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15VAC Dual adjustable regulated PSU 5V-25VDC

Discussion in 'Power Electronics' started by fader08, Apr 12, 2013.

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  1. fader08


    Apr 12, 2013
    Is this Design is right?? I'm just getting 5V-20VDC..
    how could I make it 5V-25VDC?? please help me.. thanks.

    Attached Files:

  2. duke37


    Jan 9, 2011
    To get more output you will need more input.
    The input will need to be at least 2V above the output.

    C2 and C6 put input ripple to the regulator reference. This will negate the purpose of the regulator.
  3. fader08


    Apr 12, 2013
    ahm, soo what should I do?? make a voltage doubler?? pls. help., I'm kinda new in this things. thanks.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    1) get a proper design that used an adjustable output regulator (LM317/LM337).

    2) get a transformer with 20-0-20 volt output (or thereabouts)

    You will need to be careful that the unloaded input voltage to the regulators does not exceed their maximum voltage.

    The power dissipation at high currents and low output voltages will be up to 80% higher, so a larger heatsink may be required.
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    These capacitors need to go to GND of the circuit, not the regulator IC.

    You set up the output voltage by adjusting the voltage at the GND pin of the regulator IC. But this setting relies on the GND current of the IC. This current will vary with temperature and load current. So this design is not usable.
    Use an adjustable regulator like the LM317 (positive) and LM337 (negative).
    If you stick to the 7805 and 7905, use an OpAmp to set a defined voltage at the GND pin of the regulator.

    Use another transformer. 15V (rms) are 21V(peak) at the capacitor (C1, C5), regardless of the size of the capacitor.
    If you use the LM types I recommended, you need at least 3V across the regulator foroperation. This translates to 28V (min) at the capacitors (C1, C5) for 25V output. Allowing for ripple and tolerances, I's set the capacitors operating voltage at 30V. 30Vpeak is 21V rms, so your transformer should be a 230V / 22V type. The electrolytic capacitors should be rated at 36V or higher.

    4th: Follow Steve's advise regarding the heatsink (he beat me to a mere minute with his reply) :)
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