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Damaged 7805 regulator?

Discussion in 'Electronic Basics' started by Mike, May 17, 2007.

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  1. Mike

    Mike Guest


    I've got a small PIC project with a 7805 regulator. It was working fine
    until I added ICSP (I soldered in the ICSP header and reprogrammed the uC
    without checking that it still worked in between :/ ) I have checked all
    my new connections and can't see any bridges.

    After reprogramming via ICSP the regulator doesn't appear to function
    correctly. With input around 8V I get about 3V out, with input around 12V
    I get almost 4V out.

    Is it possible that the 5V that is fed into the circuit by the programmer
    could have damaged the regulator (the regulator was not connected to a
    power source at the time)?

    The project works ok if I connect 5V from an external source.


  2. As from your description I'm almost (99.99%) sure the 7805 has been gone.
    They do not survive a voltage on the their output that's more than 1-2V
    above the voltage on their input. (AFAIK it's not in the specifications, and
    values may vary.) When you replace the 7805, add a backward diode between
    the input and the output. For small supplies a 1N400x will do.

    petrus bitbyter
  3. Reverse biasing the regulator can indeed damage it. That's why the output
    shouldn't have a huge cap if the input doesn't, and why you sometimes see a
    diode across the regulator in/out to make sure the output pin voltage never
    excedes the input pin voltage by more than .6V.

  4. Thanks for your help guys. I've re-read my programmer manual and
    apparently it is able to steal power from the circuit being programmed. I
    assume that is to prevent exactly this situation.

    I'll replace the reg and be a bit more careful next time.


  5. Mike

    Mike Guest

    In the end I don't think it was the regulator (I let the smoke out while I
    was debugging so I'm not 100% sure). New regulators showed similar output
    to my "broken" one, turns out my bench power supply is a bit dodgy, I put
    a bigger cap across the input and all is good now.

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