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Running an LM337 backwards ? — Parallax Forums

Running an LM337 backwards ?

samsn4samsn4 Posts: 49
edited 2012-07-17 11:48 in General Discussion
I'm building an LED taillight, so two LM317's feeding a single LED array. Well during the build I grabbed a LM337 by accident, but the result was exactly what i wanted for the output. So my question is can i run the regulator 'backwards'?

pin LM317 LM337 my setup
1 adj adj adj
2 Vout Vn Vout (to LED array)
3 Vn Vout Vn

12Vdc, 20mA max

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-07-16 13:53
    Is this what you mean?

    attachment.php?attachmentid=94231&d=1342471860

    If it is, then, yes, you did the right thing.

    BTW, when asking a question like yours, it's always better to include a schematic. If you don't have a schematic CAD program, use DigiKey's online version, which I talked about here:

    -Phil
    pos_neg_current_limiters.gif
    440 x 167 - 3K
  • Duane C. JohnsonDuane C. Johnson Posts: 955
    edited 2012-07-16 16:29
    Wow, that just looks weird!!!
    But ya it works.
    BTW, the pin outs of thesePositive and negative regulators are different.
    Regulator_Adj_LM3XX.jpg


    Also, I have an excel spreadsheet useful in calculating the resistors for these:
    "Regulator_Adj_LM317.xls"

    Duane J
    Regulator_Adj_LM3XX.jpg
    209 x 271 - 13K
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-07-17 00:54
    Simply put, there are two kinds of voltage regulators. Those that die a sudden death if you put reverse polarity on the input; and those that have reverse polarity protection.

    What you seem to call 'backwards' is trying to use a Negative regulator in lieu of a Positive regulator. The reason for the two kinds is to provide dual power for op amps, RS-232 output and so on.

    In short - yes, you can make use of a negative regulator in a pinch for a single voltage supply. But it may get confusing. And, the LM317/LM337 are NOT protected from reverse polarity hookup. One wiring mistake is likely to destroy the device.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-07-17 09:14
    As two-terminal current regulators, the two circuits are completely equivalent. There's no reason to prefer the '317 over the '337 or vice-versa, whether it's "in a pinch" or by design.

    -Phil

    Addedum: BTW, since the regulator/resistor combo is a two terminal device, the LED can go on either side of it. Also, since you've got 12V to work with, there's plenty of overhead available to add several LEDs in series.
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-07-17 11:48
    I missed the fact that these are used as CURRENT regulators, not voltage regulators. My bad.
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