of thumb for high reliability design in military and industrial applications:
    Capacitor voltage rating is at least twice the peak voltage applied.
    Transistor voltage rating is at least twice the peak voltage applied.
    Resistor power rating is at least twice the average or RMS power applied.
    Diode current rating is at least twice the peak current applied.

    Etc.

    ak
     
    Roderick Young and Hamlet like this.
  5. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    I recently repaired an old radio shack 12v 2A supply for a friend. It had single 2200uf
    16v smoothing cap. I thought that was low, but I wasn't sure. I had a 35v 4700 in my
    junk box, so I replaced it with that. That got me to wondering whether the ac
    voltage or the peak voltage was the one in consideration. I guess the lesson is
    this: I shouldn't let radio shack engineering influence proper application.
     
  6. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    A 16 volt capacitor will survive just fine with 12 volts DC applied to it. Were you thinking the 12 volt DC label meant Radio Shack started with a 12 volt AC transformer?
     
  7. Hamlet

    Thread Starter Member

    Jun 10, 2015
    100
    2
    Yes, that was my assumption.
    Come to think of it, that unregulated radio shack
    power supply sure couldn't keep
    the voltage up under even a modest load.

    9v transformer * 1.414 = 12.726v...

    Oh, the irony.
     
  8. RichardO

    Well-Known Member

    May 4, 2013
    1,238
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    Note that the secondary voltage of an unloaded transformer will be about 25 percent higher than the voltage at the rated load. So your 9-volt transformer could put out as much as 16 volts into an open circuit.
     
  9. #12

    Expert

    Nov 30, 2010
    16,343
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    True.:(
    Maybe not exact, but true.
    Two of the difficulties in designing linear voltage supplies are the allowable variation of the power line voltage and the transformers' voltage output change from unloaded to loaded conditions. Over-designing for these variables is a big factor in how much energy is wasted in a linear regulator.
     
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