Wirewound pot wiper bounce?

[oparr@hotmail.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.

 

[Lostgallifreyan]

"oparr@hotmail.com" <oparr@hotmail.com> wrote in
news:9039cd6c-9017-42c9-9eb7-65a4f2907e6b@b35g2000yqi.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%20
SERIES.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.

How momentarily? I imagine that if it has a 3:1 turn ratio, and more than one
turn of wire in the element is in contact with the wiper, then unless the
mass of the wiper is large compared to the spring tension used to maintain
contact, you'd be hard put to make it lose contact completely for more than a
microsecond or two. If you're finding large fractions of a second, I think
there would be something wrong with it.

Try a test for noise, putting it in series with a battery, a resistor, and a
pair of headphones, see how much wiper noise you get at various speeds of
turning. I have no idea what it should sound like but if you do it should be
easy to spot it if it isn't behaving.

 

[Lostgallifreyan]

"oparr@hotmail.com" <oparr@hotmail.com> wrote in news:b4f7c5a6-a8ae-4aa2-
9adb-1773c7c14c1d@b5g2000vbl.googlegroups.com:

How momentarily?

I'm using the pot to feed the VCO input of a function generator. The
momentary disconnect causes the frequency to momentarily spike from
say 10KHz to somehwere close to 100KHz which is easily detectable.
Only happens while knob is moving and doesn't happen very often but
once is more than enough based on the app.

Why not apply treatment anyway? If it works you have your diagnosis.
Capacitor to ground to smoothe the voltage on the wiper terminal. It wasn't a
demanding situation when I tried that, but it certainly silenced a pot and
gave me a control as smooth as an expensive optic greased with kilopoise. If
you don't want so much of the kilo, use a smaller cap...

Try a test for noise

I'll just connect wiper output to a storage oscilloscope. That way
I'll tell you what momentary really is....Later!

Well, I decided not to assume you had one of those. But go to it, it will
tell you.

 

[Sjouke Burry]

oparr@hotmail.com wrote:

How momentarily?

I'm using the pot to feed the VCO input of a function generator. The
momentary disconnect causes the frequency to momentarily spike from
say 10KHz to somehwere close to 100KHz which is easily detectable.
Only happens while knob is moving and doesn't happen very often but
once is more than enough based on the app.

Try a test for noise

I'll just connect wiper output to a storage oscilloscope. That way
I'll tell you what momentary really is....Later!


On Jul 13, 11:33 am, Lostgallifreyan <no-...@nowhere.net> wrote:

A wirewound pot can have a bit of oxide on the windings, it might
have been in storage for years.
I would "massage" it by applying a lot of rotations to it.
Using a bit of rope around the axis , you can apply them quickly.

Then test again.

 

[oparr@hotmail.com]

How momentarily?

I'm using the pot to feed the VCO input of a function generator. The
momentary disconnect causes the frequency to momentarily spike from
say 10KHz to somehwere close to 100KHz which is easily detectable.
Only happens while knob is moving and doesn't happen very often but
once is more than enough based on the app.

Try a test for noise

I'll just connect wiper output to a storage oscilloscope. That way
I'll tell you what momentary really is....Later!


On Jul 13, 11:33 am, Lostgallifreyan <no-...@nowhere.net> wrote:
>

 

[Jamie]

oparr@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.
You may see step level changes when the wiper crosses over from one

turn to the next while the 2 turns are in contact. There should not
be any gap opening at all, if so, you have a crap pot!

 

[Peter Dettmann]

On Tue, 13 Jul 2010 20:55:14 +0200, Sjouke Burry
<burrynulnulfour@ppllaanneett.nnll> wrote:

oparr@hotmail.com wrote:
How momentarily?

I'm using the pot to feed the VCO input of a function generator. The
momentary disconnect causes the frequency to momentarily spike from
say 10KHz to somehwere close to 100KHz which is easily detectable.
Only happens while knob is moving and doesn't happen very often but
once is more than enough based on the app.

Try a test for noise

I'll just connect wiper output to a storage oscilloscope. That way
I'll tell you what momentary really is....Later!


On Jul 13, 11:33 am, Lostgallifreyan <no-...@nowhere.net> wrote:

A wirewound pot can have a bit of oxide on the windings, it might
have been in storage for years.
I would "massage" it by applying a lot of rotations to it.
Using a bit of rope around the axis , you can apply them quickly.

Then test again.

It may be an oxide type problem, but more usual is the way in which
the wire is lubricated (to prevent oxidation) can cause problems when
the greasy lubricant changes with time, and gets small hard high
resistance lumps. The cure is to take the pot apart, clean the
winding, and re-lubricate.

Peter

 

[Grant]

On Tue, 13 Jul 2010 16:21:57 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

oparr@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.
You may see step level changes when the wiper crosses over from one
turn to the next while the 2 turns are in contact. There should not
be any gap opening at all, if so, you have a crap pot!

I agree with this poster, wirewound do have step value changes, you'll

see a resolution value in the specs that gets better as resistance goes
up, due to more turns of finer wire.

If running the pot back and forth over that trouble spot a few times
doesn't clear it, I'd suspect old stock, ask the supplier for remedy,
they're not supposed to go open like that in normal use. The wiper
shorts more than one turn to prevent that.

Doubt I'd want to open one to clean it

Grant.

 

[oparr@hotmail.com]

Well, I decided not to assume you had one of those. But go to it, it will
tell you.

With 10VDC connected to the 10K element and the scope's probe
connected to the wiper, I'm able to see the sudden drops in voltage.
See picture;

http://www.pbase.com/eldata/image/126505306


Drop in voltage is around 4V and occurs at around 6V output. Worst
case wiper bounce duration is around 2ms. Here's the puzzle, why is
the waveform so square as though a solid state device is involved?







On Jul 13, 2:50 pm, Lostgallifreyan <no-...@nowhere.net> wrote:

"op...@hotmail.com" <op...@hotmail.com> wrote in news:b4f7c5a6-a8ae-4aa2-
9adb-1773c7c14...@b5g2000vbl.googlegroups.com:

How momentarily?

I'm using the pot to feed the VCO input of a function generator. The
momentary disconnect causes the frequency to momentarily spike from
say 10KHz to somehwere close to 100KHz which is easily detectable.
Only happens while knob is moving and doesn't happen very often but
once is more than enough based on the app.

Why not apply treatment anyway? If it works you have your diagnosis.
Capacitor to ground to smoothe the voltage on the wiper terminal. It wasn't a
demanding situation when I tried that, but it certainly silenced a pot and
gave me a control as smooth as an expensive optic greased with kilopoise. If
you don't want so much of the kilo, use a smaller cap...

Try a test for noise

I'll just connect wiper output to a storage oscilloscope. That way
I'll tell you what momentary really is....Later!

Well, I decided not to assume you had one of those. But go to it, it will
tell you.

 

[oparr@hotmail.com]

I would "massage" it by applying a lot of rotations to it.

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. Hopefully, the "massaging" only has to be done
once with this pot.


On Jul 13, 2:55 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll>
wrote:
>

 

[Lostgallifreyan]

"oparr@hotmail.com" <oparr@hotmail.com> wrote in news:8e58d871-d831-45a0-
9b92-f9dde534842f@q12g2000yqj.googlegroups.com:

With 10VDC connected to the 10K element and the scope's probe
connected to the wiper, I'm able to see the sudden drops in voltage.
See picture;

http://www.pbase.com/eldata/image/126505306


Drop in voltage is around 4V and occurs at around 6V output. Worst
case wiper bounce duration is around 2ms. Here's the puzzle, why is
the waveform so square as though a solid state device is involved?

Probably because the contact breaks cleanly apart from a moment of noise.
That suggests that it isn't oxide, but possibly the other explanation someone
came up with fits, there could be an excess of viscous grease there. Apart
from that it would have to be clean as a whistle to produce that sharp
transition.

I recently bought some preset pots that failed due to stiffness, and the
cause was similar, a thickening of grease (in this case in pots whose wiper
block was set into a space too tight for it anyway most likely) in pots that
were new, but had been on the shelf a while. I suspect they all ought to be
using a PFPE grease like those used in underwater breathing gear but in a
cheap preset pot bought on eBay there is a risk that the maker scrimped on
this and used a hydrocarbon grease that stiffens when heat evaporates off the
finer oils out of it. I can't imagine an expensive multi-turn wirewound pot
being scrimped on the cost of a smear of PFPE grease, but I imagine there
might be some tiny bits of insulating debris caught in it.

 

[Lostgallifreyan]

"oparr@hotmail.com" <oparr@hotmail.com> wrote in news:9f2c74f7-8c03-4c04-
a80d-af4e2f36e0e9@i28g2000yqa.googlegroups.com:

Digikey is sending a replacement. Even though the problem has subsided
for now, I'm not happy with that pot...."Things that go away by
themselves tend to come back by themselves" lingers somewhere in the
recesses of my mind. If the replacement behaves the same way then my
conclusion would be that either Vishay WW pots are crap or some
applications are not suited for WW pots.

Good. If the next one does it, let the maker agonise over it.. Given that
wirewound pots are aimed at stability and low noise, they shouldn't be doing
that. Small discrete steps come with their nature but they shouldn't break
contact.

 

[oparr@hotmail.com]

Apart from that it would have to be clean as a whistle to produce that sharp
transition.

Grant suggested in the design forum that it's due to the "probe only"
no-load condition. Also, the shot I posted may be misleading. Took
several shots while wiping back and forth in between. This sequence
may be more informative;

First in sequence (time stamp 17:22);

http://www.pbase.com/eldata/image/126519337

Wiping back and forth in the bounce area......Check.....etc.

Second in sequence (time stamp 17:27);

http://www.pbase.com/eldata/image/126505305

Wiping back and forth in the bounce area......Check.....etc.

Third in sequence (time stamp 17:30), same as posted previously;

http://www.pbase.com/eldata/image/126505306

Wiping back and forth in the bounce area......Check.....etc.

Unable to reproduce again. As you can see, the first in sequence was
noisy as hell.

I can't imagine an expensive multi-turn wirewound pot
being scrimped on the cost of a smear of PFPE grease, but I imagine there
might be some tiny bits of insulating debris caught in it.

Digikey is sending a replacement. Even though the problem has subsided
for now, I'm not happy with that pot...."Things that go away by
themselves tend to come back by themselves" lingers somewhere in the
recesses of my mind. If the replacement behaves the same way then my
conclusion would be that either Vishay WW pots are crap or some
applications are not suited for WW pots.


On Jul 14, 5:03 am, Lostgallifreyan <no-...@nowhere.net> wrote:
>

 

On Wed, 14 Jul 2010 08:54:34 +1000, Grant <omg@grrr.id.au> wrote:

On Tue, 13 Jul 2010 16:21:57 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

oparr@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.
You may see step level changes when the wiper crosses over from one
turn to the next while the 2 turns are in contact. There should not
be any gap opening at all, if so, you have a crap pot!

I agree with this poster, wirewound do have step value changes, you'll
see a resolution value in the specs that gets better as resistance goes
up, due to more turns of finer wire.

If running the pot back and forth over that trouble spot a few times
doesn't clear it, I'd suspect old stock, ask the supplier for remedy,
they're not supposed to go open like that in normal use. The wiper
shorts more than one turn to prevent that.

Doubt I'd want to open one to clean it

Grant.

Almost agree. Due to differences in manufacturing, pots from 30 years
ago or more are much more repairable.

 

[Grant]

On Sun, 18 Jul 2010 17:22:47 -0700, JosephKK@yahoo.com wrote:

On Wed, 14 Jul 2010 08:54:34 +1000, Grant <omg@grrr.id.au> wrote:

On Tue, 13 Jul 2010 16:21:57 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

oparr@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.
You may see step level changes when the wiper crosses over from one
turn to the next while the 2 turns are in contact. There should not
be any gap opening at all, if so, you have a crap pot!

I agree with this poster, wirewound do have step value changes, you'll
see a resolution value in the specs that gets better as resistance goes
up, due to more turns of finer wire.

If running the pot back and forth over that trouble spot a few times
doesn't clear it, I'd suspect old stock, ask the supplier for remedy,
they're not supposed to go open like that in normal use. The wiper
shorts more than one turn to prevent that.

Doubt I'd want to open one to clean it

Grant.

Almost agree. Due to differences in manufacturing, pots from 30 years
ago or more are much more repairable.

I got a busted one from a friend, one end of element open circuit[1],
but the thing is glued together, and, it's still useful as a 10 turn
rheostat.

[1] That happens is one doesn't look at the picture on the pot, and
assume middle lug is the wiper, instead of the offset lug, oops...

Grant.