LM317 pot overheating

Discussion in 'General Electronics Chat' started by Garurumon, May 25, 2013.

  1. Garurumon

    Thread Starter Member

    Mar 17, 2013
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    http://i210.photobucket.com/albums/bb204/n00blord1/25052013075_zps769fcba0.jpg

    I had a spare transformer so I decided to make a small power supply for my desk projects.. The plan was to go from 1.25-about 14V. The scheme above is what I came up with. I used LM317 online calculators to get the resistor values.

    LM7805 part (intended for some chips) works great, no complaints there. But every pot i put in LM317 part ends up in flames. I also used fixed resistors, and they also burn.

    What am I doing wrong? Why could it draw so much current?
    I tried it with load too, still the same, resistor gets destroyed.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    I guess you have the 317 connected wrong. Also the 1k resistor should actually be 120 ohms.
     
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,809
    The adjust pin should only pass about 50 microamps, so I think your 317 chip is bad or you did a mis-wire.
    If you have an external load of 10ma, the 1k resistor will work. Otherwise, reduce the 1k to 120 ohms and shunt the pot with 1200 ohms.
     
  4. Garurumon

    Thread Starter Member

    Mar 17, 2013
    99
    1
    I checked it about zillion times, nothing wrong with the connections...

    It's bad if it's 1k?

    I used 1k only because they had no pots smaller than 10k in the store...
     
  5. Garurumon

    Thread Starter Member

    Mar 17, 2013
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    1
    And yeah, one more thing, my lm317 doesn't go down to the 1.25V but to 6.4V instead when ADJ is grounded...
     
  6. LDC3

    Active Member

    Apr 27, 2013
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    The first difference I see in your configuration and the data sheet is that you are using capacitors 100 to 1000 times larger, but that should have little effect unless it creates some sort of resonance in the voltage regulator.

    Are the resistors burning with a load?
     
  7. Garurumon

    Thread Starter Member

    Mar 17, 2013
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    1
    Yes, they also burn when load is connected.

    So I should reduce caps too?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  9. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Just checking:

    You have pin 1 connected to the pot (it is the adjust pin), pin 2 is your output voltage, and pin 3 is the input. You're sure you have these connected properly?

    The resistor values should be okay. I can't imagine reducing the capacitance would make much of a difference.

    Matt
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Remember, The pin assignments on the LM317 are not the same as the LM7805. The center pin on LM7805 is ground, the center pin on LM317 is output!
     
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