Pull Up/Down Resistor

Thread Starter


Joined Sep 12, 2005
What is the purpose of the pull up/down resistors and how it will affect the ckt

and on what basis the values of those are calculated.




Joined Apr 26, 2005
There are some devices whose outputs that are open collector.

Draw a circuit that is open collector and think about the output when the transistor is ON and the transistor is OFF.

Now, add a pull up resistor between the open collector and Vcc.

Think about the output when the transistor is ON and the transistor is OFF.

The basis for the calculation is how much current you want to sink.


Joined Jan 23, 2006
Originally posted by Surendar@Jan 7 2006, 01:08 AM
What is Source current and what is sink current.
[post=12937]Quoted post[/post]​
The source current is the current provided by the component when the output pin of that component is high. The source current can reveal fanout capability. Fanout means how many logic gates can be connected to the output of the gate being considered.

Lets consider a AND gate output tied to a 1K resistor with the other side of the resistor tied to 5 volts. The reason for the resistor is to ensure that the high logic level at the output pin of the AND gate is an acceptable high logic level for the component being fed by that output. When the output of the AND gate goes low, the gate must accept or sink enough current to create a 5 volt drop across the resistor. Since there is a 5 volt drop across the resistor, the voltage at the gate output is 0 volts. Hence the sink current is required to lower the output voltage of the gate.

The reason for providing a pull up or pull down resistor on an input pin of a digital logic IC is to ensure that a random floating level on that pin does not cause unwanted outputs from the IC.



Joined Jan 23, 2006
Originally posted by zhi_yi@Jan 26 2006, 09:13 AM
what is the meaning of open collector??

thank you :)
[post=13514]Quoted post[/post]​
When an IC has open collector outputs, it means that each collector is not internally tied to the DC power supply. Therefore the user of that IC should provide external pullup resistors to the DC power supply. Without the external pullup resistor, when the logic inside the IC tries to generate a logic high, the open collector output pin will be floating.