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Can a trim-pot wiper or contact point gets oxidized over time? For example when a potential (high) current is flowing and the contact point of the wiper on the resistive element heats up a little. I don't mean hot, just a little warm because of resistance. The effect takes years before it will be noticable.

For example this trim-pot, it is connected to a laser (CD-player).

Trim-pot example

After years of working properly, most of the time when the wiper is moved, just a fraction, the laser will work again. Maybe it works again because more current is flowing and boost the laser diode (above limits), however could it be possible that previous position was oxidized (by heat and vaporization of something - maybe resistive element, carbon, vaporised by heat), the resistance increased (a thin film is formed on contact point) and because of this the laser is reduced in power. So the laser is not in bad shape, the pot is in bad shape and caused the problem.


A couple of months ago I have taken apart some equipment and on the lid I discovered some black dust/dirt (?) positioned above elements that heating up when operating. I don't know where the black dust came from or what it is however it is caused by components that produces heat.


So I wonder if this is possible inside a trim-pot when it produces some heat, does the contamination (vaporization of something) cause malfunction/changes the value of the pot. Is this possible?

This is all curiosity, it is nice to know.

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  • \$\begingroup\$ for circuits that last for decades and decades, you use sealed-pots. Or design to avoid such adjustments, such as binary-weighted resistors for trimming. \$\endgroup\$ – analogsystemsrf Aug 15 at 6:06
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    \$\begingroup\$ @analogsystemsrf : Thanks for the comment however you answered the "how to avoid the problem" instead of the subject of the question. \$\endgroup\$ – Codebeat Aug 15 at 6:45
  • \$\begingroup\$ In the fullness of time everything oxidizes. \$\endgroup\$ – Andy aka Aug 15 at 7:24
  • \$\begingroup\$ not only oxidises, but sulphides as well (with city pollution). Neither are good. \$\endgroup\$ – Neil_UK Aug 15 at 7:52
  • \$\begingroup\$ some "contacts" use rare-earths, such as rhodium, to delay the oxidation. Can mankind build a spaceship to cross between solar systems (6 lightyears at the closest) without contact-oxidation, which means NOTHING WORKS after 10,000 years of traveling? \$\endgroup\$ – analogsystemsrf Aug 15 at 17:46
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Yes.

From "BEST of the TRIMMER PRIMERS" by Bourns:

DRY CIRCUIT CONDITIONS

AS FAR AS POTENTIOMETERS GO, DRY CIRCUIT CONDITIONS RESULT FROM EXTREMELY LOW VALUES OF WIPER CURRENT. PAST STUDIES SHOW THAT UNDER EXTENDED TIME AND TEMPERATURE CONDITIONS, OXIDE FILMS CAN FORM AT THE JUNCTION OF METALLIC ELECTRICAL CONTACTS, SUCH AS THE POINT OF CONTACT BETWEEN MECHANICAL SWITCHING ELEMENTS. USUALLY THIS DOESN’T CAUSE A PROBLEM IN TRIMMER APPLICATIONS, SINCE NORMAL OPERATING CURRENT LEVELS PRODUCE ENOUGH "PUNCH THROUGH" VOLTAGE AT THE WIPER JUNCTION TO BREAK DOWN OXIDE OR CONTAMINANT FILMS.

The best ways to avoid or remedy oxidation in a trim pot:

  • Mechanical cleansing. The wipers are typically designed to scrape away oxidation when the pot is adjusted. (But fat lot of good that does here, where the pot doesn't get adjusted.)
  • Run a small amount of current through the pot. I've commonly heard this called wetting current for relays, and I think the same term applies here since it works well with the "dry circuit" terminology above. But the same Bourns reference does recommend this too:

HOWEVER, WE STILL RECOMMEND THAT YOU PROVIDE A LOW ENOUGH LOAD RESISTANCE ACROSS THE WIPER OF THE POTENTIOMETER TO INSURE AN ABSOLUTE MINIMUM WIPER CURRENT OF 25 MICROAMPS AND PREFERABLY OVER 100 MICROAMPS. THIS SHOULD GIVE YOU ENOUGH CURRENT THROUGH THE WIPER TO HELP YOU AVOID DRY CIRCUIT PROBLEMS OVER EXTENDED PERIODS OF TIME AND TEMPERATURE.

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  • \$\begingroup\$ Hi thanks for the nice answer! Wow, the document is a long read.I think the first part of your answer explains it all however the wetting current article is a very good read. As far I understand, oxidation is possible especially when there is no current flowing (for example device is turned off most of the time) and the environment conditions are not optimal, high humidity for example or smoke etc. So when a device is not frequently used (in bad environment conditions), oxidation take place, it could be possible it cannot rectify the situation because of a bad state (no current can flow).... \$\endgroup\$ – Codebeat Aug 16 at 2:53
  • \$\begingroup\$ ... That means a device can start to malfunction even when it is not frequently used. This has nothing to do with aging, this has something to do with oxidation (caused by environment condition). So it could be possible (refer to my question) that the trim-pot was oxidized and the laser was still in great shape. That explains that moving the wiper a fraction solve the problem. Nice conclusion, nice to know, thanks! \$\endgroup\$ – Codebeat Aug 16 at 2:53
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Oxidation happens with both heat and moisture. The heating likely comes from the diode itself. But really, it’s just age and time taking their toll on a vulnerable component (exposed wiper and all that.) For example, old audio gear will usually need its scratchy pots cleaned or replaced.

And, yes, you might be tweaking the diode bias current just a smidge and compensating for its age. Maybe more folks should know how to do that when their old CD and DVD players start to act up.

The pot itself modifies the photodiode feedback to set the laser diode bias. So the pot itself isn’t carrying a high laser current.

(Full disclosure: in a past life I designed these things. Not the pickups, but whole DVD and Video CD players.)

Anyway, good on you for keeping a piece of gear going like that and keeping it out of landfill, if only for a bit longer before the laser finally shuffles off its mortal coil. That’s a rare thing these days.

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  • \$\begingroup\$ Hi Thanks for the answer however it is a little offtopic because it is not about the laser (could be also something else) but about the trim-pot, Anyway, don't want to repeat the question here. It is true that I try to repair a lot of stuff, not only to learn from it but also to try to extend the life. You are right people throw many things away without give it a second change these days, or just because it is old or obsolete by software. I pick up many devices from the dumpster and many things are still usable or need just a little fix. Frequently I found one of the latest players ...... \$\endgroup\$ – Codebeat Aug 16 at 2:11
  • \$\begingroup\$ .... blu-ray drives all with almost the same problem: the drive. On newer models there is almost nothing you can do because there seems no trim-pot on the laser unit. Some models have drive bay problems, mostly Philips. Hard to fix because the design is pretty bad, made or designed to fail on some day. \$\endgroup\$ – Codebeat Aug 16 at 2:11

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