pot /w series parallel resistors not working properly...

[bigjoncoop]

Im trying to modify a potentiometer to go from 80 ohms to 115 ohms by adding series / parallel resistors.

I have a assortment of pots:
1K 2K 5K 10K 20K 50K 100K 250K 500K 1M

(Screenshots Below) pp
In the simulator if I have a 1k pot. I connected a 35Ω resistor between pins 1 & 2 and a 10Ω resistor between pins 2 & 3. Then I have one side of a 80Ω resistor conneced to pin 1.

If I take a reading between the 80Ω resistor and pin 2, the pot does exactly what I need it to! It goes from 80Ω to 115Ω...

Unfortunately after building it for real it doesn't work like that at all! The pot for some reason measures 115ohm turned either direction... I triple checked all my connections and made sure that the wiper in the simulator is the same as the pot I'm using. The wiper is in the middle.

Below are some screenshots and a short 5 second video of the pot working in the simulator.

If some can help me I would greatly appreciate it! Maybe point out what I'm doing wrong or suggest a simpler method to achieve this...  THANKS!

[tooki]

You sure you’re measuring the real-world pot at the wiper, and not the other end? Not all pots put the wiper in the middle pin (particularly multiturn pots and some trimmers).

Have you made sure the pot works properly to begin with, without the parallel resistors installed?



FYI, uploading animated gifs as attachments doesn’t work, the animation is lost entirely.

[SteveThackery]

I've just worked it out on pen and paper, and I get a resistance range from 80 ohms to 113.8 ohms. So, I think you have made a mistake when you did the physical wiring up.

Also, the 10 ohm resistor does nothing and you can replace it with a link.

[ledtester]

Also, the 10 ohm resistor does nothing and you can replace it with a link.

In fact, it is shunted by the wiper and can be removed entirely.

Update: Here is a Falstad simulation -- with a current source used to measure the resistance.

(Link to Falstad Simulation)

You can adjust the pot using the slider on the right panel. Note that no current flows the 10R resistor.

The total resistance is indeed highly non-linear -- at the pot midpoint the resistance is already 33 Ohms.

[SteveThackery]

Here is the equivalent circuit, without the 10 ohm resistor.



Unfortunately the resistance varies in a highly non-linear manner. Graph to follow.

[SteveThackery]

Sorry about the terrible quality! But as you can see, the action all happens in the first few degrees of rotation of the pot.

[DavidAlfa]

Of course, 1K in parallel  with 35ohms makes barely any difference.
Only when the pot reaches 100omhs it's start to make some work, which is just the last 10% of the trim.