Precision wirewound pot wiper bounce?
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From: oparr on 13 Jul 2010 10:34 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.
From: Tim Williams on 13 Jul 2010 10:53 Funny, I recently discovered that in my old Heathkit oscilloscope. The horizontal position knob is loose for almost all of its travel: on closer inspection, the contact broke away from the conductive plastic. The funny part is, you turn it and it does nothing, except at certain points. When it's open, the voltage just hangs there (held in place with bypass cap; the input uses a FET follower, so leakage is negligible). So as you turn it back and forth, trying to find the right position, you pass it over the "active" areas, and it slews a little bit, so you keep working it, ohh, too far, go back the other way... I've seen old wirewounds that are scratchy. I'm a little surprised a new one would be. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms <oparr(a)hotmail.com> wrote in message news:26b2ce0a-3c7b-4b2a-afb2-0f1bff1232ed(a)5g2000yqz.googlegroups.com... > 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.
From: oparr on 13 Jul 2010 11:15 > I've seen old wirewounds that are scratchy. I'm a little surprised a new one would be. Maybe it's just my luck....Back in the early 70ties when "build your own audio amp" was popular in my neck of the woods, I built this harmonic distortion meter and the circuit called for wirewound pots. Worked fine except for the wirewound pots. Had to wipe then back and forth a few times for everything to work smoothly even when they were new. I bought this recent wirewound pot with great reluctance based on that previous experience. On Jul 13, 10:53 am, "Tim Williams" <tmoran...(a)charter.net> wrote:
From: AM on 13 Jul 2010 11:25 On Tue, 13 Jul 2010 07:34:21 -0700 (PDT), "oparr(a)hotmail.com" <oparr(a)hotmail.com> wrote: > 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? With wire wound, it can be the other way around. The wire faces that the wiper hits are initially more rounded, providing a smaller tangential surface than is there after wear has taken place. You should stick with then name brands, as it were, for multi-turn pots. There are only about three or four. Bourns is pretty common, and should have addressed such a problem decades ago. Another thing you can do is put a pot across one leg of your 'bumpy' pot to give a fine resolution adjustment of that leg, which should make the 'bumps' less prevalent.
From: AM on 13 Jul 2010 11:28
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? Also, on the other, it should not matter about the bumpy ride on the other scenario if the final position of the pot is all that matters. It would matter if it is a setting where the pot is going to be used constantly, like in a power supply voltage setting knob or an audio amp volume control. |