Reading potentiometer value in high voltage circuit externally

Question

I want to be able to read the value of a potentiometer in an existing high-voltage circuit externally with a microcontroller, but am concerned how the existing circuit would affect the Arduino and conversely how the Arduino could affect the existing circuit.

Here is my first idea for how a circuit could be constructed to measure potentiometer value in an already existing circuit:

My worry with this circuit is that the voltage going into the potentiometer from the already existing circuit would get shorted through the Arduino, frying it in the process.

My next idea was the addition of a diode to assure voltage would go across the potentiometer and not short through Arduino itself, but I still worry in such a circuit that the 5V supplied by the Arduino would get boosted depending on voltage across the potentiometer, frying the Arduino as well.

Will either of these circuits would work or is there a better way of going about measuring the value of a potentiometer in a given circuit externally?

Answer 1

Assuming that what you want to do is measure the voltage at the wiper of a potentiometer that is connected to 40 VDC with an ADC that has an input range of 0 to 5 VDC what you need to do is reduce the voltage from the pot to what your ADC can handle.

You can do that in this case with a simple resistive voltage divider.

^{simulate this circuit – Schematic created using CircuitLab}

For a 10:1 divider you want 1 part out of 10, so if the value of R3 is 10k you need a total of 100k between R2 and R3, or put another way, make R2 9 times the value of R3. This give you a range of 0 to 4 V to the ADC, if you want the full ADC range of 0 to 5 V you can adjust the divider values a bit so that 40 V in gives 5 V out, and then have your software deal with that.

A resistive divider does affect, and is affected by the rest of the circuit, the divider loads down the potentiometer, and the ADC loads down the divider. The higher the value of the pot, the higher the values in the divider need to be, and the lower the input impedance of the ADC the lower the divider values need to be. It's a trade off at best, and at worst you need to do some buffering with opamps. If there is the possibility of voltage spikes higher than 50 V on the voltage feeding the pot you also need to have some way to protect the ADC input from excess voltage.