please explain pull up/down resistors...?

Question

I have read about them over and over again but rereading just isn't making sense to me..So I tried different sources and it still didn't answer my confusion so I am trying to ask on here.

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there is a shcematic of a pull up resistor..My confusion here is when the button is pressed, I don't know why all the electricity would go to the GND through the button and not get divided between the button and the resistor...Usually if you have something like a power source and 2 resistors the electricity is DIVIDED between them so why wouldnt it be divided in here? Why would it all go through that button, i understand its lowest resistance but still, shouldnt some go through that resistor making it a floating state?

In a pull down resistor that 10k ohm resistor and button are swapped...Once again, why in the world will ALL of the electricity go through the shortest path and not divide through the resistors like it does in some circuits?

Lastly, why do they decide to use a 10k ohm resistor? why not 5k or 100k, like how did they determine the value to use?

Answer 1

Let's work out the pull-up circuit. And even though it's not stated, let's say that the switch has a resistance of 1 ohm. This is usually a much-higher resistance than most switches, but we'll play it safe and call it 1 ohm. Let's also assume that the input to R1 (100ohm resistor) is ideal with zero source impedance, and can be either 0V or 5V. The output is where all three components meet.



-Case 1 - Input is 5V, switch open: In this case, you have 5V supplying both resistor, so it is the same as if both resistors are in parallel and being feed from 5V. A 100 ohm and a 10 kohm resistor in parallel works out to a single 99 ohm resistor. So as long as the ouput has a resistance of at least 99 ohms, the output will be 2.5V or greater. If the output load is say 1 kohm to common, the output will be about 4.5V, a definite logic high.



-Case 2 - Input is 0V, switch open: Now the circuit is the same as a 10 kohm and a 100 ohm in series, going from 5V to common. The 10k will drop 100 times the voltage as the 100 ohm, so the unloaded output voltage will be 0.0495V, a definite logic low. If you add a 1 kohm load resistance and recalculate, the output will be even a bit lower (something like 0.045 or so) and still a definite logic low.



-Case 3 - Input is 5V, 1 ohm switch closed - Now you have something similar to Case 1, but with a 1 ohm resistor to common from the output. Total, you have a 99 ohm resistor in series with a 1 ohm resistor, connected from 5V to common. That works out to an output of 0.05V, definite logic low. And if you add ANY load resistance, the output will be even lower and still a logic low.



-Case 4 - Input is 0V, 1 ohm switch closed - works out to an even lower output voltage, still definite logic low.



Like I said, most mechanical switches are much lower than 1 ohm, so when closed, your output will actually be lower, like in the microvolt range rather than the millivolt range above. So yes, there is some voltage drop across the switch when closed, but it is such a low value that it virtually is an ideal, zero-ohm connection. An ideal zero-ohm switch, of course, will yield a zero-volt output when closed, no matter what the inputs are.



Doing the same sort of analysis on the pull-down circuit will give similar results. Outputs may not be exactly either zero or 5 volts, but will be so close to one or the other as to be virtually the same.



As far as the values of the resistors, these are pretty typical values for TTL logic, and based on -



- The output load resistance, which for a TTL input could be something like abut 1 megohm, and

- The drive capability of the input, which could be something like up to 30 milliamps for TTL



To actually select the resistor values, you consider both, as well as the switching threshold of the load (as in, supply no more than say 1 volt for a logic low and at least 4 volts for a logic high).

Answer 2

Pull-up resistors are resistors used in logic circuits to ensure a well-defined logical level at a pin under all conditions. As a reminder, digital logic circuits have three logic states: high, low and floating (or high impedance). The high-impedance state occurs when the pin is not pulled to a high or low logic level, but is left “floating” instead. A good illustration of this is an unconnected input pin of a microcontroller. It is neither in a high or low logic state, and a microcontroller might unpredictably interpret the input value as either a logical high or logical low. Pull-up resistors are used to solve the dilemma for the microcontroller by pulling the value to a logical high state, as seen in the figure. If there weren’t for the pull-up resistor, the MCU’s input would be floating when the switch is open and brought down only when the switch is closed.





Pull-down resistors work in the same manner as pull-up resistors, except that they pull the pin to a logical low value. They are connected between ground and the appropriate pin on a device.



The pull-down resistor must have a larger resistance than the impedance of the logic circuit, or else it might be able to pull the voltage down by too much and the input voltage at the pin would remain at a constant logical low value – regardless of the switch position.



https://www.electrikals.com/

Answer 3

pull up and pull down resistors are used in transistor circuits. they are usually associated with some type of switch, and prevent a short circuit of the power supply when the switch changes state.



a pull up resistor commonly keeps the base of a transistor at Vcc until the switch closes, which grounds the base, but doesn't short the power supply.



a pull down keeps the base at ground until the switch closes, which connects the base to Vcc. again without a short.



the different uses have more to do with the type of transistor. NPN or PNP, and what the circuit is trying to accomplish.

Answer 4

If the USB microcontroller is comfortable with floating pins then there must be no concern connecting a pull down change to it. once you connect it this type and the gadget is powered up i could attempt it by ability of measuring the voltage at each and each of those pins as you turn the switches. you ought to work out the voltage flow from a voltage close to to the dc offered to the chip while the change is open and nil while the change is closed. If the voltage while the change is open isn't close to to the dc offered to the chip then i could connect a pull up there of possibly 1k or bigger. this could advance the flexibility your circuit makes use of each and every time the change is interior the closed place. i wish this facilitates