Recently bought the 3-turn wirewound pot below to replace a worn 1- turn conductive plastic pot;
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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.
" snipped-for-privacy@hotmail.com" wrote in news: snipped-for-privacy@b35g2000yqi.googlegroups.com:
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.
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.
I'll just connect wiper output to a storage oscilloscope. That way I'll tell you what momentary really is....Later!
" snipped-for-privacy@hotmail.com" wrote in news:b4f7c5a6-a8ae-4aa2- snipped-for-privacy@b5g2000vbl.googlegroups.com:
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...
Well, I decided not to assume you had one of those. :) But go to it, it will tell you.
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!
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.
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.
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;
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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?
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.
" snipped-for-privacy@hotmail.com" wrote in news:8e58d871-d831-45a0- snipped-for-privacy@q12g2000yqj.googlegroups.com:
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.
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);
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Wiping back and forth in the bounce area......Check.....etc.
Second in sequence (time stamp 17:27);
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Wiping back and forth in the bounce area......Check.....etc.
Third in sequence (time stamp 17:30), same as posted previously;
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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.
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.
" snipped-for-privacy@hotmail.com" wrote in news:9f2c74f7-8c03-4c04- snipped-for-privacy@i28g2000yqa.googlegroups.com:
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.
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