Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Strange phenomenon

  1. Oct 17, 2010 #1
    I just watched at a strange phenomenon. One of my PCB's (Cmos+TTL) is connected to my PC through a Centronics port. No power supply is provided by this wire (only data in and stobe signals). With main power supply off and filtering caps empty, this board runs perfectly during hours and hours with a Vcc at 3.5V coming from nowhere. If someone may explain to me ...
  2. jcsd
  3. Oct 17, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    Maybe a charged capacitor? We can't tell without knowing what the PCB looks like.
  4. Oct 17, 2010 #3
    If the TTL input on your PC is normally high, it is a current source, and may charge capacitors on your PCB. The corresponding TTL output on your PCB is a current sinking (pull down) output (think npn open collector).

    Bob S
  5. Oct 17, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    This is not uncommon at all. The power supply rail can be powered up through the internal protection diodes in ICs if there is activity on the I/O pins of said ICs.
  6. Oct 19, 2010 #5
    Thanks for yr replies. That was also my own explanation, but anyway if I'm not surprised for CMOS IC's, it's more amazing for TTL Ls and NE585 (not Cmos) which are big consummers.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook