microcontrollerpotentiometerresistance
When I hook up a 5k rotary potentiomoeter to the analog input pin of a microcontroller it will read values from the pot as integers 0-1023.
If I switch the 5k out for a 500k the microcontroller will still read values as 0-1023.
How is the system able to abstract the reading from the potentiometer down to the same range and distribution of integers when the max value of the potentiometer changed?
A simple resistive voltage divider will achieve what you want.
The formula to calculate the output voltage is:
So if we assume your input voltage ranges from 0-50V, we need to divide it by 10 to achieve 0-5V. If we also assume we want to load the input voltage with 100kΩ, then the calculations would something like:
Vout / Vin = R2 / 100kΩ
0.1 = R2 / 100kΩ -> R2 = 10kΩ
R1 = 100kΩ - R2 = 90kΩ
So R1 = 90kΩ and R2 = 10kΩ
For an ADC requiring a maximum source impedance, you must make sure the voltage divider impedance is below this level. The impedance at the divider can be calculated as R1||R2.
For <2.5kΩ, the above won't meet this requirement as 10kΩ||90kΩ = 9kΩ
If we use 9kΩ and 1kΩ though, we get 1 / (1/1000 + 1/9000) = 900Ω
Bear in mind the lower the resistance the higher the wattage rating resistors you need. 50V / 1k = 50mA -> 50mA * 45V = 2.25W across the top resistor (0.25W across the bottom)
In these cases it's best to use an opamp buffer in between a high resistance divider and the ADC. Or use a 2kΩ and 18kΩ divider, which is not quite as power hungry as the 1k/9k version.
If you want to have your potentiometer going from 10k to 20k, simply hook a 10k after the potentiometer (e.g. where you get your changed Voltage).
Formula: Potentiometer range: From x to 10k+x: Add a resistor of x to the potentiometer. If you want to extend the range, then you have to use another approach.
Best Answer
Apparently you hook up the potentimeter as a proper potentiometer, that is: the wiper supplies a fraction of the toptal voltage to the A/D ipnput. That fraction does not depend on the Ohmic value of the potentiometer: when you tae a higher value, then for a given position of the knob bot 'sides' will get higher values, but the ratio will not change.
Note 1: the voltage supplied by the wiper will not change when you take another Ohmic value for the potentiometer, but the impedance (effective series resistance) does change. As long as this impedance (worst case 1/4 of the value of the potentiometer) is lower than what the A/D input specifies as maximum this is not harmfull.
Note 2: the most common potentiometers are linear, but there are also logarithmic (and anti-logarithmic) potentiometers, which will (for a given position) give a very different ratio.