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LM317 @ 3.3V or 5V only with jumper ?

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by blueroomelectronics, Jan 15, 2008.

  1. blueroomelectronics

    blueroomelectronics Well-Known Member

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    I'm sure it's trivial but I'd like to find COMMON resistor values that with a simple switch or jumper would select between 3.3V or 5V on an LM317
    [​IMG]
    I don't wish to use a pot like above, but a simple jumper and two resistors.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The more expensive LM117 uses a 240 ohm resistor for R1. The LM317 is supposed to use a 120 om resistor or less so that its output voltage doesn't rise without a load if it has its max rated operating current and if its input voltage is high.

    With 120 ohms for R1 and 360 ohms for R2 the output is 5.0V.
    with 120 ohms for R1 and 200 ohms for R2 the output voltage is 3.333V.
    These resistors are common 5% values.
     
  3. Blueteeth

    Blueteeth Well-Known Member

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    Check out Sparkfun..they've got a breadboard PSU with a switch for 3.3v and 5v. I've implemented one on a simple AVR prototboard, that just uses a 2-pin header for a jumper. Pretty accurate. But it used 4 resistors as the jumper 'shorted' a resistor in parallel with another. 240 ohms between out and adjust (two 120's in series) with a 390 and 330 in series from adjust to GND. The jumper just shorts the 330 ohms.

    probably an over complicated way of doing it, but I built it without designing it first, just a 15 minute job with the parts I had. Roughly 4.97V and 3.29V.

    Blueteeth
     
  4. dave

    Dave New Member

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  5. bananasiong

    bananasiong New Member

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    R1=120 :eek:hm:
    R2=360 :eek:hm: for 5 V
    R3=470 :eek:hm:. Close the switch for R3 to parallel with R2 to get 3.37 V
    A spst will do.
     
  6. Roff

    Roff Well-Known Member

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    If you're going to do it by putting R3 in parallel with R2, R3=430 (instead of 470) will yield 3.29V.
     

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