pull up&pull down resistors

Hi,who can tell me why & when use pull up & pull down resistor?

The Pull-up Resistor
This time in Very Basic Circuits, I would like to talk about pull-up resistors. The basic function of a pull-up resistor is to insure that given no other input, a circuit assumes a default value. Actually, there are two flavors of this circuit. A pull-up and a pull-down. Their function is the same, to create a default value for a circuit, but one pulls the line high, the other pulls it low.

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rgds,
ianp

Usually it refears to a uncommited transistor , that is a collector with no conection and you can put a pull up resistor in order to work, or a emitter with no conection and you can use a pull down resistor.
Tha use of that kind of resistors are from transistors to digital logic and voltage comparators.

The pull up and pull down resistor can be used for many purpose. Whenever, you want the circuit to be logical 1, use pull up resistor, if you want logical 0, use pull down resistor.

Except use for the transistor, we also use many pull up and pull down resistors on microprocessor ports.

Pullup and pulldown resistors can be used for different purposes. Generally speaking they keep the line to which they are connected to a fixed level through a quite high impedance generator. Due to this high resistance the level driven from the pull up/down can be easily overdriven

the pull up or pull down resistors are basically used to convert an undefined state (floating state) in to a stable state.
pull-up will provide stable state of logic high, where as pull down will provide stable state of logic low.

Pull up are used to increase current capacity of the port pin by certain amount.
e.g. id you connect a device to port pin it may require 50 mA for its operation. but controller pins have around 25mA max current. So these pins will be loaded because of the load of the other external device. This can also cause damage to the controller and its pins.
So in order to make the other device work properly, we put a pull up resistor whose one end is connected to controller and other end to Vcc. applying KCL to this. current meeting at nodes gets added up. We have some amount of current flowing from controller pins and since resistor is connected to Vcc, some amountof current flows from there and both currents gets added up and flows to the device, which makes the device operational, without loading the controller port pins.

Similarly Pull down resistors work in the reverse way, Hope this answers your question to some extent.

electronc_elementary said:

Hi,who can tell me why & when use pull up & pull down resistor?

Click to expand...

One good point missed out (I think) is that a PULL UP resistor is used to PULL the voltage to the supply (usually +5 VDC) and a PULL DOWN resistor is used to PULL the voltage to the reference (usually O VDC).