LM317 output current?

Discussion in 'The Projects Forum' started by khawarspirit, Feb 13, 2011.

  1. khawarspirit

    Thread Starter New Member

    Feb 13, 2011
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    0
    So I'm simulating this circuit... And with the values of resistors I'm getting the desired output voltage but with the 10k resistor at the load point, I'm not getting current (as you can see). It's about 2mA however, isn't the LM317 supposed to give a current of 1A at any voltage with those resistor values? I don't get it...

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  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The LM317 is normaly capable to have an output voltage between 1.2 and 37 Volts.

    See the datasheet for more info:
    lm117_lm317.pdf

    Bertus
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    By Ohm's Law, V=IR: a current of 1Amp into 10kΩ would therefore imply a voltage of 10 thousand Volts!

    LM317 is for low voltages , and as it is not a switched-mode device its output voltage cannot exceed its input voltage.

    In general, any linear regulator can only deliver output voltages up to the limit of its input voltage. Actually, the maximum output is normally the input voltage less an amount called the "drop-out voltage" which differs according to the device type.
     
    Last edited: Feb 13, 2011
  4. khawarspirit

    Thread Starter New Member

    Feb 13, 2011
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    0
    Okay, right. Got it. Thanks.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    What is the intended output voltage?
    When you measure 2 mA with the 10 kOhm resistor,
    you will have an output voltage of 2 mA X 10 kOhm = 20 Volts.

    Bertus
     
  6. khawarspirit

    Thread Starter New Member

    Feb 13, 2011
    3
    0
    I need 25-26V at output. And the current was 2.7mA with the 10k resistor. My mistake, I thought that was a peak value. I simulated with a motor connected and it drew 3A... Of course I won't work a motor on it when I make it, but now I know it can get a high value. :]
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Diode D1 is connected backwards.
     
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