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LM317T

Discussion in 'General Electronics Discussion' started by nevcams, Mar 31, 2012.

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  1. nevcams

    nevcams

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    Mar 31, 2012
    Hi,
    Im new to electronics and have just wired a lm317 up to test out put voltage, ive got a 1K variable resistor in place of R2, it works fine. but when I put a Heat-sink onto the 317 I can only adjust voltage slightly... any ideas why??:confused:
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Can you post the schematic? Without it is rather impossible to judge what might have gone wrong.

    What has the heat sink to do with voltage adjustment? DOes the heat sink possibly create a short circuit to another part of your circuit?

    Harald
     
  3. nevcams

    nevcams

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    Mar 31, 2012
    Here is the schematic ive followed..
    12-to-5-volt-dc-to-dc-converter-circuit-diagram.png
    Its on a bread board so it should not be shorting out, I was told to use a heat sink if the voltage regulated was above about 3 volts (im going from 12 down to 5)
     
    Last edited: Mar 31, 2012
  4. davenn

    davenn Moderator

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    Sep 5, 2009
    or maybe shorting a couple of the lm317 pins

    Dave
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    You can't follow that circuit it's not even for a lm317 and even if you were using a fixed regulator as mshown, the lm7805, the shouldn't be a series diode on the input

    Google a lm317 and get the data sheet, it has plenty of sample circuits for its many uses, including a variable voltage supply

    Dave
     
  6. nevcams

    nevcams

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    Mar 31, 2012
    Sorry Guy's,
    The LM317 was faulty... I've just tried 4 more and they all work fine.
    I'm very new to this and it's just as a hobby but Im getting to enjoy tinkering about and burning my fingers.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The corerct circuit is here: http://www.national.com/mpf/LM/LM317.html#Overview

    The series diode in the input is not necessary, but neither should it pose a problem.

    Since you talk about adding a heat sink to the LM317, I assume you're using the TO220 package. In That case note that the metal cooling tab is connected to Vout (datasheet http://www.ti.com/lit/ds/symlink/lm317-n.pdf page 3). Therefore the heat sink will be at Vout, too. You have to make sure that the heat sink is not connected to any part of the circuit that is not Vout. Otherwise you need a thermally conductive insulator between the TO220 package and the heat sink - plus plastic screws to avoid any contact.

    Harald
     
    Last edited: Mar 31, 2012
  8. nevcams

    nevcams

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    Mar 31, 2012
    I also posted schematic for 7805 instead of lm317
     
  9. nevcams

    nevcams

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    Mar 31, 2012
    Thank you so much Harald..and yes its the T=220 I'll go to link youve provided.
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    We noticed that :)

    Harald
     
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