Linear power supply w/ multiple outputs

Discussion in 'The Projects Forum' started by Trip1, May 27, 2012.

  1. Trip1

    Thread Starter New Member

    Jan 28, 2012
    Hello, I am attempting to build a linear voltage regulated power supply with the following requirements:

    -3 outputs:
    • 1 Variable 0-30Volts output up to 1A
    • + / - matched (tracked) output for powering various op-amps (up to 22V for ua741)

    I have the following design that only does the first variable output, and I think this part of the design is pretty solid:


    -Note that the second switch off the secondary of the transformer is basically used to control heat dissipation in the LM317. I will hit the switch when I need high current at low voltage to reduce the voltage drop across the regulator.

    I'm wondering how I could modify this to include + / - supplies for powering op-amps. I want to be able to adjust them from say 15V up to 22V, and I want them to be tracked (adjust them both with 1 pot)

    Would a second transformer be the best (easiest) solution?
    Last edited: May 27, 2012
  2. crutschow


    Mar 14, 2008
    What is the purpose of the two diodes in series with the output? They will just degrade the regulation of the regulator.

    Here is a dual tracking regulator. It probably would be easiest to use another transformer for this circuit.
  3. Trip1

    Thread Starter New Member

    Jan 28, 2012
    Thanks for the response. The two diodes in series with the output are a quick way of obtaining 0 Volts from the LM317.

    How would the second transformer be wired in? Is there a way to do it so that the power supply would only require 1 plug?
  4. crutschow


    Mar 14, 2008
    You should use a larger diode than the 1N4007 for that purpose since it has a 1A max rating and you should not run any device. A larger diode will be more reliable and also have a lower forward resistance so the voltage change with load current will be less.

    The voltage change from no load to full load will still be over a volt, if that's no problem(?). Remember that a diode forward drop versus current is logarithmic so you will still a small minimum output voltage at no load.

    Just connect the two transformer primaries in parallel to the mains power through your switch and fuse.
  5. mcasale

    Active Member

    Jul 18, 2011
    You didn't give a load current for the dual supplies. If they are just driving a few op amps, you won't need much.

    Here's a chip that might do what you want (SMT only, looks like) ...
    There are fixed versions and an adjustable version.

    With such a small load (?), I'd doubt you need a second transformer. Can you find one that has another set of secondaries? That might improve the heating you're going to get in the 1A supply.
  6. wayneh


    Sep 9, 2010
    +1 It's a common mistake to assume diode drop ∆V is a constant. It's not, and is also a function of temperature.
  7. upand_at_them

    Senior Member

    May 15, 2010
    I've seen a bunch of people talking about getting their LM317-based power supply to go down to 0V. What are people doing with it that they need to go below 1.25V?
  8. takao21203

    AAC Fanatic!

    Apr 28, 2012
    They don't use LM317 for that.
    Typically, OpAmp as error amplifier + pass transistor.

    Reference voltage to one OpAmp input, and feedback to the other.
  9. absf

    Senior Member

    Dec 29, 2010

    I think the polarity of LED D2 in the schematic is wrong.

    Last edited: May 29, 2012
  10. Wendy


    Mar 24, 2008
  11. upand_at_them

    Senior Member

    May 15, 2010
    LED2 or D2? They're both correct. D2 is a reverse protection diode to prevent a reverse power spike from the load from killing the voltage regulator.