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I have 16 potentiometers of the same value (with some tolerance) like this. The wipers go to muxers and then to ADC pins of a micro-controller.

On the breadboard this worked fine (using only a few potentiometers), but now that I soldered it together and testing each pot, 2 pots in the middle behave weird. When I disconnect the circuit from power and use a multimeter to measure their resistor value while dialing from min to max, then the value goes up for a while and then all of a sudden goes down again. It never reaches 4.7k at all.

Since I am not an electronic engineer, I wonder if I just managed to buy some broken pots or if I did something stupid here (in which case buying new pots and swapping them won't help).

parallel potentiometer

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  • \$\begingroup\$ sounds like you have a couple of defective pots .... maybe check if the part numbers are all the same (if there is a part number printed on the pots) \$\endgroup\$ – jsotola Feb 2 '19 at 21:10
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    \$\begingroup\$ How exactly are you measuring the resistance? If you have 16 identical pots wired in parallel as you show, and measure the resistance between any one of the wipers and ground, you'll get the maximum measurement in the middle of the range -- and this value will be about 1.25k. As you move the wiper above this point, the current flows through the top half of the pot and then through the other 15 pots in parallel to ground again. \$\endgroup\$ – Dave Tweed Feb 2 '19 at 21:15
  • \$\begingroup\$ I measured between ground and wiper. \$\endgroup\$ – ensonic Feb 3 '19 at 18:26
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The equivalent circuit.

You have 15 x 4k7 pots in parallel with the one you are testing.That means that there is a resistance of 4k7 / 15 = 313 Ω between the rails due to the other pots. If you measure resistance between the wiper and ground and rotate clockwise your should go from zero at the bottom to 313 Ω at the top.

In the centre you will have 2.35 kΩ to the bottom due to the bottom half of the pot in parallel with 2350 + 313 Ω due to the top half of the pot plus all the other pots. This should result in a reading of about 1248 Ω.

If your soldering is OK then turn on the power and you should get a linear voltage increase from 0 to 5 V as you rotate the pots.

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  • \$\begingroup\$ I've now powered the circuit and the pots don't seem to be defective. It is still a bit weird, the first 9 are okay and then 4 of then reach their 'max' value already when I dialed them to ~ 75%. The last 2 pots are good again. I checked the soldering over and over, it looks good. \$\endgroup\$ – ensonic Feb 3 '19 at 18:29
  • \$\begingroup\$ "* 4 of then reach their 'max' value already when I dialed them to ~ 75%.*" Do you mean 5 V on the wiper at 75% rotation? A photo, perhaps? \$\endgroup\$ – Transistor Feb 3 '19 at 18:31

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