connecting input to logic ic

Thread Starter

fugrammer

Joined Apr 1, 2007
4
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?
 

John Luciani

Joined Apr 3, 2007
477
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 *)

-----
http://www.luciani.org
 

thingmaker3

Joined May 16, 2005
5,084
Furthermore, even if a add a resistor in the path, the voltage across the resistor is zero, what does it imply?
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.
 
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