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LM317 voltage drop

Discussion in 'General Electronics Discussion' started by JS7777, Apr 23, 2015.

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  1. JS7777

    JS7777

    17
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    Apr 23, 2015
    Hello,

    I bought 5 lm317 for 1$ on ebay, wanted to make a bench power supply from a laptop charger, first for safety i take an 9v battery but there is a problem :

    i adjust the potentiometer to 5V to power up an arduino but the voltage goes down to 4.80V and for me this is not normal.

    I tried to figure it out, i tried others lm317 (the one i bought on ebay) but it's the same.

    The voltage input is a 9V battery.

    I tried the lm317 test circuit too but always the voltage drop at the output.
    page 5 : https://www.fairchildsemi.com/datasheets/LM/LM317.pdf


    Are these lm317 are fake ?

    Thanks by advance
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    Hello
    How much current is the Arduino drawing do you know? Can you draw your circuit showing the connections or take some close up pictures and post them here.
    Thanks
    Adam
     
  3. davenn

    davenn Moderator

    13,589
    1,872
    Sep 5, 2009
    If they were just those small pp3 9V batteries, the battery was probably failing under load
    as they have a very small current capability
     
  4. Arouse1973

    Arouse1973 Adam

    5,164
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    Dec 18, 2013
    Yes I did think of that, could they really fall that much? I have never loaded one to be honest, I am guessing the Arduino would be drawing a small amount of current maybe 10 mA or so. He might have some additional load on the circuit he didn't mention.
    Thanks
    Adam
     
  5. JS7777

    JS7777

    17
    0
    Apr 23, 2015
    Thanks for your response

    i don't know how much current arduino is drawing, i will mesure tomorrow.

    Here is the schematic :
    [​IMG]
    LOAD is the arduino or other stuff.
    R2 is a 2k potentiometer but i don't have one, i use 10K.
    For R1 i use 220ohm, not 150.


    I've done more test :

    -I adjust the potentiometer to max (7.90V). With a motor at the output the voltage goes down to 7.10V.

    -I adjust the potentiometer to min (1.30V). With a resistor of 30ohm at the output the voltage goes down to 1V.

    It's strange !

    Pictures of my circuit :

    [​IMG]


    [​IMG]

    Thanks
     
  6. davenn

    davenn Moderator

    13,589
    1,872
    Sep 5, 2009
    well you have the loading of both the actual load as well as the LM317 to think about
    those small pp3's are only capable of around 300 - 350 mA
    its going to take very little to load them down and cause a voltage drop at the battery
    and therefore at the output of the LM317
     
  7. davenn

    davenn Moderator

    13,589
    1,872
    Sep 5, 2009
    your bottom 2 circuit images are not showing


    that Q1, R3 and R4 are not needed
     
  8. JS7777

    JS7777

    17
    0
    Apr 23, 2015
    Q1, R3 and R4 is for limiting the current at 1A and to protect to ciicuit too because this is going to be a bench power supply and i like a protection for output.
     
  9. JS7777

    JS7777

    17
    0
    Apr 23, 2015
    [​IMG]

    [​IMG]

    The picture are not great, sorry
     
  10. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    Yep Dave's right. PP3 batteries have about 18R internal resistance. The output will drop to approx. 4.7 Volts when drawing 120 mA.
    Thanks
    Adam
     
  11. JS7777

    JS7777

    17
    0
    Apr 23, 2015
    It's the same with a 12V drill battery but less(1500mah)

    when i drain 150ma i loss 0,10V

    I like to have a output protection, how can i do ?

    Thanks
     
    Last edited: Apr 23, 2015
  12. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    You are using breadboard which has resistance, skinny wires which have resistance, the LM317 will have a certain internal resistance, this all adds up. You only need 0.6 Ohms of resistance to drop 100 mV at 150 mA.
    Thanks
    Adam
     
  13. JS7777

    JS7777

    17
    0
    Apr 23, 2015
    Thanks for your reply

    Then the circuit limiting is enough to protect the circuit ?
     
  14. BGB

    BGB

    154
    11
    Nov 30, 2014
    for running something like running a Raspberry Pi, I have a 9v brick of AAs I had been using as a backup. this can supply a little more current. a PP3 battery may not be sufficient for this.

    another possible issue is the regulator overheating, in which case it may not be able to supply enough power, and the output voltage will drop. this sort of thing may require using a heatsink and a cooling fan.
     
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