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Author Topic: pull-up/pull-down resistors  (Read 1916 times)

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pull-up/pull-down resistors
« on: February 05, 2014, 07:33:17 AM »
Hello guys,
I was wondering if you could explain how exactly pull-up/pull-down resistors work in a circuit, and also how sinking and sourcing currents are generated.
Thank you,


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Re: pull-up/pull-down resistors
« Reply #1 on: February 06, 2014, 09:32:25 AM »
Hello Xristo,

here is the big deal about pull up and down resistors. These are only fancy names of a simple resistor with one of its ends connected to either +V or ground. Example: Suppose you want to make a microcontroller circuit with one PCB Pushbutton button input. These pushbuttons have typically one Normal Open contact, which means that when the button is not pressed no current flows through, when the button is pressed current flows through. So you connect one end of the button to the ground and the other end to the microcontroller input. When the button is pressed, the microcontroller clearly reads "LOW" and runs the routine accordingly. But when it is not pressed, the input of the microcontroller is as we call "floating". A floating input is neither HIGH or LOW, is just.. floating. This is where the pull-up resistor takes part. You connect a 10K or 22K resistor from +V to the input of the microcontroller. So when it is not pressed, the micro reads "HIGH" through the resistor. When it is pressed, the internal resistance of the switch is many many times smaller than the pull-up resistor and therefore the micro reads LOW through the switch. Same applies for the pull-down. The same circuit with the button connected the +V needs a pull-down resistor.

As for the sink/source currents: This is a way to explain how an output provides power to something. When we say that an output can "sink 25mA of current" it means that if you connect something to +V and then to the output, when the output goes LOW it will allow curretn to flow through and this current must NOT exceed 25mA. Same for source but with different polarity. When we say that an output is capable to source 25mA we mean that when we connect something to ground and then to the output, when the output goes HIGH it will provide current to flow and this current must NOT exceed 25mA.