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Very Odd potentiometers

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LAN_403

EEYORE

Sat Mar 07 2009, 01:09AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello all,
I just got some nice Bournes 10k potentiometers off of ebay. They look so very nice and turn so smoothly...Just one hitch. They are continuous? Instead of being a single turn or multiturn type, they just turn an unlimited amount of times which varies their resistance. So for example:
0 degrees: 0ohms. 10 degrees: 2kohms. They continue up to 10kohms and then immediately revert back to zero ohms and you can keep turning the knob and watching the resistance climb and then hit zero again. Whats with these? How is that useful for anything?

Ive never seen such a thing! Very odd! It makes it VERY hard to use when you want to go from 0-max and not accidentally go to zero again. (My application is a high current laser driver). It would be very bad to go up to say 35A and then very quickly go to zero again. I will have to see if this causes any nastiness for the lasers.

So, what are these used for?
Matt

rp181

Sat Mar 07 2009, 01:33AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Maybe the cycles are counted. I have opened one up, and its just a ring. Its exactly the same, just with no block.

aonomus

Sat Mar 07 2009, 01:41AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Perhaps it was supposed to be a single turn pot but someone worked it too hard and broke the internal stop?

Maybe a picture and model number would help

Proud Mary

Sat Mar 07 2009, 02:00AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Potentiometers with continuous mechanical rotation are widely used in instruments and process control circuits.

If you move away from the idea of volume control model of a potentiometer, and think of using it to search for something - a particular point or value for example - then the merits of continous mechanical rotation become obvious. Why would you want to turn it back in the direction from which you have just come?

Servo driven pots also often have continuous mechanical rotation.

Some pots have optional locks and stops so continuous rotation can be disabled if necessary.

If you go to the Bourns site, you'll find numerous examples of potetiometers with continuous mechancial rotation.

EEYORE

Sat Mar 07 2009, 07:18AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

I guess they could be made to work. I would have to set up a very accurate range selection to ensure that the maximum current will not exceed the laser diode's current. That ought to be easy enough...Now comes the question of whether or not the laser likes having its current potentially change as it may. (I guess it shouldnt really).

It would be easier to use other pots. but these are the nicest ive seen! So smooth and precise!

Bourns
6639S-1-103
10KÎ©, 1W, Â±15%
Continuous Turn
Precision Potentiometers

Matt

Ken M.

Thu Mar 19 2009, 05:37AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Long time no jib jab, as for purposes I'd place my bets on audi equipment like home stereos, I have a small son 3disc changer stereo and the volume knob just turns till i get bored of turning it, that my only thought. =P If your gonna use it on a laser I'd imagine u could make a stop for it around the max ohmage spot.

LithiumLord

Thu Mar 19 2009, 12:44PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Actually the continious potentiometers were originally the predecessors of the modern encoders, nowdays they are to be used in the applications where extensive accuracy is required so encoders are tough to be used. For your application you'd better lock off the "jumpy spot", therefore just place an arrow knob and put a screw through your front panel that would limit the rotation cycle

Steve Conner

Thu Mar 19 2009, 12:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yup, they are position feedback pots for servo systems. The stop is left off because if the servo drive went wrong, the motor would just smash it off anyway.

Also some systems rotate more than 360'. I have an old engine testing handbook that describes a test rig for valve timing investigations. It used a 360' pot attached to the camshaft, and the pot output used to provide a timebase on an analog scope that could be calibrated directly in degrees.

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