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5v regulator L7805

Discussion in 'General Electronics Discussion' started by lewis, Jan 23, 2012.

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  1. lewis

    lewis

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    Jan 23, 2012
    the problem i'm having is that when i put it into my circuit i have a 7v output to my ic , even when i issolate it and it into series i still have a 7v output , i havent shorted it to ground , nor have i blown it up , i have check the resistance is 1mohm and tested it on a scope and dmm. the vcc is 12v and ground is connected .what do you recomend i do next?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,482
    2,830
    Jan 21, 2010
    (1) show us a circuit diagram

    (2) ensure you haven't reversed the input and output (or shorted them)

    (3) do you have a load (or is the device stable without a load?)
     
  3. lewis

    lewis

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    Jan 23, 2012
    dont worry sorted it out now :) thannks for someadvice though :)
     
  4. Rleo6965

    Rleo6965

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    Jan 22, 2012
    Can you share solving your 7805 problem.? Just curious. :)
     
    Last edited: Jan 24, 2012
  5. lewis

    lewis

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    Jan 23, 2012
    no i conected it because otherwise i would of had a 12v or somewhere close to that on the output , but i have 7v , i dont know what it was but i think it was something to do with the wires / short circuit on the bottom of the board as when i changed the wires and move the board it worked ?!?!? strangley enough , but im glad it does :)
     
  6. Rleo6965

    Rleo6965

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    Jan 22, 2012
    Accident does happen in building projects in electronics. Even simple circuits. But it does help us to learn and not to repeat the same mistake.:)
     
  7. lewis

    lewis

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    Jan 23, 2012
    yes agreed, but i still cant believe wires / short circuit made that all happen :/ bad times :(
     
  8. Resqueline

    Resqueline

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    Jul 31, 2009
    A lack of decoupling capacitors can sometimes make a regulator oscillate at a high frequency and thus lose regulation.
     
  9. lewis

    lewis

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    Jan 23, 2012
    i had 2 caps on the input and out put of the 5v reg and a cap on the vcc + to start with, also i tested it on the oscilliscope to see if that was it and it wasnt
     
  10. Rleo6965

    Rleo6965

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    Jan 22, 2012
    or ground terminal of LM7805 was not connected to ground. Therefore LM7805 act as resistor when measuring its Output Terminal (7V).
     
  11. lewis

    lewis

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    Jan 23, 2012
    look above to answer that question..... it says "i have check the resistance is 1mohm and tested it on a scope and dmm. the vcc is 12v and ground is connected"
     
  12. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Taking it literally 1mohm means 1 milli-Ohm or 0.001 Ohm. Do you have an Ohm-meter that accurate?
    And you don't say where you measured that resistance. For all we know you could mean 1 Mega-Ohm in-to-out, in-to-gnd, or out-to-gnd.
    Rleo was thinking about tab-to-gnd. Is this what you measured?
     
  13. lewis

    lewis

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    Jan 23, 2012
    Sorry I meant 1 Mohm :L woops 2 spelling errors isn’t good :/ 1 Mega ohm = in - out //// out - ground = 5k12ohms //// in - ground = 4k98ohm
     
  14. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Hehe, that's better.. Details do matter.
    I tested the broken ground theory just now on some 5V reg's, and with no load (11M) they delivered around 11V, with a 3k3 load most of them delivered around 9-10V.
    One type only delivered a little more than 3V into the 3k3 load however, so depending on type & load it seems that your results could be reproduced that way.
    I therefore suspect that you had/have a microscopic break in the ground connection to the regulator IC, and that moving the circuit happened to "fix" it.
     
  15. lewis

    lewis

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    Jan 23, 2012
    yes, i totally agree with you there , thats esactly what i thought
     
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