PullUp and PullDown Resistor

[scientificpakistan]

pull up resistor calculation

Dear Forum Members,
Tell me about how to calculate the value of a PullUp and PullDown Resistors, Tell me from very basic. Also how these resistors works.

 

[IanP]

pull down resistor calculation

: What is the purpose of pullup and pull down resistors?

Pullup resistors are used to pull logic signals up
(to logic 1). So when some inut signal needs to be
set to logic 1, but might need to be changed for
some reason to 0 at some other time, a pullup resistor
can keep the signal at logic 1 until the signal is
pulled down by something. Typical application for
pullup resistors is to connect 4.7 kohm (or some
other suitable resitor value usually between 1 kohm
and 10 kohm) from the circuit operating voltage (+5V usually)
to the input pin. This resistor keep the signal at logic 1.
When the signal needs to be set to 0 it is pulled
down by conencting that input pin to ground
(usually throuhgh a button, DIP switch or open
collector output of some other part of circuit).

Pulldown reistors work in the opposite way.
They keep signal a logic 0 until somethign
connect te signal input to +5V (or whatever
the operating voltae of the logic circuit is).

Regards,
IanP

 

[scientificpakistan]

pull up register

But I Have Seend a Circuit In which 100 Ohm Resistor used with +5V Supply.
Also It Depends Upon the Requirement of a Circuit. The Resistance values 4.7K or 10K are not fixed. I want to know that, is there any formula to calculate the resistance for PullUp and PullDown.

 

[IanP]

calculate pull up resistor

I am not aware of a single and universal fomula that can be used for these calculations ..
Here, however, you can find examples on how to select/estimate pullups:
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html

Regards,
IanP

 

[paulux]

pull down resistor

If the pull-up or pull-down resistor is required to be large in Mega ohm order to save current, would the mosfet do the same as the pull-up or -down resistor?

 

[IanP]

how to calculate pull up resistor

Instead of a resistor you can use so called "pullup transistor" or "depletion mode transistor" (see picture below) ..
More details on transistor loads can be found at:
https://cnx.org/content/m1028/latest/

Regards,
IanP