Setting the min Vout on a LM317 regulator

Discussion in 'The Projects Forum' started by Al Vince, Feb 25, 2015.

  1. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    I have a basic LM317 circuit with a pot, giving me a Vout range of 1.2v - 12v, as expected.

    I was wondering if there was a simple way to set the minimal Vout to 4v. So in essence, have a range of 4v-12v, 4 volts being the min Vout and 12v being the max Vout with the pot.

    Circuit I am using...
    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    yes put a preset in series with the pot, and set it to 4V.
     
  3. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Do you mean another pot, like a trimmer pot?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,148
    3,058
    No, a fixed resistor would do the job and is actually preferred, to prevent an inadvertent turn of the dial. It's Murphy's law that ALL dials will eventually be turned to the most damaging settings.
     
  5. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Awesome... I tried putting another pot inline and dialed in the setting I needed, and found a resistor with that setting and it seems to work!

    Thanks for the speedy reply.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,793
    Regardless of what you do, as the input ramps down you will see the output also ramp down as the input drops below the minimum required to produce a given output voltage. What I am saying is that the output will not go from the minimum regulated output down to zero but will follow the input down with an offset.
     
  7. Al Vince

    Thread Starter New Member

    Feb 24, 2015
    27
    1
    Yes I understand what you are saying... the input will be set at a fixed volt. How I have it setup seems to work... with a fixed input of 13v, I get about 4v-12v. The device I am using to power does not work below 4 volts, so no need for the lower settings.
     
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