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Electronics => Beginners => Topic started by: little_carlos on December 17, 2014, 05:45:49 pm

Title: how much resistance for a pulldown resistor?
Post by: little_carlos on December 17, 2014, 05:45:49 pm
hey guys
how much resistance do i have to apply at minimal so it can work properly as a pulldown resistor for a microcontroller?
Title: Re: how much resistance for a pulldown resistor?
Post by: tggzzz on December 17, 2014, 07:07:19 pm
hey guys
how much resistance do i have to apply at minimal so it can work properly as a pulldown resistor for a microcontroller?

Sufficient so that, with the current flowing through it, the voltage across it is within limits.

Without furthur information, that could be from 0.01ohms to 10Mohms.
Title: Re: how much resistance for a pulldown resistor?
Post by: c4757p on December 17, 2014, 07:19:57 pm
Without furthur information, that could be from 0.01ohms to 10Mohms.

I think he was wondering what further information might be needed, not how cleverly sarcastic a response he could get.

little_carlos: The pulldown resistor has to be able to pull the line down (obviously) - in two ways. First, it has to be able to hold it down against things which might tend to pull it up accidentally. This includes leakage current and external electromagnetic noise. There's really no solid answer to that - generally don't go too far north of 100k. You'll often have to determine this experimentally. If the line tends to pick up noise, drop the resistance.

Second, you have to pull it down in a certain amount of time. This is determined by how quickly it can charge the capacitance present on the board and on whatever inputs are connected to the line. You'll have to determine an estimate for that capacitance, and then know that the line will be pulled down in about 5*R*C, where C = the stray capacitance and R = the pulldown. Figure it out based on how fast the signals are going.

Both of these set maximum limits. Third, you want to keep from wasting too much power. That's mostly up to you - obviously 0.01 ohms will blow things up, but even 1k could be wasteful. Generally you'll stay well away from the "might blow things up" limits, and just be concerned with not wasting power. This is more or less up to your own judgment, and really only factors in when there are a lot of them on the board, or in low-power situations like when powered by batteries.

In short, play with it. Build a few circuits and experiment with values.

Got more details?
Title: Re: how much resistance for a pulldown resistor?
Post by: Kappes Buur on December 17, 2014, 07:22:07 pm
There is an app (lication note) for that:  :)

http://www.ti.com/lit/an/slva485/slva485.pdf (http://www.ti.com/lit/an/slva485/slva485.pdf)
Title: Re: how much resistance for a pulldown resistor?
Post by: tggzzz on December 17, 2014, 07:44:28 pm
Sufficient so that, with the current flowing through it, the voltage across it is within limits.
Without furthur information, that could be from 0.01ohms to 10Mohms.
I think he was wondering what further information might be needed, not how cleverly sarcastic a response he could get.
...
Got more details?

Your response appears to presume that the pulldown resistor is on an input. Without more information, it could well be on an output.

And that's the reason for trying to educate people to give sufficient information.
Title: Re: how much resistance for a pulldown resistor?
Post by: c4757p on December 17, 2014, 07:49:26 pm
I would hope to find at least an input and an output at either end, or else I've terribly misunderstood the purpose of inputs and outputs...
Title: Re: how much resistance for a pulldown resistor?
Post by: Richard Crowley on December 17, 2014, 08:11:46 pm
I would hope to find at least an input and an output at either end, or else I've terribly misunderstood the purpose of inputs and outputs...

The pull-down situation for something like a microcontroller INPUT is rather different from what would be needed (for example) for a FET output driver transistor driven from a microcontroller output. Yes, you can lump them both together into the general case, but that doesn't seem helpful for someone at the perceived level of the OP.

I didn't think the response from tggzzz was all that "sarcastic".  It was a simple statement that a specific resistance value (or even range) is not possible without better details.
Title: Re: how much resistance for a pulldown resistor?
Post by: tggzzz on December 17, 2014, 08:27:30 pm
I would hope to find at least an input and an output at either end, or else I've terribly misunderstood the purpose of inputs and outputs...
Yes you have misunderstood; I suspect you missed a word.

Here is the sentence again, with emphasis for clarity.
"Your response appears to presume that the pulldown resistor is on an input."