# Old potentiometer as a special voltage divider model

In an Adruino-like project that is built to deal with worn potentiometers (trimmer pots inside thumbstick, of alps alpine type), I am noticing a consistent set of input reading patterns among all old, overused potentiometers.

The patternis best described as having the input tending to prematurely reach both the lower and higher values away from the mid value, which appears not to be affected.

This is a quick drawing to explain the behavior: I'd like to know what could be the probable explanation of this behavior of an old potentiometer as a voltage divider; In other words, could the wearing effect actually change the thicknesses of the resistor hence its electric resistance, leading to change in voltage assessment. Or could the resistance of the old wiper is relatively higher or lower than that of the resistor, leading to such results?

• Odd that max resistance is mid-way...but reasonable if pots are run end-to-end. Wear or carbon migration to the ends would be a reasonable guess. Oct 22, 2021 at 23:11
• Do you know what type of potentiometers they are? What if they had non-linear curve to begin with? Not all pots are intended to be linear. Oct 22, 2021 at 23:31
• do the worn potentiometers measure the same resistance as their label suggests they should? Oct 22, 2021 at 23:32
• @Justme it is linear. Oct 22, 2021 at 23:35
• @Jasen Those are tiny components, that I don't have the proper tools yet to measure, I've just ordered from Amazon said tools for that very purpose. This question is actually aimed at tackling that issue; Based on input readings, can we know how worn pots behave as voltage dividers? i.e. what is going on with their resistance? Oct 22, 2021 at 23:46

Maybe the older pot just has a different electrical angle, nothing to do with wear.

Normally there's a region in the mechanical range of motion at either end where the resistance does not change much. The electrical angle, the more-or-less linear portion between those end bits, is less than the mechanical angle, but how much less can vary.

For example, this datasheet from Alpha specifies as follows:

On the other hand, if you're actually using it as a voltage divider, the pot element resistance (and to a much less extent) the wiper resistance can cause an S-shaped curve (normalized and put overtop of a straight line) when loaded. In other words, a 10K pot with a 100K load will show more nonlinearity than a 1K pot with a 100K load.

Edit: Assuming localized wear is the issue (see comments) then Bourns in their Potentiometer Handbook (1975) specifically comments that:

They do not provide information on the nature of the reduced performance. Servo pots that experience hunting about a single point tend to wear out that particular region of the element, as might be expected.

• The old pots are compared against the exact model of new pots. Moreover, the effect started to appear progressively in mid-aged pots as well, during weeks of testing. Those alps pots' life is 2000000 cycles, and it is used tens of times more. youtube.com/watch?v=OC0eEOHtX7A Oct 23, 2021 at 3:16
• Well, maybe you're just mechanically wearing away the resistive element where the wiper contacts it. I assume the motion is not evenly distributed over the pot element, yes? Oct 23, 2021 at 3:19
• The nature of change is gradual, and sort of uniform across 20 different old potentiometers. I'd like to know what is the change in resistor/wiper's electrical resistance that could lead to such predictable readings I get from 20 different pots? Oct 23, 2021 at 3:28
• @Physician Maybe you could sacrifice one to science and have a look at the worn element. If most of the motion is around center you'd expect the resistance to increase around that point and therefore the slope of divider voltage vs. angle will increase in the most worn region(s). Oct 23, 2021 at 3:30
• Of course I'd do. Thanks for you help. Oct 23, 2021 at 3:56