# Potentiometer terms

Discussion in 'Electronic Basics' started by Ron J, Feb 6, 2006.

1. ### Ron JGuest

Hi all,

I was reading the datasheet for a potentiometer and came across this
listed feature:
"High resolution 25-turns enables precision adjustment easily."

Does this mean that there are only 25 available resistance?

Thanks!

2. ### Dan HollandsGuest

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

Dan

--
Dan Hollands
1120 S Creek Dr
Webster NY 14580
585-872-2606

www.QuickScoreRace.com

3. ### Ron JGuest

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.

4. ### John LarkinGuest

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

John

5. ### Don BoweyGuest

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

6. ### John FieldsGuest

---
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
10k pot with 300 degees between stops will have a sensitivity of:

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
0.333%.

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.

7. ### John LarkinGuest

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
*really* futz with it, by maybe as much as 2:1, but nothing like 25:1.

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

8. ### John FieldsGuest

---
I wasn't arguing the actual settability of the pots as much as I was
the difference between 'resolution' and 'sensitivity', and I agree
with you, generally, on the stability matter.

there's not much which is going to affect the pot but temperature,
and if it's being used as a voltage divider unto itself, it's
internals will be pretty nearly isothermal, so the ratio of the two
resistances on either side of the wiper will stay constant, as will
the putput voltage.

Especially if it's a _good_ pot, like some of the Vishays with a
tempco of 5PPM/C°

Don't get me wrong; I don't care much for pots, but they _do_ have
their place.

9. ### Pooh BearGuest

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

Graham

10. ### Bob MastaGuest

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

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

11. ### Ron JGuest

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

12. ### Rich GriseGuest

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

13. ### Rich GriseGuest

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

14. ### Rich GriseGuest

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

Cheers!
Rich

15. ### John LarkinGuest

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