Pull Down Resistors

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Hey r/explainlikeimfive could someone explain to me how a pull down voltage circuit works? I tried googling it but I can’t make sense of the answers I’m getting.

In: Technology

To use the analogy of water flowing in pipes, a pull down resistor would be like having a tiny pipe that empties into a drain attached to a larger one. If water is flowing in the large pipe, sure some escapes down the tiny pipe, but most goes where the big pipe goes because it is easier. If the big pipe stops flowing, like from closing a valve somewhere, the little pipe will empty the big one of water because no new water is coming in.
Expanding on this a little, there are also pull-up resistor circuits that work the opposite way keeping the big pipe full of water (at some voltage potential) if it isn’t allowed to drain.

The super simple answer: A pull down (or a pull up) resistor is a way to set a “default” voltage on a pin/wire when nothing else in the circuit is trying to impose a voltage on it. A pull down resistor defaults the pin to the low/ground state.

An example: Lets say you have an output on a microchip that in the “on” looks like a 50-ohm resistor connected to 5V, and in the “off” state looks like a 50,000,000-ohm resistor connected to 5V.
Now what you really want is when the output is “on” that it has ~5V on it, and when it’s “off” it’s at ground. So what you do is put in a decent sized (say 50,000 ohm) resistor that connects that pin to ground (i.e. it’s a resistor that “pulls down” the voltage if the only inputs on a line are very high resistance). So in the on state you have:
5V – (50 ohm) – pin – (50,000 ohm) – ground.
Which means the pin will end up very close to 5V.

On the “off” state you will have:
5V – (50,000,000 ohm) – pin – (50,000 ohm) – ground
Which would mean the pin is very close to ground.

You want to have the input line to a chip at a known value even if you do not have an input signal. If you do not force it to a voltage is can float around and randomly change from high to low and back if the load draws very little current as a CMOS transistor gate do.

So if you have a chip and a switch that you can press to put is to lest say +5V how does the line go to 0V to indicate no button pressed? A line at +5V will remain at that level until charges are drained and with no direct connection to ground, you will only have small leak currents. The line is also sensitive to outside interference and it could change for low to high even if the button is not pressed by the capacitive effect.

The solution is a pulldown resistor. You connect the conductor with a resistor to the ground so there is a way for the charges to get away and it will as long as you do not connect it to +5V remain at 0V. There will be a current through it all the time when you press the button so the input will not be at 5V but a bit less but as long as that is in the range that it considers high it do not matter.

The value of the resistor will determine how the fact it returns to zero but a faster return also results in higher leak current and reduce voltage when you press the button. What you use is a compromise.

So you can use a simple close open switch and a resistor so the wire voltage depends on the position ow the switch

Another usage is when a wire can have input from multiple sources. Each chip could send a +5V signal for something but when they do not send the output is not 0V but disconnected from the wire so multiple chips can rise the wire to +5V. In this case, you need to pull it down with an external resistor or one inside the chip that reads the signal.