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# What is the purpose of pull-up resistor?

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#### darlling5147

##### Member level 3
why we need pull-up resistor? to control current flow? how it can help in maintaining the voltage?

pull-up resistor??

the out of some chip is drain floating, so if you want get a voltage you must place a pull-up resistor.
the other use of pull-up res is to enlarge driver current.

Re: pull-up resistor??

the circuits fabricated in ICs can source only a particular amount of current before their internal transistors saturate and hence the extra current sourced by the pullup resistor helps increase the output voltage....

### darlling5147

Points: 2
Re: pull-up resistor??

Sometimes the output of an IC is open drain or open collector. In thios case You not have any voltage variation but only variation of impedance.
With the pull-up resistor you can have a a voltage variation.

Also sometimes is used to reach exactly the supply voltage, avoiding the saturation voltage.
Last is to reduce the impedance and to avoid noises.

Mandi

Re: pull-up resistor??

but how come a resistor can did so? this is what i dun understand? resistor is not just able to limit current ?

Re: pull-up resistor??

you are talking about load which is very low in impedance i.e. then only the drop across the resistor would be high....

pull up resistor would work fine for a normal load...

Re: pull-up resistor??

If the current in the resistor is zero, the voltage drop is zero too, so the output goes to +Vcc (In most of the cases 5V). I hope this will help you. I will in the same problem a lot of time ago. (Remember that the oputput is floating, so it takes the voltage you put externally).

I hope this will help. Bye and good luck.

Re: pull-up resistor??

You can choose your own Vdd, as simple as that.
This means that without a pull up resistor, a logic 1 might represent 5V, with it, it might represent 20V. Watch out, you might damage the IC.

Cheers

Re: pull-up resistor??

You can choose your own Vdd, as simple as that.
This means that without a pull up resistor, a logic 1 might represent 5V, with it, it might represent 20V. Watch out, you might damage the IC.

Cheers

THANKS YA~

diegobb said:
If the current in the resistor is zero, the voltage drop is zero too, so the output goes to +Vcc (In most of the cases 5V). I hope this will help you. I will in the same problem a lot of time ago. (Remember that the oputput is floating, so it takes the voltage you put externally).

I hope this will help. Bye and good luck.

by the way, how to make the current in the resistor to be zero? under any condition the resistor will do so ?

THANKS AGAIN~

Re: pull-up resistor??

the current through the resistor will become zero only when the output voltage of the IC and that of Vdd are equal... this is achieved only based on the output swing of the IC.....

Re: pull-up resistor??

A.Anand Srinivasan said:
the current through the resistor will become zero only when the output voltage of the IC and that of Vdd are equal... this is achieved only based on the output swing of the IC.....

oic......THX LECTURER....haha
now i understand de.

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