Precision wirewound pot wiper bounce?
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From: John Larkin on 13 Jul 2010 11:34 On Tue, 13 Jul 2010 08:28:15 -0700, AM <thisthatandtheother(a)beherenow.org> wrote: >On Tue, 13 Jul 2010 07:34:21 -0700 (PDT), "oparr(a)hotmail.com" ><oparr(a)hotmail.com> wrote: > >> >>Other new conductive plastic and cermet pots work just fine, however, >>can't get them in anything other than 1-turn. > > Can you think about why they are only one turn devices? I have seen hybrid pots, multiturn wirewounds that have a cp coating over the wire element. That smooths out the discrete wire steps and sort of lubricates the motion. But to the op's question, a wirewound pot wiper will show electrical steps as you cross turns, but it shouldn't go open at any point. Wiper contact resistance will vary a lot, so don't load the wiper. John
From: George Herold on 13 Jul 2010 11:43 On Jul 13, 10:34 am, "op...(a)hotmail.com" <op...(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/533,534,53... > > 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. We use a lot of the Bourns 3590 sereis ten turn pots. These are wire wound, but I've never seen any problems. If you look real close you can see little steps in the output, presumablly as the wiper moves from one wire to the next. George H.
From: oparr on 13 Jul 2010 11:46 > Bourns is pretty common, and should have addressed such a problem > decades ago. 10-turn seems to be all they offer...Too fine... I chose Vishay because of the 3-turn option. In fact, Vishay is the only one offering less than 10-turn at Digikey. Haven't looked elsewhere, other search engines are such a PITA to use. On Jul 13, 11:25 am, AM <thisthatandtheot...(a)beherenow.org> wrote: >
From: AM on 13 Jul 2010 11:47 On Tue, 13 Jul 2010 08:34:48 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >but it shouldn't go open at any point. Wiper >contact resistance will vary a lot, so don't load the wiper. That is actually a rule. Think about it. There must not be an open wiper circumstance at ANY time during the operation of the pot... ever.
From: AM on 13 Jul 2010 11:58
On Tue, 13 Jul 2010 08:43:31 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jul 13, 10:34�am, "op...(a)hotmail.com" <op...(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/533,534,53... >> >> 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. > >We use a lot of the Bourns 3590 sereis ten turn pots. These are wire >wound, but I've never seen any problems. If you look real close you >can see little steps in the output, presumablly as the wiper moves >from one wire to the next. > >George H. Yes. There are actually several wiper "elements" and more than one turn in contact at any one time. The "bumps" are spikes in resistance as one wiper is walking off the top of a wire, and the next wiper is walking onto the top of another. The old way was to make a single wiper span more than one wire top. They later arrived at the multi-fingered approach to make the transitions as 'smooth' as possible, without setting up losses within the pot itself at the junction node (wiper contact 'point'). Another situation where the ideal circuit diagram and the real part do not match exactly. Make a single wire finger, and get transition issues. Wipe enough fingers to kill the transition issues, and you get contact node issues. I think the happy medium is three fingers and always falling off one and riding up on another as the center follows along on whichever wire top it happens to be on. Another example of this is an optical chopper wheel,where they double the wheel's resolution by staggering in a second sensor one half hole pitch distance from the first, resulting in two pulses per hole pitch lineal traverse. |