Idea: Trimmer Screwdriver & Reg. Potentiometer vs TrimPot - Page 1
Forum section: electronics / projects

author: Altair8800
author_rank: Regular Contributor
timestamp: December 20, 2023, 05:57:20 am
content: Just an idea...Wouldn't in some situations it be better to have aTrimmer Screwdriverand a regular potentiometer than a more expensive TrimPot (Trimmer Potentiometer).My idea of a Trimmer Screwdriver is a Screwdriver that either mechanically or electromechanically can do controllably very small turns. A regular screwdriver basically has a 1:1 ratio. You turn the handle one turn and it turns the head of the screwdriver one turn. But if you had a screwdriver that could have say a 10:1 or 100:1 (handle turn to head turn) then you can just use a less expensive regular potentionmer but have the fine adjustments of a TrimPot.It just seems a bit wastefull to have all that hardware of a TrimPot (case, wormgear, toothed worm wheel, maybe lubricant) to be used most likely only once at the factory to make some fine tune adjustments and never turned again.Does such a screwdriver exists? And I'm curious how accurate could a simpler regular potentiometer be made to behave like a dedicated trimpot?What your thoughts?Note: I can see some situations (like high vibration or heavy g or critical) where maybe a trimpot will still be better because it has a worm gear mechanism that prevents slipping thus locking the fixed resistance better.

author: moffy
author_rank: Super Contributor
timestamp: December 20, 2023, 06:06:09 am
content: Doesn't sound like such a bad idea, the problem with multiturn trimpots is backlash, notoriously difficult to change turning direction without a jump in value, at least the budget value pots. I seem to remember Bob Pease doing a test between multiturn and single turn pots, and with fine control the single turn pots had the same resolution as the multiturn.

author: Gyro
author_rank: Super Contributor
timestamp: December 20, 2023, 09:35:35 am
content: The devil is in the detail. It depends whether you can keep the mechanical parts, both in the 'screwdriver' and the single turn pot sufficiently resistant to normal knocks and vibration. This is a problem with Chinese fake Bournes multi-turn pots - the worm gear and internal wiper are poorly toleranced so they randomly change fine adjustment value with handling of the equipment (UT61E, I'm looking at you!).Itispossible, but you would need a high quality pot and gearing, probably not economical against the cost of a decent multi-turn pot.

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2023, 10:51:38 am
content: The problem is: making the multi-turn mechanism external and removable demands extreme precision for the interface between it and the pot body and rotor. Any angular slop in the interface is magnified by the reduction geartrain, giving a lot of backlash at the control knob. You are still going to need a case round the track to provide bearing surfaces for the rotor (as riveting a wiper to the track substrate is unlikely to provide good enough concentricity to provide comparable accuracy to a conventional multi-turn preset), and to provide anchor points to take the reaction torque from turning the rotor, so all you are eliminating is the little brass adjuster screw.https://www.robotroom.com/Trimpots-2.htmlHowever the ultimate in set once at the factory trimmers is the laser trimmable resistor. No housing, no moving parts, just a resistance layer on a ceramic substrate!

author: Gyro
author_rank: Super Contributor
timestamp: December 20, 2023, 11:37:46 am
content: The other thing is to keep the pot adjustment range to the absolute minimum needed to overcome circuit component tolerances. Use the lowest possible pot value and pad the ends with appropriate value fixed resistors.

author: donlisms
author_rank: Frequent Contributor
timestamp: December 20, 2023, 06:38:43 pm
content: I assume a practical resolution of about 1/100 per turn fir a typical pot or trimmer. The "steps" are not linear, of course. As has been mentioned, the ideal of infinite adjustability isn't realistic due the mechanicals.That said, I have some vintage Helipots that do close to ten times better than that. They're about 75mm in diameter. They're wonderful. And huge.I also have some Vishay foil trimmers that were extremely expensive when new that are much (much) better than all the others, in all respects. For 50x cost.So... my conclusion is it's possible to do better than 100 parts, but you have to really want it!For adjustment, I think it's MUCH easier to just add a radius arm of some kind to the screwdriver. Cheap, simple, do-it-yourself in a minute. Then you will likely find that the limit is the pot or trimmer after all.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 20, 2023, 07:01:49 pm
content: Your basic blue box with a screw head multi turn trimmer doesn't really have much more in it than a regular pot of similar size. I'd guess in terms of material/production cost increase the screw probably dominates, and it's just a little brass screw, cheap.

author: jpanhalt
author_rank: Super Contributor
timestamp: December 20, 2023, 07:15:13 pm
content: With any sort of gear mechanism and manual adjustment, you will need to secure the "gear box" to keep it from turning. You might consider a long spiral like used in the Yankee screwdriver, which can be done with one hand, but it requires considerable pressure on the screw being driven. That's an advantage when putting screws into wood, but probably a disadvantage when adjusting a pot.Have you considered a stepper motor driven screw driver blade? You could make single or fractional steps. A very small one as is used in some meters might have enough power.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 20, 2023, 07:41:44 pm
content: Quote from: Altair8800 on December 20, 2023, 05:57:20 amTrue, tho these days many production adjustments are entirely electronic. It is rare to see trimpots on anything made in volume.Quote from: Altair8800 on December 20, 2023, 05:57:20 amIt's not just mechanical wiper angle, you also need to consider the temperature coefficient, and cheap 'regular potentiometer' can be terrible there.An exposed wiper is also going to be less long term stable, than one inside a housing.There are also many modern alternatives :There are quite good Non volatile digital pots (1024 steps,1%), and TI and ADi have non volatile DACS, and of course MCUs have become very, very cheap with 12b ADC & PWM DAC on even 20c models.