curious observation in using LM350/LM317

Discussion in 'General Electronics Chat' started by sureshparanjape, Feb 22, 2013.

  1. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
    2
    I am fiddling with developing 1.2 V to 30 V DC power supply using LM350/Lm317, circuity being same whether one uses either IC.With the transformer(12-0-12) 220 t0 12 V ac, I get converted 34 V dc. I am using fix resistance of value 222 between Iadj and Vout reducing maximum Vout to 30 V. I am using 5K pot between Iadj and earth. The circuit works fine for both IC with a curious observation- As POT resistance goes down, a stage comes for both ICs, when Vout goes on increasing! If I use transformer to get 220 to 12 v ac( actually, with the transformer I got I get 17 Vac!) and reducing to 15 v Dc as input for both ICs, the above observation doesn't hold. Why so? Can any expert find a reason for this curious observation?
    I checked my POT and it is working OK.
    I am using circuit given in data sheet of LM350/LM317.
    sureshparanjape
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    The 220 Ohms at the output to Adjust pins is to high.
    The regulator needs to be loaded with 10 mA minimum.
    A resistor of 120 Ohms between output and Adjust will do this.
    Also decoupling capacitors are needed for stable working.

    Take a look at this drawing made by Bill_Marsden:

    [​IMG]

    Bertus
     
  3. sureshparanjape

    Thread Starter Member

    Feb 10, 2012
    64
    2
    Thanks a lot.
    I will try with less value, as you have suggested, for the fixed resistaance and determine maximum vlaue of POT resistance so that I shall get desired resistance and let me see whether I don't observe that I had.
    Thanks again.
    I will post back this experience.
    sureshparanjape
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    I have seen your email:

    As I do not answer emails, I will try doing it here.

    Do you use a heatsink on the LM chip?
    If so, does it get warm, when you add the load?
    If it gets really hot, the LM chip will go in shutdown.
    The maximum rated powedissipation of the TO220 version is only 20 Watts for the LM317.

    It would also be nice if you could post a full schematic of your set-up.

    Bertus
     
    Last edited: Feb 23, 2013
  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    The 20W dissipation rating for a TO-220 is a bogus spec as it assumes an infinite heatsink. In reality, it can only dissipate about 1.5W with no heatsink and it can safely do maybe 8W maximum for the typical kind of heatsinks used with a TO-220 before it goes into thermal shutdown.
     
  6. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    If you look at a LM117/LM317 data sheet, you will see that the 117 part only requires 5ma flowing through that resistor so it can theoretically be as high as 240 ohms 1.2/.005 = 240.
    The 317 part wants 10ma so the the resistor will 1/2 of the 117's value.
     
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