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Topic: Pull-down (Read 413 times) previous topic - next topic

JMD1

Most likely there is any similar topic. But anyway.
Why its always said 'Use 10k ohm for pull down"
Cant I use 100 ohm or 1M?
Does it matter what value you choose?

septillion

It's not critical no.

But a lower resistance means a higher current and whatever is driving it needs to be able to supply it. Also, more current is more heat so the resistor needs to be able to handle it. For example a switch might be able to handle the current with 100Ohm (50mA @ 5V) but that means 250mW of heat which is at the border of a normal resistor and will get very hot.

On the other hand a higher resistance means less current. But that also means it's more susceptible to noise. It doesn't take a lot to draw the line up again / to overcome the pull down current. Also, at the higher range you might get in the same order of resistance as the input resistance which might influence it, especially with pull ups.

So all in all it's a trade off. So lower resistance means better noise immunity but high current / high power. Less resistance means less current but more susceptible to noise. 10k usually is just the go to value. All 1 values are (just like 100Ohm, 1k, 10k etc). And the order of 10k works for most "normal" situations and has little current draw (0,5mA).
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JMD1


INTP

Yes, it is a tradeoff. Ideally you want some magical resistor number that will give you 100% reliability (of knowing what state the pin is in) while using the least amount of current.

I would say the priority is on the side of having the reliability, since using a few extra mA of current won't keep you up at nights while a program not behaving as it should due to a floating pin likely will.

Jiggy-Ninja

#4
Jul 02, 2017, 07:43 pm Last Edit: Jul 02, 2017, 07:44 pm by Jiggy-Ninja
I would say the priority is on the side of having the reliability, since using a few extra mA of current won't keep you up at nights while a program not behaving as it should due to a floating pin likely will.
For stuff plugged into a wall, power consumption is a non-issue and a bias toward reliability is definitely best. Just stay out of the obviously stupid extremes (like 10 ohms or 100 megaohms) and you're fine.

It starts being an issue when you are dealing with a restricted energy supply, like small batteries or energy harvesting. Every coulomb counts, and pissing away more than necessary in a pullup is wasteful. This is especially important if the switch might remain in the closed position for extended periods of time, like a reed switch detecting if a window is open or closed. Increasing the pullup value conserves scarce energy, and other advanced methods can be employed like disabling the pullup when you aren't reading the switch.

There was one thread I saw on EEVBlog's forum where the guy was trying to push his microcontroller to the limit and ended up using 10 meg pullup resistors. He still wasn't able to reach his intended power target.
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