Electrical Engineering Stack Exchange
Electrical Engineering Stack Exchange

Questions tagged [pulldown]

A pull-down resistor brings a line down to a voltage (typically ground potential) when the line isn't being driven to a specific voltage. It prevents the line from "floating" with an unknown voltage on it.

A pull-down resistor brings a line down to a voltage (typically ground potential), while still allowing the line to be driven to a higher voltage.

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When to use pull-down vs. pull-up resistors

After learning and experimenting with microcontrollers, I've understood the concept of pull-up- and pull-down resistors. I now understand when and how to use them, and how they work. I've mainly used pull-ups because I was taught to, but it has…
osvein
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Which configuration is better for pulling down an NPN transistor's base?

I was discussing on pull down resistors with a colleague of mine. Here are the two configurations for transistor as a switch. The input signal can either come from a microcontroller or an another digital output to drive a load, or from an analogue…
abdullah kahraman
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What makes a pull-up/down resistor strong or weak?

A "strong" pull (up/down) resistor would be one of a relatively low value, while a "weak" one would be of a relatively high value. For example, a pull-down resistor would be used to keep an I/O pin low, but a button connected from that pin to VCC…
JYelton
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Heuristic explanation of the usage of a pull-up (or pull-down) resistor needed

First, I acknowledge that there are several questions regarding this topic in the forum, however, the answers assume too much background knowledge of electronics to be of use to a true beginner (like myself). That being said, if you choose to…
Zak
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Why I2C designed to work with pull-up resistors and not pull-down ones?

I understand that in I2C, SCL and SDA lines use pull-up resistors and the pin drivers are open collector NPN devices which can drive pins to ground. This gives I2C an advantage that the same bus now can be shared with multiple slaves, and even if…
Ashutosh
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Pull-up and Pull-down Resistor Usage on Input or Output MCU Pins

Are pull-up/down resistors (whether internal or external) only needed for MCU INPUT pins? In contrast, an MCU pin configured as an OUTPUT "knows what level it's at" because it does the driving - a "floating" MCU OUTPUT pin tied to some input of…
The_Ders
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Pull Down Resistors

In my quest to understand electrical engineering, I have stumbled across this tutorial: http://www.ladyada.net/learn/arduino/lesson5.html I have understood the diagrams until I got to switches. I am not sure how switches work on the breadboard or…
LunchMarble
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Why do we need external pulling resistors when microcontrollers have internal pulling resistors?

Microcontrollers do have internal pull up-pull down resistors yet most of the circuits have external pulling resistors. I looked on Google for answers and a few sites said that those resistors are not that strong but I thought they were good enough…
Gunjan Gangwani
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Considerations when using internal pull-up/down resistors

I am working with STM32 micro-controllers. These micro-controllers have the option to set the input pin as pull up or pull down: Earlier I used to give external pull up or pull down resistor wherever required but now I am wondering whether I can do…
Whiskeyjack
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Recommendation for default settings for unused pins on an STM32 (ARM Cortex M3) - pull up/pull down?

We are currently using various variants of the STM32 microcontroller family. I would like to know the following: What are the recommended default settings for pins of the microcontrollers in general if you have the option to select both pull ups…
IntelliChick
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Why are pull up resistors more common than pull down resistors?

I noticed that pull up resistors are much more common than pull downs, why? For example, the Arduino's MCU has internal pull ups but these tend to invert the physical logic of things you are working with (such as working with switches) whereas a…
Faken
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Placement/location of pull-up/pull-down resistor?

I have connected a microcontroller GPIO pin, which is intended to be an output, to a DC-DC converter's active high Enable input pin. Since this pin is active high, and because I do not want this converter to be ON at power up or before it is…
LoveEnigma
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Is a pull up/down resistor really necessary?

Logical reading (1 or 0) of a micro-controller input GPIO pin may float if it is neither connected to VCC nor GND; someone said it is because of surrounding RF interference. When a floating GPIO input pin voltage is affected by RF interference that…
mannok
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Why does a pull-up resistor not stop current flowing to the input pin when the switch is open?

Sorry this is such a silly question, but I can't seem to understand this. In the third diagram here it shows a pull-up resistor. I understand that when the switch S1 is closed, current is pulled down to ground and assumes a value of 0. This…
Mark Robinson
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Must IO pins each have their own pull-up/down resistor?

A very basic question: Must each IO pin of a microcontroller that is serving as an input from a switch or jumper have its own pull-up/down resistor? In this example, each pin is pulled down by a resistor, then pulled up by a 1P3T switch to VCC. It…
JYelton
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