What are Pull Up and Pull Down Resistor with Applications.

Hi Mentor ,
I want to know about Pull Up and Pull Down Resistor with Applications.

Thanks in Advance
DigitalWork

As far as I am aware, they are used to ensure a node is at a particular level rather than just flapping around without a connection.
The circuit on the left demonstrates a pull down resistor (R1). It is used to ensure the base of the transistor is at 0v when the switch is open, otherwise the base would not be connected to anything and a that is supposed to make a transistor's behavior unpredictable, so when the switch is pressed, the base is supplied with current and the lamp lights up. In the circuit on the right, R3 is a pull-up resistor, it is used to pull the base of the transistor up so that current flows into the base when the switch is open, turning the transistor on and lighting the lamp, when the switch is closed, the current instead flows to ground and the base falls to 0V and the lamp turns off, the opposite effect the the circuit on the left.

More experienced members please feel free to correct me, as I'm only a novice myself.

**broken link removed**

That's more or less correct.

A pull-up is used at the input of a circuit (like the base of the transistor in MD68's diagrams) to ensure that if nothing else is driving, it assumes a high level.
A pull-down is used at the input for the same reason but to make it assume a low level.

The terms pull-up and pull-down are more often used in terms of logic circuits where there are only two active levels, high and low. The value of the resistor is normally chosen so it is high enough to ensure the logic level is reached but not so low that something else can overcome the 'pull' if necessary.

You will also see them used on open collector and open drain outputs where the logic circuit can only pull the pin low because it has no internal high side driver. The pull-up is used to provide some current to make the output high unless the device pulls it low.

Brian.

DigitalWork said:

I want to know about Pull Up and Pull Down Resistor with Applications

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The main feature achieved on such output topology is the ability to allow connect several outputs in parallel at same bus yielding no effective load conflict.


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Pull-up resistors are resistors used in logic circuits to ensure a well-defined logical level at a pin under all conditions.

Pull-down resistors work in the same manner as pull-up resistors, except that they pull the pin to a logical low value.