Multi turn Pot failures - Page 1
Forum section: electronics / projects

author: xavier60
author_rank: Super Contributor
timestamp: January 13, 2020, 11:02:51 pm
content: I'm having trouble with the cheap 3590S 10K Pots from ebay sellers. After some use, the resistance element goes open circuit.This can be bad when the Pots are used in a bench power supply as the voltage set point will jump from 0V to full reference voltage(5V) as the wiper passes over the break. Three have failed during the last 6 months.Has anyone had the same type of failure?I'm now using "BOCHEN" branded WXD3-12 10K Pots, hoping that they are better.

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2020, 12:15:42 am
content: I have never seen one of these fail.Also note, that if the PSU goes berserk when the pot is open circuit, then I'd say the designer of that circuit shall think twice if his design is correct. Pots failing open is I'd say quite common type of failure mode for them. Even a "scratchy" and worn potentiometer may cause spikes in outptut voltage, that may damage the connected load.Always think when designing with any pots or trimpots about making the design fail safe when the trim/pot goes open circuit.

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2020, 12:58:54 am
content: Quote from: Yansi on January 14, 2020, 12:15:42 amYes, after the first Pot failed, I add protection to the micro-controller board,https://www.eevblog.com/forum/projects/linear-lab-power-supply/msg2373084/#msg2373084The CC setting Pot failed last. It happened over night while the PSU was on and sitting idle. The break occurred at the high end of the range which I rarely use.

author: Yansi
author_rank: Super Contributor
timestamp: January 14, 2020, 01:23:50 am
content: QuoteThat unfortunately is not the correct fix to the problem; what would be a correct step is to change the pot role in the circuit in such way, that the open circuit wiper terminal automatically puts the output voltage to zero (or any controlled variable to zero, otherwise safe state). For example to wire the potentiometer in a different place of the feedback loop that he impact of it going open circuit gets the opposite effect.Note that in some circuits this may not be possible, which indicates such topology is not fit for the purpose and different one shall be chosen.And btw, cool PSU build, I like when people have the fine skill to work with some nice metal, instead of just the dumb 3Dprinter everything.

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2020, 01:49:22 am
content: I'm using them as potentiometers, dividing the 5V reference rather than as VRs. I already had a bleed resistor from the CV Pot's wiper to ground in case the wiper lost contact with the resistive element.But this precaution doesn't help when the resistive element goes open. When I dismantle a failed Pot, I find a break in the resistance wire that makes up the compound helix resistive element.The failed Pot acts like a switch. When the wiper is below the break, 0V is selected. When the wiper is moved above the break, the full 5V reference is selected.The hair thin resistance wire is surprisingly strong.I'm not going to abandon the design, if not for the failing Pots, it works perfectly.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 14, 2020, 05:15:25 pm
content: You're buying junk electronic components on eBay. Use Bournes (or a couple other quality vendors) from an authorized distributor and you won't have that problem. If you have space, you can do a resistor network that you adjust by clipping out resistors. Cheap, highly reliable and won't change with vibration.

author: Le_Bassiste
author_rank: Frequent Contributor
timestamp: January 15, 2020, 02:18:49 pm
content: Quote from: Conrad Hoffman on January 14, 2020, 05:15:25 pm+1

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: January 15, 2020, 04:03:50 pm
content: There is a good reason military loves and uses them over conductive plastic or ceramic.Leo

author: bd139
author_rank: Super Contributor
timestamp: January 15, 2020, 05:04:31 pm
content: They're damn expensive though. I have about 6 here used for various RF tuning things and they were £15 a pop from RSAnother option is use two cheaper single turn pots (Alpha etc) for separate coarse and fine controls. This is actually more convenient from experience for voltage or current setting. Getting from 0 to Vmax on a 10 turn is laborious when you have to do it several times a day

author: jbb
author_rank: Super Contributor
timestamp: January 15, 2020, 07:33:40 pm
content: Coarse and fine pots work OK, but they can be very annoying when you want to slowly sweep the voltage. I often start at one voltage, twist the fine pot all the way over, don't reach the target and have to (very carefully!) adjust the coarse knob.

author: bd139
author_rank: Super Contributor
timestamp: January 15, 2020, 07:36:24 pm
content: Use both hands

author: jmelson
author_rank: Super Contributor
timestamp: January 15, 2020, 09:11:13 pm
content: Quote from: xavier60 on January 13, 2020, 11:02:51 pmWe have gear with lots of 10 turn trimpots and some panel pots that have been used (and abused) for DECADES! Generally, any failed pots show signs of mechanical abuse from misaligned screwdrivers, etc. These are all US-made name-brand units.Jon

author: BravoV
author_rank: Super Contributor
timestamp: January 16, 2020, 03:37:15 am
content: Quote from: jmelson on January 15, 2020, 09:11:13 pmYep, example, a 10K 15w 15 turns and 0.02% linearity.

author: John B
author_rank: Frequent Contributor
timestamp: January 16, 2020, 05:13:31 am
content: The genuine 3590S aren't that expensive, especially when purchased from somewhere like LCSC. However you're limited to only a few values, like 5k or 10k. I guess it's about supply/demand - the 3590 are the most common ones, some of the special size/shape/values can get crazy expensive.The ebay ones are fake/counterfeit. The turning mechanism has a totally different feel ie harder to turn, more binding.

author: xavier60
author_rank: Super Contributor
timestamp: January 16, 2020, 05:38:18 am
content: Quote from: John B on January 16, 2020, 05:13:31 amLCSC's photo's show signs of the Pots being genuine. Amongst other observations, I think I can see the "C" stamped on the 3rd pin which none of my ebay Pots have.I have also noticed tight spots. Maybe this leads to the broken resistance wire.The "BOCHEN" branded WXD3 series POTs are also very cheap so are they genuine or a counterfeit of something else? They do feel very nice.And this. Bochen also make 3590S series Pots,https://www.alibaba.com/product-detail/RoHS-BOCHEN-3590S-2-103L-10K_60238488487.html

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 16, 2020, 03:01:28 pm
content: I had problems with some of 1k..10k 3590S 10-turn potentiometers too. Have bought em at about $2.5-3.0 at local shops. It's like some virus eat them.

author: xavier60
author_rank: Super Contributor
timestamp: January 16, 2020, 09:47:56 pm
content: Quote from: Vovk_Z on January 16, 2020, 03:01:28 pmHow did they fail?

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 17, 2020, 12:00:12 am
content: They start to "jump" in resistance (for 10-20%) at some point.

author: H713
author_rank: Regular Contributor
timestamp: January 17, 2020, 07:38:59 am
content: I've had good luck withused10 turn pots. I'm not brave enough to buy the knock-offs. The genuine helipots are really nice. I will say, I think I've only replaced one 10 turn pot.My main bench supply (made with modified Power-One Linears) uses a pair of concentric wirewound Clarostat pots for fine and course adjustment. They're the ducks guts but not the easiest to find.If you have resistive tracks going open (you mentioned having a carbon track go open?!) take a look at how much current you're putting through the pot. They aren't rated for all that much and I don't recommend pushing them.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: February 13, 2021, 03:39:12 am
content: Even Bourns can have bad batches, I suppose. Just replaced a bunch of 3590S on several power supplies that were about 10 years old and hardly used. The wiper was loosing contact almost everywhere on the track. The power supplies were from a reputable US company, so I assume they had genuine parts.

author: sleemanj
author_rank: Super Contributor
timestamp: February 13, 2021, 05:37:00 am
content: I use the wxd3 ones in a few things of my own for years, and sell them, I have not had a problem with them other than some scratchyness on the oldest one, and none of my customers have ever reported an issue either, I expect I might have sold a hundred give or take over the years.

author: xavier60
author_rank: Super Contributor
timestamp: February 13, 2021, 05:51:10 am
content: Quote from: sleemanj on February 13, 2021, 05:37:00 amThat's good to hear. The WXD3 Pots in my bench supply have so far survived a year of heavy use.A few drops of PAO oil permanently fixes scratchy Pots. But nothing will fix the broken resistance wire problem that I was having with the knock off "Bourns" Pots.

author: jonpaul
author_rank: Super Contributor
timestamp: February 13, 2021, 10:25:06 am
content: Hello: Used 10T posts from Beckman and Bourns since 1970s,The wire slide elements get oxidized, they can be opened and lubed with contact cleaner, DeOxit D-100, etc.The dials fail mechanically and can be repaired.The Chinese clones are junk, as always poorly made to appear the same but every material and component is the lowest quality.Suggest genuine used Bournes or Beckmann, I find them on ebay and at Ham Radio fleas."You get what you pay for" "Buyer Beware"DONT BUY CHINESE JUNK AND EXPECT PROFESSIONAL QUALITYKind Regards,Jon

author: wizard69
author_rank: Super Contributor
timestamp: February 14, 2021, 01:35:31 am
content: This may sound hilarious but it always pays to read the manufacture technical docs. For example some trimmers have surprisingly low usage figures. I'm talking 5 or maybe 20 rotations.Back in the day we use to send a bunch of A&B servo drives back for repair. It was common for the drivers to come back with all new trimmers. Even more conventional potentiometers have life spans (the number of turns) that vary dramatically.By the way, buy name brand if you expect high performance. Your still need to read the manual though.

author: David Hess
author_rank: Super Contributor
timestamp: February 14, 2021, 03:56:39 am
content: Quote from: xavier60 on January 13, 2020, 11:02:51 pmThat was a common problem even 40 years ago with "off brand" multi-turn wirewound potentiometers. Either design the circuit to be tolerant of that failure or buy better parts like from Bourns.

author: floobydust
author_rank: Super Contributor
timestamp: February 14, 2021, 04:05:05 am
content: Do an autopsy on one of the failed pots, take it apart.

author: xavier60
author_rank: Super Contributor
timestamp: February 14, 2021, 04:52:04 am
content: Quote from: floobydust on February 14, 2021, 04:05:05 amI did. The helical wound wire that is the resistance element, gets broken.https://epci.eu/wp-content/uploads/2019/10/Figure-R4-31-Cutaway-view-of-a-multi-turn-bushing-mount-potentiometer.jpgThe Pot then functions as a switch, outputting either 0V of full reference source, 5V.

author: maxwell3e10
author_rank: Frequent Contributor
timestamp: February 14, 2021, 04:55:57 am
content: Quote from: David Hess on February 14, 2021, 03:56:39 amYea, that's why I was surprised a bunch of (what I believe were genuine) Bourns turned bad. Luckily the supply was designed properly, so no damage done.

author: xavier60
author_rank: Super Contributor
timestamp: February 14, 2021, 04:59:34 am
content: As well as the WXD3 series which have proven reliable so far, Bochen also make the a 3590,http://www.bochen-cn.com/3590.htmlMaybe they are good quality?

author: floobydust
author_rank: Super Contributor
timestamp: February 14, 2021, 07:46:27 pm
content: What is breaking the wire? Just weird. Is it just trashy quality or corrosion or maybe you have an overload damaging it (but rated 2W). The wire is NiCr I thought.Most wire metallurgy from china is pretty bad, lots of impurities and lumpy diameter.This might be the reason the Bourns are $20. Those fail from old age oxidation so I have taken them apart and cleaned them.A shorted LM317 will smoke the potentiometer so I add a fuse/PTC after realizing how expensive a quality multi-turn pot is, much more than the IC.

author: Pawelr98
author_rank: Regular Contributor
timestamp: February 15, 2021, 04:49:17 am
content: For one, doing a slight redesign of a control loop to stop the voltage from shooting up is a good start.With simple feedback I just connect a pullup resistor, if the potentiometer breaks, then the voltage at the feedback input goes high (higher than usual feedback voltage) and the output goes to 0V.And for my designs that is a basic thing, because I often use two pots in series to get that rough/precise adjustment.Those 20$ multiturn pots are more expensive than the devices that I build, so I just buy old military stock stuff, that I know is fairly high quality.Mainly older US-made, USSR-made and other non-chinese things.In fact often the prices are lower than the chinese things.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 15, 2021, 05:51:33 am
content: It were times when milty-turn 'Bourns' (?) cost 2 eur and worked fine. Now we have the same 2 eur pots which fail after a short time, and there are $10-$15 ones which I don't know how reliable are (because it seems too costly for me).

author: xavier60
author_rank: Super Contributor
timestamp: March 13, 2025, 12:17:41 am
content: The Bochen WXD3-13-2W 10K voltage adj pot eventually failed in my bench PSU after 5 years of heavy use.The insulation on the wiper fly-lead cracked in 2 places, leading to the wire strands breaking.For now, I have replaced the wire with 28AWG Teflon coated hook-up wire which Im hoping wont crack.

author: donlisms
author_rank: Frequent Contributor
timestamp: March 13, 2025, 10:38:57 am
content: I'm wondering if the resistance wire is melted, rather than broken.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: March 13, 2025, 01:04:48 pm
content: Quote from: donlisms on March 13, 2025, 10:38:57 amA measurement of the current going through the pot(s) has to be measured before anymore'guesses' are made. A 10k ohm potentiometer rated at 2 watts, the maximum current it can handle is around 44 milliamps (0.044 amps).

author: xavier60
author_rank: Super Contributor
timestamp: March 13, 2025, 11:01:09 pm
content: Mine has 5V across it,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3582664/#msg3582664The repaired Bochen pot is working fine so far.The wiper wire breaking is actually a safe failure. It would cause a properly designed PSU to go to 0V output.OTH, broken resistance wire can cause the output to go to full voltage. The actual PSU has protection for this by detecting abrupt increases in wiper voltage.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: March 14, 2025, 01:05:41 pm
content: Quote from: xavier60 on March 13, 2025, 11:01:09 pm

author: xavier60
author_rank: Super Contributor
timestamp: March 14, 2025, 02:02:31 pm
content: No. Look at the linked schematic. The potentiometers are configured as variable potential dividers, not as variable resistors.

author: TimFox
author_rank: Super Contributor
timestamp: March 14, 2025, 04:24:38 pm
content: Quote from: xavier60 on March 14, 2025, 02:02:31 pmA three-terminal voltage divider is apotentiometer.A two-terminal variable resistor is arheostat.Obviously, a potentiometer (pot) can be connected as a rheostat.This is not pedantry, but careful technical language to avoid putting 5 V across 2 k\$\Omega\$ by accident, to get 2.5 mA, not 50 mA.Math is hard.

author: Zoli
author_rank: Frequent Contributor
timestamp: March 15, 2025, 06:16:06 am
content: Quote from: BILLPOD on March 13, 2025, 01:04:48 pmIf you run 44mA over a 10kΩ potentiometer rated for 2W, it will end up in smoke, since you've dissipating ~20W over(P=I2R=0.0442*10,000=0.001936*10,000=19.36W).And to get 44mA over 10kΩ, you will need 440V.I agree with TimFox: math is hard , but physics is harder