This question already has an answer here:
I am building a simple voltmeter circuit that needs to measure up to about 35 volts. Looking around the web, I am finding all sorts of suggestions for voltage divider values, like 1 Mohm / 10 Kohm, and the one I am currently experimenting with, which is 10K/2K. I am having issues with allowing enough time between readings for the Arduino Nano to settle down, of course, but that is not the question. Using 10K/2K it maxes out at a reading of 1020 on A6 with an input of 27 volts.
I do not understand why one posting says to use resisters in the Mohm range and the other (that I am using) is a total of 12k.
Question: Is there a rule of thumb that should be used to decide what resistances should be used for the divider for measuring this level of voltage?
And how important might it be to keep the resistance between Vcc and GND higher than what I am using?
-- EDIT: EE asked me if this was the same as another suggested Q/A - and this is only different because it is specific to the Arduino's ADC input impedance. The other answer only talked about the ratios.
The answers to this are great for my particular application.
--FURTHER EDIT: Based on the collective berating from the powers that be, I would mention that this started in an Arduino forum where I was concerned about the particular ADC, then it got moved here. Calling this a duplicate is incorrect. It is very specific to the Nano application. I wanted to make sure my small values wouldn't cause problems by being between Vcc and GND. Plus, I did not know the proper input impedance for the Nano's ADC. It all got answered here, nicely.
Thus I believe it stands as being unique here. I can find no duplicates of this particular question. Being marked this way with no explanation is frustrating. If there really are duplicates then I would like to see their answers. But finding none, this one adds to the community and I believe it should not be marked as a duplicate here.
And the good thing is that one respondent brought up smoothing capacitors. So now I am using one, and it helps stabilize the readings. So this has been a positive experience.
The problem now is that Arduino people won't necessarily know to look here for this answer when they try to use their Arduinos this way. So there will likely be duplicate questions there which just got answered here.