Voltage Divider

I understand the very basics of using two resistors to make a voltage divider to supply a lower voltage such as how the ratio of the sizes of the two resistors determines the Vout. But is it important to know what size of resistors to use as there are infinite possibilities that match the correct ratio? E.g. I’m hooking up a humidity sensor (discussion here: http://www.tinyclr.com/forum/topic?id=8847 ) that outputs 5V so need to pare it down to 3.3V for analog in. Farsa helps out by suggesting R1= 11k and R2 = 20k and calculators give similar suggestions of 10.3k and 20k. So I trot down to my local electronics store and they are fresh out of 20k and 11k resistors. Can I use any resistors that match this ratio, why would Farsa recommend 20k and 11k vs some other pair, do i need to match some current limit, or?


You can substitute any resistors that maintain the ratio, within reason. The other thing is that in most cases, you don’t need to be ultra accurate; if you used resistors that resulted in 3.2V or 3.1V then that would be ok, it would just reduce the range of your voltage reading (you only have 1024 divisions across 0 to 3v3, and if you only went to 3v1 you would never be able to use the upper 62 divisions and your range would effectively be only 0 to 962 and you would need to adjust your calculations).

Of course you can also use multiple resistors in series to get specific values if you want to be accurate; and you might need to go to 1% tolerance resistors too if you want ultra accuracy…

Thanks, Brett. Helpful, including the idea of using resistors in a series to zero in on the desired rating. At the risk of being totally pedantic, what does “within reason” mean? (I don’t have much in the way of reason in this field.) Could I use a 11 ohm and a 20 ohm?

I mean within reason as my way of saying there’s science behind it but you probably don’t want to know :slight_smile:

Actually I can’t find a good reference, and don’t have a good level of reason either.

Wikipedia says

What that says to me is you want large R values to get more stable output, so 11k and 20k are preferable to 11R and 20R. But in general when talking about uC signal levels you’re probably not worried too much about the current levels anyway as they’re pretty small; it’s not like you’re trying to power an LCD or anything that will consume a lot of power over this :slight_smile: