Precision wirewound pot wiper bounce?
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From: Michael Karas on 14 Jul 2010 10:11 In article <gcnp36tduqq5ne21gcde9aue51mp4b88g2(a)4ax.com>, thisthatandtheother(a)beherenow.org says... > > On Wed, 14 Jul 2010 07:09:41 +1000, David Eather <eather(a)tpg.com.au> > wrote: > > >On 14/07/2010 12:34 AM, oparr(a)hotmail.com wrote: > >> Recently bought the 3-turn wirewound pot below to replace a worn 1- > >> turn conductive plastic pot; > >> > >> http://media.digikey.com/photos/Vishay%20Spectrol%20Photos/533,534,535%20SERIES.jpg > >> > >> Out of the box, the thing exhibits wiper bounce or something similar. > >> Every now and then the wiper appears to momentarily lose contact with > >> the element while turning the knob. I would expect that with a worn > >> pot but not a new one. Is this a bad pot or is this a caveat with > >> wirewoud pots? > >> > >> Other new conductive plastic and cermet pots work just fine, however, > >> can't get them in anything other than 1-turn. > > > >These are 3 turn conductive plastic - 10k and 5k > > > >http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6023740 > > > > > >http://australia.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=6023756 > > > What is it, a small helix that the wiper climbs? Some pots that work in the range of 3 turns are built with small planetary mechanism of three ball bearings that roll in a groove in the end of the adjuster shaft. Pressure applied in the mechanism makes the bearings roll nicely on the shaft giving a 3-1 turns ratio of shaft to element. Sometimes one can force a slip between the bearings and the shaft by applying excess force at the end of the normal rotate range. -- - mkaras
From: Mycelium on 14 Jul 2010 10:40 On Wed, 14 Jul 2010 07:11:12 -0700, Michael Karas <mkaras(a)carouselDASHdesign.com> wrote: >Some pots that work in the range of 3 turns are built with small >planetary mechanism of three ball bearings that roll in a groove in the >end of the adjuster shaft. That is a cheat on the claim then. It is STILL a single turn pot at that point, regardless of any "hat" mechanism they plant onto the top of it. That is really sad. Unlike a car's shaft rotation, which is incremental applications of torque, this scenario is advertised as some greater wiper lineal traverse, which would make for a finer resolution incremental 'step' in resistance at the wiper. And it is not that at all. There is no gain over a single turn device at all, in fact. The user can make fine adjustments without backlash better than a planetary gear setup can. That is also the whole reason that ten turn pots were made to begin with. They make the number of bumps steps greater in number, but smaller in size. When he said that "ten turns is too fine", but he wants more than one, I thought to myself that he should just go for the ten turn then. Especially if it is simply a set point pot on a circuit. We use the little ten turn guys on all our stuff in that realm, and they are considered "instrumentation class" devices by the industry, and that is their purpose. Folks simply did not build high class products with el cheapo car stereo class parts from the seventies. Single turn pots are out. Ten turn pots are cheap. Problem solved.
From: oparr on 14 Jul 2010 11:43 > Especially if it is simply a set point pot on a circuit. The pot in question is not a trimmer pot. Long story short....I would not be able to simulate fast acceleration/deceleration, using fingers alone, with a 10-turn pot. This is a somewhat "outside the box" use of a function generator. On Jul 14, 10:40 am, Mycelium <mycel...(a)thematrixattheendofthemushroomstem.org> wrote: >
From: Mycelium on 14 Jul 2010 12:41 On Wed, 14 Jul 2010 08:43:17 -0700 (PDT), "oparr(a)hotmail.com" <oparr(a)hotmail.com> wrote: >> Especially if it is simply a set point pot on a circuit. > >The pot in question is not a trimmer pot. Long story short....I would >not be able to simulate fast acceleration/deceleration, using fingers >alone, with a 10-turn pot. This is a somewhat "outside the box" use >of a function generator. > > >On Jul 14, 10:40�am, Mycelium ><mycel...(a)thematrixattheendofthemushroomstem.org> wrote: >> OK, then make a chain and sprocket reducer/overdrive. Then you can make it turn ten times to your three-on-the-fingers or whatever ratio you come up with. THAT scenario is far better than buying a pot where they took a single turn pot and made it take 3 turns to turn it. Taking three turns to turn a ten turn pot is a far higher resolution adjustment from a 'noise' POV, with 'noise' being transition spikes made between wire contact nodes. They should sell ten turn pot assemblies where the user can make the ten in less turns based on what gears he puts in the box. Just like the drill press pulley arrangements. Naaahh.. too much like mechanics for the engineers to grasp that one.
From: oparr on 14 Jul 2010 13:02
> THAT scenario is far better than buying a pot where they took a single > turn pot and made it take 3 turns to turn it. The pot in question spreads the resistance linearly over 1080 degrees rotation of the knob. I think you misunderstood the poster who mentioned the planetary mechanism used on **some** multiturn pots. > Taking three turns to turn > a ten turn pot is a far higher resolution adjustment from a 'noise' POV, > with 'noise' being transition spikes made between wire contact nodes. You didn't read enough of the thread. > They should sell ten turn pot assemblies where the user can make the > ten in less turns based on what gears he puts in the box. Your fixation on 10-turn pots is somewhat silly and irrational IMO. On Jul 14, 12:41 pm, Mycelium <mycel...(a)thematrixattheendofthemushroomstem.org> wrote: > |