7805 regulator voltage drops

Discussion in 'General Electronics Chat' started by mxabeles, Jun 20, 2009.

  1. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
    182
    0
    hi,
    for some reason when ever i have a circuit w/ a 555 timer and 595 shift register (im too tired to go into details right now) the voltage on the regulator swings regularly from around 2 volts to the usual 4.9v.
    Anyone experience similar? Any words of wisdom?

    Thanks,
    Max
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    If you are "too tired" to supply some detail you should not be too surprised if you don't get any help.

    Without a circuit diagram, input voltage, filter capacitor values, output current (for starters) no one can make any sort of educated guess or offer a sensible suggestion as the cause of your problem.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    That would be the expected behavior if the input voltage drops below about 6.5 volts. The output follows the input down in a linear fashion. They are called linear regulators for a reason -- LOL

    Also 4.9V sounds a bit low for a 5V regulator. How much current are you drawing with your circuit? A plain vanila 7805 should be good for 1 Amp. You could be drawing that much if there is a wiring mistake. If there is a wiring mistake and you are drawing large currents then maybe the regulator is shutting down to protect itself. Check the datasheet to see if thermal shutdown is possible.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Thermal shutdown is possible for a 7805.
     
  5. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    Basically anything is possible, input dropping below the regulator dropout voltage, output current too high, thermal shut-down, etc, etc. Without more input from the OP its just a waste of time speculating on this problem.
     
  6. mxabeles

    Thread Starter Active Member

    Apr 25, 2009
    182
    0
    i have tried both 12v 1 Amp and 12v 700mA adapters as input to the regulator. I have a 555 connected as clock/latch to x2 595 shift registers. Are the adapters chosen large for the 7805?
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Does the 7805 get warm/hot?
    If so a heatsink may solve the main problem.
    Did you also add the capacitors at in and output of the 7805 ?

    [​IMG]

    The capacitors prevent the 7805 from oscillating and getting instable.

    Greetings,
    Bertus
     
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