connecting input to logic ic

Discussion in 'General Electronics Chat' started by fugrammer, Apr 10, 2007.

  1. fugrammer

    Thread Starter New Member

    Apr 1, 2007
    why is it that i can directly connect the power supply to the input of a logic ic without damaging it? Is it because the IC has great internal resistance? Furthermore, even if a add a resistor in the path, the voltage across the resistor is zero, what does it imply?
  2. John Luciani

    Active Member

    Apr 3, 2007
    The IC has large input resistance as long as your input voltage does not go too far
    above the power supply voltage or too far below ground. There are usually protection
    diodes that will start to conduct when the input voltage is 0.3V higher than the power supply voltage or 0.3V below ground. Even with the protection diodes you can still
    draw sufficient current to destroy the IC.

    Since the resistance is very high and I = V/R then I is very low.

    (* jcl *)

  3. thingmaker3

    Retired Moderator

    May 16, 2005
    Depends on your definition of "zero." Could be that your resistor is negligable in comparison to the chip's i/p resistance. Could be you've got a MOSFET chip and your measurement isn't quick enough to catch the gate charging up. Could be that you would read voltage across the resistor if you changed the resistor i/p. Could be you're using the wrong scale. Could be a lot of things.