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How easy is it to fry an LM317?

Discussion in 'Electronic Basics' started by Jeffrey C. Dege, Feb 24, 2004.

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  1. I'm trying to get an LM317 hooked up to generate 5.0V.

    R1 = 1k and R2 = 3k,

    Vo = 1.25 * (1 + R2/R1) = 5.0

    Except that it doesn't. I'm getting Vo of about .25V less than Vi.

    (I've tried Vi from 2.5V to 12V).

    Any possibility that I've fried the thing?
     
  2. CFoley1064

    CFoley1064 Guest

    Subject: How easy is it to fry an LM317?
    This kind of thing happens if it's not hooked up correctly (View in fixed font
    or M$ Notepad):

    .---------.
    | |
    | o |
    | |
    |---------|
    | LM317 |
    | FRONT |
    | VIEW |
    | |
    '---------'
    | | |
    | | |
    | | |
    | | |

    A O I
    D U N
    J T

    You can fry 'em if you hook them up incorrectly -- seen it many times. The
    first time I hooked up an LM317 (back when the earth's crust was still
    cooling), I wired it like a 7805, and got similar results. If you've got a
    spare, best to substitute -- *after* you check your wiring. But who knows --
    it might still work.

    By the way, if you're using an R1 greater than the standard 240 ohms, you want
    a minimum output load current. The data sheet says you'll probably want at
    least 4 mA of load current. I'm not sure if that would cause your problem, but
    you might want to check it, just in case.

    Good luck
    Chris
     
  3. John Larkin

    John Larkin Guest

    Is that voltage measured with no load? A 317 has a minimum load
    current spec, and the 4K load of the divider may not be enough to keep
    it down. Usually the divider is much lower in resistance for this
    reason.

    John
     
  4. Joe

    Joe Guest

    Jeff,

    You need at least 7.5volts in to get 5volts out. Use a 240 ohm (or 270 ohm)
    resistor for R1, and for R2 use a 2K potentiometer (one side and wiper only,
    as in variable resistor). Tweak the potentiometer till the output is at
    5Volts. Then turn power off, and measure the potentiometer resistance. Use
    the closest standard value resistor to the potentiometer resistance (as R2),
    or just keep the potentiometer in the circuit.

    Quiquid latine dictum sit altum viditur

    hth,
    Joe
     
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