Resolution of a single-turn or 10-turn pot? - Page 1
Forum section: electronics / projects

author: anfang
author_rank: Regular Contributor
timestamp: October 03, 2016, 08:45:01 pm
content: What is the real-life resolution of a pot? I mean, how much do I have to turn a (normal, average quality) pot in order to see a change?Suppose that a single-turn 300 Ohm pot has 300 degrees of maximum rotation, and that I have a very precise motor that can rotate it. If the pot has a resolution of 1 degree, I will read at most 300 different values: 0, 1, 2, ..., 99, 100. If it has a resolution of 1/2 of a degree, then I will be able to read 600 different values: 0, 0.5, 1, 1.5, ..., 99, 99.5, 100.Because they are concrete things made out of physical parts, pots must have a limit to their resolution. Is this limit quantifiable?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 04, 2016, 06:21:44 am
content: Some high-power pots are wire-wound and have inherent resolution, but normal signal pots are practically unlimited in resolution coming from the material itself. Of course, they are inaccurate, and the contacts may be noisy -- even when you turn it perfectly smoothly, you can record sequences like 50.1, then 50.5, then 50.3, and so on., but you asked about resolution; that is unlimited, there is no quantization at all (on a humanly measurable level, that is). This means the physical material-defined resolution is so high it will be very difficult if not completely impossible to measure and quantify electrically.

author: JPortici
author_rank: Super Contributor
timestamp: October 04, 2016, 10:20:58 am
content: though there is friction on the axis and that will limit your resolution. there is a white paper by tektronix, IIRC, that demonstrated that you couldn't get more than 10 or 12 bits of resolution

author: danadak
author_rank: Super Contributor
timestamp: October 04, 2016, 04:21:26 pm
content: In a continuous film type pot noise would ultimately limit resolution. To do due diligencecalculate it for the T and Ohmic value at full scale to get worst case.Regarding earlier comment about infinite resolution.Regards, Dana.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2016, 12:32:29 am
content: On a normal* pot the 'steps' are the size of the molecules in the device.Good luck measuring that!{*} ie. not wire wound pots.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 05, 2016, 04:12:08 am
content: You get into questions like how many angels can dance on the wiper of a pot when you ask a question like this.Depends in part on your definition of resolution. Due to things like friction, springiness in the contact fingers and the shaft moving around in the hole it may be difficult to get a value smaller than delta x when trying to move a pot by the smallest possible increment. But the resolution of the substrate may be much higher and it may well be possible to reach values from further away that you can't creep up on. delta x can often be made smaller by vibration, or tapping on the pot.

author: jitter
author_rank: Frequent Contributor
timestamp: October 05, 2016, 05:43:51 am
content: My job partly consists of calibrating instruments strewn with multi turn pots many of which are expected to have a far greater range than they IMHO should.The result is that even the smallest fraction of a turn of the adjusting screw overshoots the target. You then find yourself going back and forth around the target constantly overshooting it either side.If that keeps happeing, I just "age" the pot by turning to and fro around the target 25 times or so. The wiper seems to smooth that part of the track a little and then - with the tongue at the right angle - I find I can set it to the target value.So, in a way you can increase the resolution of an unwilling pot.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 05, 2016, 03:27:29 pm
content: Quote from: jitter on October 05, 2016, 05:43:51 amIt's the wrong pot for the job, the surrounding resistors are bad, or the requirement are too strict for the hardware. In any case, this should not be happening.One thermal cycle will undo your work.

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2016, 03:43:48 pm
content: Quote from: jitter on October 05, 2016, 05:43:51 amThen they're badly designed instruments. Trimmer pots should have resistors on either side so that the full range of the pot covers a very narrow output voltage range.If you're doing micro-adjustments with a multi-turn pot then something is very wrong.

author: jitter
author_rank: Frequent Contributor
timestamp: October 05, 2016, 04:54:46 pm
content: All the pots are the same make (probably Bourns), but different values.But some pots they have designed to be able to adjust a range of +/- 60 mV where +/- 20 mV would have sufficed.Other pots are not asked to do that wide a range and are much easier to adjust.Most of the "easy" pots are for offset nulling, but some for setting gains are tricky. Trimming a 16 bit ADC can also require patience.I have communicated this with the customer on several occasions, but to no avail.