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I am trying to replace a 2-D analog joystick assembly that no longer works, because I dropped it. The assembly looks just like the one pictured in this forum post.

My question is about the trim potentiometers in the assembly.

I removed each of the potentiometers from the circuit and measured the resistance across each pin when the potentiometer is centered (pin 1 to pin 2, pin 2 to pin 3, pin 1 to pin 3). I recorded the measurements I took, but they are not what I expected:

R_1-3 for each potentiometer is ~7.5kohms, but R_1-2 and R_2-3 are measuring ~5kohms each.

Do these measurement make sense? I was expecting that, given the same centered position of the potentiometer, R_1-2 and R_2-3 should sum to the value of R_1-3. Am I incorrect?

I included a screenshot of all of my measurements below, if it helps.

enter image description here

1 Answers1

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R_1-2 and R_2-3 should sum to the value of R_1-3. Am i incorrect?

This is the ideal case. However, carbon potentiometers and trimpots are not perfect devices due to their construction. So this inherent non-ideality brings some unexpected results such as you have.

The centre leg is connected to a wiper that moves along the resistive element (can be a strip or similar) as you turn the knob:

enter image description here

Img Src: Vareias.com

(The image above does not show the internal structure of your trimpots but the construction is similar.)

The basic logic tells you that the total resistance should be the sum of the resistances at the left-side and the right-side portions of the wiper. But in your measurements, the contact resistance comes into play. If the contact is not good, it'll show resistance. The worse the contact, the higher the resistance.

Now take a look at your measurements again and see the contact resistance: When the wiper is positioned at min or max (clockwise or counterclockwise, depending on your perspective), you should have seen nearly zero resistance at the respective side but you saw a few hundreds of ohms or even around a kiloohm. This is because the wiper cannot go to extremum points plus there's a contact resistance.

And also, when the wiper is at centre the sum of left and right side should be equal to the resistance between the outer legs, which is the actual pot/trimpot resistance. The sum is higher because it contains the contact resistance "twice" (both R12 and R23 contain the contact resistance).

The contact resistance can be decreased to some level by using some cleaner chemicals but don't expect to see zero.

Rohat Kılıç
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  • Thanks for the detailed answer, that helps a lot. So, I have a follow up based on your answer: Looking only at the resistance measurements, would you agree that it can be concluded the total resistance of the potentiometer is ~7.5kohms? Furthermore, looking at the measurements taken while the potentiometer is in the "rest" position, the ~2.5kohms additional resistance present when summing 1-2 and 2-3 measurements can be attributed to contact resistance? – aflyingcougar Apr 20 '22 at 17:42
  • @aflyingcougar please see the updated answer. – Rohat Kılıç Apr 20 '22 at 18:33