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# pull-up and pull down resistors

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Anonymous
pull-up and pull down resistors

what are pull-up and pull down resistors? where they are used?

kaushil
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Last seen: 7 years 7 months ago
Joined: 19/08/2010 - 00:09
i understand the function of a pull up or pull down resistor, pull up increases the voltage at a node and a pull down decreases the voltage 1. If a pull-up resistor is being used and the pin it's pulling on is grounded, the pin becomes a direct path to ground (a short for the power-supply). The resistor prevents it from being a direct short. Same thing, applies for a pull-down resistor, although slightly less direct. And, 2. The resistor is necessary to create 'potential'. On one side of the resistor is the power-source (either ground or positive). On the other side is the pin and whatever current potential is on it. If the two are different, they will try to equalize by flowing through the resistor towards the power-supply (effectively pulling the signal in that direction). Hope that helps....
nileshgupta
thanks a lot, it helped
muepenistgast
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Joined: 21/11/2011 - 19:40

????????, ?????? ???????? ??????

AJISH ALFRED
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Joined: 10/01/2012 - 12:10

In embedded field they are basically used to set a default input/output values. Suppose you have set a pin as input and you are trying to read the value of that pin before applying any input device on that pin. Such a case is called high impedance state and you can't predict wheather the value read will be logic high or logic low.

Here if you were using pull down resistor to the pin, then eventhough no input is connected, the pin is read as logical low  .If pull up were used then the input pin will read values as logic high eventhough no input device is attached.

It is same with the case of output pins and output devices

Tanuj Singh
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Joined: 06/02/2014 - 21:06
I got confused with the function of pull up & pull down resistor. A resistor does not changes its property whether it is used as a pull up or pull down resistor. And I'm sorry to say that the way in which you explained seems a bookish language so if possible plz explain it in easy way.
Ganesh Selvaraj
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Last seen: 2 years 7 months ago
Joined: 15/07/2011 - 16:32

Here the resistor is not changing its property! the terminal it is connected to changes. Pull up resistor are connected to the +ve terminal and pull down resistor are connected to the negative terminal.

himanshu gupta
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Last seen: 3 years 8 months ago
Joined: 07/11/2013 - 13:28

In simple language.

Pull up and pull Down resistors are needed to drop down the floating condition because sometimes when switches are connected to devices they may give floating state to device when they are in open loop. But devices sometime says i only accept one or zero so please pass me with those two things only so to lower down that problems we use pull up and pull down.[[wysiwyg_imageupload:12192:]][[wysiwyg_imageupload:12193:]]