Driving the output pin of a 7805 regulator

Discussion in 'General Electronics Chat' started by davebee, Mar 18, 2009.

  1. davebee

    Thread Starter Well-Known Member

    Oct 22, 2008
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    I have a commercially produced circuit board with an on-board 7805 regulator. It was designed to be powered from 9-12 volts input to the regulator. The 5 volts powers a bus on the board, but all the main board circuitry runs from a separate 3.3 volt regulator which is powered from the 5 volt bus.

    I want to run the board from 4 NiMH batteries, which emit from about 5.5 to 4.5 volts depending on their charge state.

    What I've done so far is to connect the NiMH batteries directly to the 5 volt bus on the board, and it seems to be working fine. The 7805 appears to have no problem receiving this varying voltage on its 5 volt output pin.

    But to charge the NiMH batteries in-circuit, the voltage on the 5 volt bus may run up into the 6 or more volt range.

    Does anyone know if applying this voltage to the 7805 output pin is a potential problem?

    I've looked over a 7805 data sheet but it doesn't say anything about this unusual usage of the regulator.

    I could unsolder and pull the 7805 out of the board but I'd rather not hack the board itself if applying this voltage to it would be completely harmless.
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287

    The 7805 has an internal pass transistor (emitter follower), which should adequately isolate your battery. If the battery voltage is higher than the regulator output, it's simply biased off. I'd say, if it ain't broke, don't fix it. :)

    eric
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    If you run the 5v bus above 5.5v or so, you may very well fry TTL components and/or microcontrollers that may be onboard. Better disconnect the batteries from the bus before charging them.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A 7805 needs a minimum input of 7.5V or 8.0V to regulate properly.
    Use a low dropout regulator that still works when its input is only 5.5V.

    Your 4 cells will make 4.0V to 4.4V when they need to be charged so no 5V regulator will work properly.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Psst... Audioguru... he's feeding the bus from the OUT side of the regulator...
     
  6. davebee

    Thread Starter Well-Known Member

    Oct 22, 2008
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    This is a very simple board; the only part on it that runs from 5 volts is the 3.3 volt regulator, which powers a 3.3 volt microcontroller and an EEPROM. The regulator is specified for up to 18 volts input.

    I could design it to remove the batteries for charging if I had to but was hoping to be able to recharge them while the board is still running, which sounds like it should be no problem.

    But still, I think I'll run a test of powering it up to 6 or 7 volts and feel that the 7805 stays cool, just in case...

    Thanks for the help!
     
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