Building a Variable Power Supply with Limiting Current Control

Discussion in 'The Projects Forum' started by blah2222, Jun 2, 2010.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi all, I'm starting out and would like to build myself a variable bench power supply that keeps current to a maximum of 1A and voltages anywhere between 0V and 30V. I have looked at many schematics but I'm not 100% sure of which to go by. I have no problem with the transformer/rectification and smoothing processes of the design but I'm not sure how to set up the voltage regulator (LM317).

    If anyone can help me out with an easy to follow schematic or help me figure it out I would much appreciate it, especially help with what kind of values for resistors/caps/pots I have to use, that's what I need most help with. The basic flow diagram I imagine is just:

    AC Input -> Step-down Transformer -> Diode Bridge Rectification -> Smoothing Capacitors -> Voltage Regulation and Safety Fuses...?

    Thank you,

    JP
     
  2. Markd77

    Senior Member

    Sep 7, 2009
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    Download the LM317 datasheet, it has examples. If your datasheet doesen't, try one from another manufacturer, the circuits are the same.
    LM317 limits at 2.2A anyway and has thermal shutdown.
     
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  3. Pich

    Active Member

    Mar 11, 2008
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  4. beenthere

    Retired Moderator

    Apr 20, 2004
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  5. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Thank you very much for the replies, much appreciated! After more research I kept on coming across this LM723 being a heck of a lot easier to use for this project.

    I came across this page that gave a nice easy laid out schematic that I embedded below:

    [​IMG]

    Just a couple questions that I have regarding this circuit:

    1) Why are there two smoothing capacitors (one electrolytic and regular), rather than just one?
    2) What is the role of each potentiometer besides being a knob that can turn on the outside of the power supply box?
    3) Why are there two different transistor models, couldn't we use two of the same model?
    4) For the other resistors and caps (560ohm/0.15ohm & 100pF/100uF), what are they doing?

    I'm sorry if my questions seem obvious to answer but I'm trying to figure this stuff out.

    Thank you again for your help!
    JP
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Click to see schematic: http://www.eleccircuit.com/wp-conte...lts-25a-variable-power-supply-using-lm723.jpg

    The 100nF (0.1uF) cap takes care of the high frequency transients, and the other cap takes care of the large, low-frequency transients.

    There is only one pot; that attached to pin 5. It sets the output voltage.

    The 2N3055 has low gain, but can output quite a bit of current.
    The BD135 can't output much current, but has relatively high gain.
    The BD135 only has to output about 1/10 of the current that the 2N3055 does.
    The resistors are establishing current levels which are necessary for the rest of the circuit to function properly. The capacitors are decoupling.

    Resistors pass current, therefore drop voltage across themselves. E=IR, or Voltage = Current x Resistance.
    Capacitors block DC, but pass the effects of AC.
    Large values of capacitance (>=0.1uF) are typically used to filter power.

    That's why the forums are here. :)
     
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  7. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Wow that's awesome, thank you SgtWookie! That cleared up a bunch of stuff!

    Is there a reason why the 100nF is electrolytic? Couldn't they of both been just ceramic capacitors? I know that electrolytic tend to have higher capacitance values than ceramic but couldn't a ceramic be sufficient?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    The 100 nF cap should be ceramic. In the schematic you posted, C1 is the electrolytic, as shown by the representation of one plate being curved. C2 is a non-polar capacitor symbol. So C2 is 100 nF, C1 is 100 uF.
     
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