Effects of pull up resistors on voltage

MrChips

Joined Oct 2, 2009
19,926
I posted an explanation about pull-up and pull-down resistors on inputs to logic circuits.
https://forum.allaboutcircuits.com/threads/should-logic-gates-have-a-pull-down-resistor.165758/#post-1461093

In brief, the purpose of the Switch closure to Ground is to force a logic LOW at the input of the Logic Gate.
When the Switch is released (Normally Open) there is nothing at the input of the gate (except for leakage current) to cause the input to assume a valid logic level.

The purpose of the pull-up resistor is to force the input to a logic HIGH. The value of the resistor needs to be calculated based on the valid voltage threshold for logic HIGH and the input leakage current. This depends on the logic family of the device.

The Logic Gate shown in your diagram is just representative of any Logic Gate. The Vout has nothing to do with Vin.
 

Thread Starter

x11

Joined Dec 25, 2019
5
The purpose of the pull-up resistor is to force the input to a logic HIGH. The value of the resistor needs to be calculated based on the valid voltage threshold for logic HIGH a
So the resistor does cause a drop in voltage? How can a single resistor cause a voltage drop?
 
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