Potentiometer terms

No - it means you have to turn the shaft 25 complete revolutions to go from one end of the pot to the other.

Dan

Hi Dan,

Thanks! Ahh.. okay. So a 25 turn compared to a single turn translates to a higher resistance resolution, right?

I thought it was 25 resistance values because the price was cheaper than a single turn.

Not really. Most multi-turn trimpots have enough backlash in the drive that their resolution is no better than a single-turn pot... just a lot more tedious to adjust.

John

Go get him Phil. You can't let him get away with this misinformation.

--- Resolution refers to the smallest increment of resistance achievable in the pot, and has nothing to do with backlash. Of course, if you go too far and then have to go even farther forward to back up to whre you need to be, it's inconvenient, but the resolution of the pot is still the the resolution of the pot,. A wirewound pot,for example, will have a resolution depending on the pitch of the helix the resistance wire is wound to, while the resistive element of a cermet pot will have essentially infinitesimal resolution. Whether that resolution can be achieved will be largely determined by the 'stiction' of the drive, but there will be no fixed resolution limit as exists in a normal wirewound pot.

I think what the OP is referring to is the _sensitivity_ of the pot, which can be described as the total resistance of the pot divided by the total angle of rotation of the shaft. That is, a single turn

Rt 10kR S = ------- = ------ = 33.33 ohms per degree theta 300°

While a 25 turn 10k pot will have a sensitivity of

Rt 10kR S = ------- = ------- = 1.11 ohm per degree theta 9000°

Which means that if the best you can do, by hand, is a one degree crank, with a 25 turn pot (with a resolution great enough to allow it) you'll be able to resolve 1 ohm out of 10k, which is 0.0111%, while with a single turn pot with 33.3 ohms out of 10k, that's

So, for the same angle of rotation, the 25 turn pot will have a sensitivity 30 times greater than that of the single turn pot.

-- John Fields Professional Circuit Designer

Your calculations are impeccable. But the pots don't work that way. I just tried a couple of Bourns trimpots, single and 20-turn. Settability was similar; both have hysteresis and both have stiction. Both are fairly easily set to 0.2%, and to 0.1% with some reasonable teasing. The 20t is actually a bit better if you're willing to

But no circuit should require a trimpot to be set to 0.1%. Trimpots aren't stable enough to justify setting them this close.

John

Multi-turn pots aren't normally cheaper than single turn ! Surplus parts ?

Graham

Maybe he's comparing a cheap multi-turn trimpot with a single-turn panel-mount pot?

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Thanks for the reply everyone!

I'm not sure about Surplus, but I saw it on Digikey.

No, it means that you have to turn the knob 25 whole 360 degree turns to get from one end of its range to the other. You need a special, gear-driven, turns-counting knob to use it effectively.

Hope This Helps! Rich

Oh! A TRIMPOT! In my other answer, I thought you were talking about a PRECISION pot! What this is sounding like - is it about 3mm by 6mm by about 20 mm long, with a screw head at one end? In that case, the screw head is actually connected to a lead screw, that moves the wiper from one end of the element to the other, like a FDD indexing leadscrew.

Hope This Helps! Rich

Maybe not, but it sure is fun adjusting them while watching your output on a spectrum analyzer! ;-)

Cheers! Rich

Bob Pease (ever the voice of moderation) in his "Analog Troubleshooting" book, claims that multiturn pots are worse longterm than single-turns, by a factor of two or three, something about storing stress and jumping when exposed to shock. He suggests 0.2% as a good working resolution for trimpots.

But a 60-cent eeprom can replace 500 pots.

John