trimpots - Page 1
Forum section: electronics / beginners

author: tatel
author_rank: Frequent Contributor
timestamp: January 28, 2024, 01:58:21 pm
content: Hi all.It looks as if, should some african wizard peregrinate to Russia, to make juju under full moon, while Prigozhin's ghost sings "Boatswains of the Volga", then those guys on audio forums could agree about trimpots.But, fact is, I haven't found so much here, either.Usually I go the Digikey way to get some cermet trimpots. But, right now, after reading what those guys wrote, I don't know even where my ears are.But I know I'm not going to buy half a dozen of €35 trimpots willy-nilly...So, what do you guys think?. Are cermets OK? Are wirebounds the only way to go (God forbid) I'm getting ready to build a Kelvin-Varley divider, don't want to ruin that build by skimming in trimpots.If not for those audio forum posts, I would go whit "cheap" cermets from Digikey.

author: Andy Chee
author_rank: Super Contributor
timestamp: January 28, 2024, 02:14:44 pm
content: What circuit are you designing?

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2024, 02:17:56 pm
content: Been audio designer since 1970s, For many decades cermet have been used. in fine audio analyzers and equipmentWe have boxes of all types from 1960s..1990s I have not bought new recentlyold line USA were best like Bourns.For fine vernier the multiturn are in cermet, WW and other.Only issues are the number turns, and scratchy in old equip esp if exposed to corrosive polluted humid.Just clean trim with control cleaner/lube or iso alcj

author: Gyro
author_rank: Super Contributor
timestamp: January 28, 2024, 02:22:49 pm
content: Quote from: tatel on January 28, 2024, 01:58:21 pmIf you're building a Kelvin Varley divider, then you want the trimpot to be as stable as possible, and preferably the same temperature coefficient as the main resistors. You need to scour the datasheets for that,but I have a feeling that Cermet might not be the answer. One thing is certain - you need to ensure that the adjustment range is as tiny as possible. That might mean putting a higher value trimpot across a low value, stable, fixed resistor.

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2024, 02:32:14 pm
content: Wirewounds were the standard for the cathode circuit of powerful audio output tubes where current is in the hundreds of milliamps. As for audio quality that can be detected with the ears as a double blind test with audio snobs there is no way in hell a trim pot could show a difference unless it was so poorly designed as to have hundreds of picofarads of capacitance or hundreds of microhenrys of inductance to cause a slight bump in the audio response. Remember, these are the same assholes who buy little wooden blocks to keep their speaker wires up off of the floor and claim to hear 'REMARKABLE' differences in the 'sound stage'. Or buy some very expensive 'wall outlet' with such low resistance contacts it 'actually improves the bass accuracy and opens the mid-range' all while connected to dubious house wiring that could be back-stabbed 3 times before it even gets to the stereo equipment outlet!!! The best ears I ever ran into was a legally blind disc jockey that I worked with for over 10 years. When he told me something didn't sound right I trusted him (The other engineers thought he was 'full of shit' and a pain in the ass) and even if it took most of a day I could eventually find a real problem. He even proclaimed the majority of 'audiophools' were indeed idiots who in real double blind tests couldn't reliably detect even real differences induced in the tests.

author: CaptDon
author_rank: Super Contributor
timestamp: January 28, 2024, 02:36:18 pm
content: Sealed trimpots are nice until they get 'scratchy' and the suggestion for your circuit of using temperature stable trimpots is right on. You will really have to dig deep in the spec sheets for that kind of data.

author: TimFox
author_rank: Super Contributor
timestamp: January 28, 2024, 03:21:23 pm
content: Wirewound trimpots are good for tempco, but have finite resolution as the wiper bumps from wire to wire. A good data sheet will specify the resolution, which is worse for lower resistance units that use fatter wire. Expensive units can use a conductive plastic over the wires that smooshes out the bumps, and reduces the noise caused by the motion when there is dc voltage across the winding. These are quantitative questions for the design.

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2024, 04:28:46 pm
content: Rather than trimpots for precision measurement like a ratio bridge, use a 10..20T Beckman Helipot, wire wound but continuous and have turns count dials available.https://www.beckman-foundation.org/about-foundation/inventions/helical-potentiometer-helipot/Jon

author: TimFox
author_rank: Super Contributor
timestamp: January 28, 2024, 08:56:49 pm
content: Wirewound Helipots(tm) still have finite resolution unless they have conductive plastic over the wires.The Bourns data sheets show this explicitly.Since the length of the 10-turn helix is much longer than the roughly 1 inch length of a trimpot, the resolution is much better with the helical device.

author: donlisms
author_rank: Frequent Contributor
timestamp: January 29, 2024, 09:11:58 am
content: My perspective is that the trimmer is going to be... icky, in various ways, so just accept that and design around it.If, for example, I put a 10k trimmer in parallel with a 100 ohm fixed resistor, the unpleasant characteristics of the trimmer are reduced by a factor of 100. That turns the icky thing into something wonderful. Temperature coefficient of 100ppm just became 1ppm. Well... yay! Noise, resolution of adjustability, all that stuff.Maybe it's not entirely a panacea, but it seems like a good place to start.

author: Gyro
author_rank: Super Contributor
timestamp: January 29, 2024, 10:35:11 am
content: Don't rule out using a high value 'select on test' fixed value resistor in parallel with the low value stable one. That's the way they do it in precision resistance standards. The best trimpot is no trimpot. It's also the reason why you sometimes see precision resistors that are marked slightly higher or lower value (series trim) than the integer value that you would expect.This thread may be helpful...https://www.eevblog.com/forum/metrology/fine-resistance-adjustment-for-standard-resistors/These threads too - lots of porn!...https://www.eevblog.com/forum/metrology/teardown-standard-resistors/https://www.eevblog.com/forum/testgear/kelvin-varley-reistor-porn/

author: tatel
author_rank: Frequent Contributor
timestamp: January 29, 2024, 01:29:35 pm
content: Well, thank you all for your answers.I'm attaching an spreadsheet with data about some 20 different trimpots. No carbon composition, I flew as soon I saw 300 ppm tempco.So, if you don't have deep pockets and have to go with cermet, tempco is 100 ppm. You get into wirewound territory as soon as you want less than that 100 ppm.Wirewound means 50 ppm tempco. To get 25-50 ppm tempco, you need metal foil. Probably Vishay. Perhaps Bourns' metal ones are out of stock. Fact is, no Bourns metal foil trimpot appeared on my Digikey searchs.Vishay has an extraordinarily hard face, speaking about 10 ppm tempco, but looking deeper, you find that it's 25-50 ppm through the wiper.Cermet means mostly all under €5; wirebound means all over €20, and metal foil means all over €35. Before taxes. To reach the best metal foil one, you need an X-wing fighter from Star Wars.A "TT Electronics" brand appeared, unknown to me. Doesn't look bad. Any comments about it?I'm worried about low current circuits, Bourns recommendations (see second attachment) say the wiper of a cermet could change its resistance if current is less than 50uA. If I'm getting it right, the current a multimeter puts out to measure resistance would be lower than that. Any comments?However, I'm almost decided to follow the cermet way, mostly for money reasons. Buy half a dozen of €40 trimpots, and you begin to be near the point where a second hand decade box makes sense. 100 ppm tempco it will be, even if resistors will be 25 or 50 tempco. That way at least most of the thing will be more stable, and one can always use it under controlled temperature. I'll try to make the adjustment range as little as possible, making it with as many fixed resistor as sensible. And will use the parallel resistor and other tricks.@jeanpaul. I'm going that way for the null-detector. To me, it makes sense to get a potentiometer with knob and display for that. Going for a cheaper one, though. Cheaper than a wirebound trimpot, BTW. And life is about 1000000 cycles, not 200. For a six decade thing, it's still expensive, and clumsy, I think. For that thing I want trimpots.Again, thank you all. If you have any other comments, please post.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 30, 2024, 04:22:07 pm
content: Avoid trimpots where ever possible. Tweak the pot, unsolder it and replace with fixed low tempco resistors.I hate them!

author: CaptDon
author_rank: Super Contributor
timestamp: January 30, 2024, 08:59:45 pm
content: Some of the worst trimpots I have ever had to deal with were in very expensive equipment!!! You guessed it, those white body with gray knob screwdriver adjust pots found in all of the Tektronix 500 series modules!!!! Touch them and they fall apart. So twitchy (V-Gain H-Gain Etc.) they are almost impossible to set correctly and then won't hold the setting even overnight. The quality we were used to with the old bench warmers (502 453 454) sure went to hell with the electrolytics in the 500 modules as well as the 'leak masters' in the TDS 500 and 600 series. I was a die-hard Tek guy but today I look elsewhere for upgrading my bench. B.T.W., I just bought the much hated Wavetek 3000 signal generator which took me 4 days to bring into perfect working order (why not? I am retired). An Ebay "Parts only, we don't know how to test it", which means "It's junk" or "We blew it up being to stupid to know how it works". This one had every adjustment way out of whack "Just turn every adjustment till it works" must have been the technician's thought process. It's all good now, even added a light to the meter which improved the sex appeal of this old dinosaur!!

author: MathWizard
author_rank: Super Contributor
timestamp: February 03, 2024, 11:17:33 am
content: I like to have lots of part values at home, and I'm very low on those little 10-turn cermet W3206 ?? types.But even those, from the cheap places like ebay, cost a fortune. And they are more set and forget, they wear out pretty quick. The daily use 10-turn pot's, their prices scare me away.

author: jonpaul
author_rank: Super Contributor
timestamp: February 03, 2024, 12:09:18 pm
content: Bonjour, in most Vintage TEK, HP, GenRad equipment circa 1970s..1990s, the cermet trimpost are Beckman, Clarostat, other old line USA brands.All are needed as the devices can have driftin semiconductors, passives etc over time.On suspected trimpot problems, We use control cleaner, Isopropyl Alchol 91..99% , apply with syringe, exercise , turn adjuster a few degreees L-R but RETURN trimpot to original position.In many decades have yet to replace a cermet 1 turn or multituen trimpot.Jon

author: tatel
author_rank: Frequent Contributor
timestamp: February 03, 2024, 05:47:44 pm
content: Quote from: jonpaul on February 03, 2024, 12:09:18 pmGood to know, thank you