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I have a usage case where I would like a potentiometer for controlling the charge going to a lead-acid battery via a IC. It would be susceptible to some (although not extreme) vibration, such people carrying it around and various bumps.

It would for most part be a set-and-forget thing hidden in a case that nobody will think of, unless the battery type was changed. I don't want the resistance to go shifting over time, it should ideally remain reasonably stable within a few percent.

How can I evaluate if a potentiometer is stable enough to use for this purpose?

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    \$\begingroup\$ Why would you not use Loctite? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jun 16, 2015 at 13:55
  • \$\begingroup\$ @IgnacioVazquez-Abrams good idea, but a little concerned I would need change the value at some point in the future. Could unsolder and replace it but that seems like a pain... \$\endgroup\$
    – MikeFoxtrot
    Jun 16, 2015 at 13:56

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Don't worry about it. If trimpots shifted around a few percent with minor vibration they'd never be used. Just about any trimpot from a reputable maker (and quite a few from disreputable ones) will be fine.

Minimize the range of the pot (don't require it to be set to 0.1%) and try to use it as a voltage divider rather than a rheostat. If you must use it as a rheostat, keep the setting away from the very ends of the range and try to use a relatively high value (avoid 10 ohm cermet for example, if you can) so that CRV (contact resistance variation) is not much of a factor.

But a few percent stability is not a high bar.

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    \$\begingroup\$ Am I allowed to add the wonders of anti-tamper laquer in the unlikely event the potentiometer is a bit too free moving after-all? It's not really glue, but it adds a decent amount of extra friction. I think I needed that about once in a thousand. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 14:54
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    \$\begingroup\$ The main reason for lacquer is to detect the presence of "screwdriver drift" and thus invalidate the calibration warranty. ;-) \$\endgroup\$
    – Spehro Pefhany
    Jun 16, 2015 at 15:06
  • \$\begingroup\$ I know what others use it for, I'm just saying that experimentally in mobile media designs I found the alternative use to be very valid as well. Though on an antenna tuning cap, but same difference. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 15:12
  • \$\begingroup\$ @Asmyldof Was it the kind of cap that squeezes down when you tighten the screw? Those make me nervous. \$\endgroup\$
    – Spehro Pefhany
    Jun 16, 2015 at 15:25
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    \$\begingroup\$ No, a spinning half-plate one. I like the squeezy ones. Allows for all kinds of ridiculous servicing fees when some nitwit turns them too tight. :-D We don't have many of those here, from what I can tell, though. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 16:18
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Solder female header to your board instead of the potentiometer. Then, if you're lazy, just stick a fixed-value resistor in the female header. If you're slick, make a series of small breakout boards with the resistor you need soldered on and silkscreen text that says what battery type and/or voltage it should be used for (i.e. - 12V lead-acid, 7.2V LiPO, etc.).

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    \$\begingroup\$ I'd avoid standard headers in a vibrating environment unless there's also screws involved (lessons you learn the first weeks of designing for a mobile media company in younger years). The idea in principle is a decent one though, especially if different types with their resistance values can be anticipated, as it leaves out the need for tuning pots later. I've always favoured plug&play options over "let me get the multimeter" ones. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 14:57
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    \$\begingroup\$ Good point. If the vibration is such that you would think that it would shake components apart like that (or even loosen the connection), add a hole to the breakout board and screw it to a standoff installed on the main board. Something like this, where the bottom plate is the main power board. \$\endgroup\$
    – Chuck
    Jun 16, 2015 at 15:02
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    \$\begingroup\$ Exactly, although I often prefer balanced snap-in if it's board-to-board. I just wanted the "idea" to dangle on your answer, since the plan for a plug-in board is a good one, but if you worry about potentiometer drift from vibration it's definitely strong enough to shake loose a 3-pin header connection. In fact, in a normal size truck I know a 20pin ribbon connector of "cheap" quality can shake loose in several weeks to 2 months. \$\endgroup\$
    – Asmyldof
    Jun 16, 2015 at 15:10
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How can I evaluate if a potentiometer is stable enough to use for this purpose?

This is what data sheets are for. Evaluate the data sheet first.

If you take one physical sample of a potentiometer and test it, you may get a very favorable result. This might lead you to believe that this pot (or that pot) works really well and is stable in end-to-end resistance, ratiometrically pretty good and generally it makes you happy and confident.

Then, you take a look at the data sheet and it's a lot worse than what you thought.

So, don't bother testing until you have read the data sheet then, you might realize that you don't need to physically test it - just buy a pot (from a reputable supplier) that does the job that you need.

Would you test every component individually you fix to a circuit board?

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