What is a pull up resistor?

current tdrive transmission npn

I want to know what concept of pull up resistor and what it does?
How to calculate the values of pull up resistors in a circuit?

Pull up resistor shifts output level of circuit toward Vcc ( supply voltage).
If NPN transistor's emiter is connected to GND, base to previous stage ( drive circuit), and collector is open, you shoud connect pull resistor between Vcc and collector. In that way when transistor is "open" ( current flows trough collector to GND), output voltage wiil be LOW ( about 1V). When transistor is "closed"( Ic=0) , output voltage will be high, and the load current will be flow trough this pull up resistor. Ammount of need output current, limits from one side pull up resistor value. When you need more current trough load - ou must use low value resistor. But when low value resistor is implemented:
1. Disipate too much power in output LOW mode
2.Transistor is heavily loaded - and it possible to became out of saturation mode.

It is useful when you :
1. Drive high voltage loads, from example 12 V from TTL open collector IC
2.To make wire OR - i.e. implement logic function.

That way you can drive output with one transistor, instead use of more complicated push-pull circuit. For example if you have

http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html

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.

for more deatails
check the url
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Hello

Pull-up/pull-down resistor

Pull-ups (or pull-downs) have two primary purposes both of which are variations on a fundamental theme which is to prevent a short-circuit by adding a resistor in the path between Vcc and GND for a particular signal.

Configuration: Many ICs have pins which must be set to a HIGH or LOW to configure the chip. Unless the IC is defined to have an internal pull-up or pull-down you typically use a pull-up (the resistor is between the signal pin and Vcc) to set a HIGH (1) or a pull-down (the resistor is between the signal pin and GND) to set a LOW (0).

Floating Signals: If a signal is not being actively driven all the time it will float (i.e. take an arbitrary and maybe changing value). To prevent this it may be pulled-up (HIGH) or pulled-down (LOW) into a default state.

Pull-ups or pull-downs are usually weak (i.e high value resistors of 4.7K, 10K (most common) or 47K) since in the case of floating signals this allows the 'driven' level to overcome the resistance with a modest current. For minimum power loss especially in configuration functions use the highest value (47K). Since higher resistance values take longer to overcome than lower values if the signal needs to be stable very quickly you may need to go as low as 1K for the pull-up (pull-down).

Maverickmax

vickychand_2000 said:

I want to know what concept of pull up resistor and what it does?
How to calculate the values of pull up resistors in a circuit?

Click to expand...

Pull-up resistors are simply resistors placed between a signal line and vcc. The reasons are all stated by others.
There aren't strict guidelines on the resistor value, but it is usually in the range of a few kiloohms. The use of excessive resistance will often lead to slower slew rates, which will affect the max transmission speed of the signal line, or bus.

Thanks

Now I have another question for my microcontroller I need to connect 2 pull up resistors to the ports.I connected ordinary 10k resistors and the controller is blown out.The microcontroler experts suggest that only metal film resistors should be used as pull up for uc.Why is that and why no ordinary resistors only metal film :? :? :? :?

The type of the pull-up resistors doesn't matter at all..
To protect the input lines use 4.7-5.1V zener diodes or transient voltage
suppressors ..

There is no such thing, that only metal film resistors can be used as pull ups. You can expect pronblems only if you use wirewound resistors, due to their higher inductance. Problem is with something diferent, in my opinion.

A pull-up ressistor is usually used ni TTL (Transistor Transistor Logic)
circuits because the out put pin of the IC can't drive a LED or LCD so to reduce the it's ampere a resistor commonly 10k ohm is used.
in the now a day IC Technology it is common to use a SC-Filter instead of a discrete resistor , because of thermal dissipation and IC possible failures.

Yes,for the collector open circuit to enhance the ability of mounting large currrent load.

vickychand_2000 said:

I want to know what concept of pull up resistor and what it does?
How to calculate the values of pull up resistors in a circuit?

Click to expand...

From a practical point of view, u will need a pull up resistor on a open-collector pin of an IC output. Some digital gates use open-collector output pins. In digital circuits, this may be useful to construct a simple wire-connect logic (is my term correct?). In other applications, this feature may also be useful to make a different high level at the output.

Value of pull-up resistor can be calculated, but usually i am too lazy to count and just use any R's i can find at my desk (i would prefer 10k).

Added after 18 minutes:

vickychand_2000 said:

Thanks

Now I have another question for my microcontroller I need to connect 2 pull up resistors to the ports.I connected ordinary 10k resistors and the controller is blown out.The microcontroler experts suggest that only metal film resistors should be used as pull up for uc.Why is that and why no ordinary resistors only metal film :? :? :? :?

Click to expand...

I dont understand "2 pull up resistors" connected to the ports. Each port pin have one pull-ups? Is that what u mean?

Events which can blow up a microcontroller pin are: giving excessive current/voltage.