resistor keeps buring out

Discussion in 'General Electronics Chat' started by ak52, Dec 31, 2014.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
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    Hello Everyone,
    I need to convert a 12v supply from a lead acid battery to a 3.3v supply using a LM317

    I have given the connections as per the datasheet
    Capture.PNG
    Where in my value of R2 is 390 ohm.
    I have added a 10 ohm resistor at the output for current limiting. and a 3.3v zenner.
    The moment I switch on supply the 10 ohm resistor burns out and soon after the zenner diode burns out as well.
    Where as the same circuit ,if i give the 12v from a regulated power supply it works perfectly.
    Should i use a high wattage resistor at the input side?
     
    Last edited: Dec 31, 2014
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    What is the purpose of the 10R resistor and the 3.3V zener? Seems to negate the point of using a voltage regulator in the first place.
    What voltage is on the output of the 317?
     
  3. ak52

    Thread Starter Member

    Oct 15, 2014
    145
    4
    The resistor and the zenner are just for precautions.
    The voltage at the regulator output is about 3.2 volts
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    There must be something wrong if the resistor and zener are burning out when the supply output is only 3.2V. Have you actually measured the voltage across the 10R resistor?
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the downstream load?
     
  6. ak52

    Thread Starter Member

    Oct 15, 2014
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    Yes i have measured it,but as i said before i have used two power sources here,
    With the regulated power supply (set to 12v) I get 3.2v

    But with the lead acid battery ,at the regulator the drop is 10v :(,which i don't understand as per the calculations mentioned in the data sheet it should come to 3.2v only.
    I am not giving any load at the moment.
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Then you have the Zener backwards :(
     
  8. ak52

    Thread Starter Member

    Oct 15, 2014
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    Nope the zenner polarity is correct,i double checked,if the zenner is backward it should have burnt out with both power supply sources..
     
  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    Well, you've got something wrong. First, get the 317 working consistently, if you're getting different output voltages with different supplies there's a problem somewhere, maybe it's died or is connected wrong.
    All you can do is carefully and methodically check you work, assume nothing.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The
    The knee on a Zener is not very sharp on low voltage Zener diodes (below 5 volts is terrible and only really sharp over 8 volts). That is, you made a big problem by trying to solve a non-existant problem. Your 317 is good enough.

    image.jpg



    http://www.microsemi.com/document-portal/doc_view/14614-zener-voltage-regulation-with-current
     
    cmartinez likes this.
  11. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Why don't you use a buck regulator instead? An LM317 is rather inefficient in comparison
     
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