LM317 power supply help

Discussion in 'The Projects Forum' started by amansammy, Apr 2, 2013.

  1. amansammy

    Thread Starter New Member

    Nov 21, 2012
    7
    0
    Hello everyone..I have been trying to make a lm317 bench power supply using this circuit. Basically I want to use a 12volt battery as input(fully charged to 13volts) and 1.25v to 11v variable output.

    I have using this circuit: http://www.ladyada.net/images/diytools/schem.jpg

    For R1, I am using 220R resistor and for R2 I am using a 1K potentiometer. But I can only vary the voltage from 1.25V to 6.88V.

    I have put the LM317 on a big heatsink.

    What may be the problem with the circuit? Please Help.

    Regards
    Aman Sammy
     
  2. BrainFog

    Member

    Jan 24, 2011
    122
    4
    I just did some quick calculations and your problem is that you used a 1k potentiometer not the 5k in the schematic.

    You need a higher resistance on R2 to get higher a output voltage. 2k ohms should be enough.
     
  3. amansammy

    Thread Starter New Member

    Nov 21, 2012
    7
    0
    Actually I changed R1 from 220R to 100R and it goes from 1.25V all the way up to 11.55V. But LM317 gets hot under a very small load. I put up Leds in parallel taking 3V and 200mA and the transistor got very hot. What is the problem?
     
  4. BrainFog

    Member

    Jan 24, 2011
    122
    4
    By transistor I assume you mean LM317. They do get hot. It is because the voltage they drop multiplyed by the current is turned into heat energy in the LM317. This is particularly high if you are running more than a few 100mA at a much lower voltage than the input voltage.

    By lowering R1 rather than increasing the value of R2 will cause it to draw more current through R1 and R2. R1 and R2 are simply a voltage divider so it may be worth upping their values to draw less current. I would imagine that R1 and R2 are getting hot which will in turn mean the LM317 is drawing more current and getting hotter.

    How big is your heatsink?
     
  5. amansammy

    Thread Starter New Member

    Nov 21, 2012
    7
    0
    My heatsink has 2cm breadth, 4cm height and 6cm width.

    I changed R1 to 470R and R2 to a 10k pot and it still works about the same although it goes from 1.25v to 11.89v. Tuning this thing to the right voltage is very hard. I guess I have to get a 5k pot. Is there any way I can do without it?

    And thanks for replying so quick.
     
  6. Tealc

    Member

    Jun 30, 2011
    140
    10
    You can use discrete resistor arrays calculated to give you the output you want, then use a switch to select the individual resistor values to get your voltages. Using the 317 calculation you can work out exactly what resistances are needed.
     
Loading...