My friend Bill Schweber sent along information from BI Technologies about a multi-turn counting dial used with 10-turn potentiometers. I hadn’t thought about a 10-turn pot in a long time but found several–and their dials–in a parts bin. One has a label from the Borg Equipment Division of The George W. Borg Corporation marked with a resistance of 20000 ohms and a linearity tolerance of 0.25%. (The successor company, Borg General Controls now manufactures timers for HVAC and appliance vendors.)
The 10-turn potentiometer serves in circuits where designers need an accurate but variable resistance. By “stretching” the resistive element over 10 turns, the pot manufacturer can spread the resistance over a longer distance, which allows for finer resistance settings. In spite of a wide variety of digital circuits, some test equipment and measurement instruments still require accurate analog resistance settings.
The 10-turn pots required a 10-turn indicating knob rather than just a knob with a pointer. Otherwise you wouldn’t know which “turn” the pot was to. I have a mutli-turn dial that incorporates a counter window with its own concentric ring of digits from 0 to 14. Moving the main dial clockwise from 90 to 0 increments the count by one. Rotating the dial counter clockwise decrements the count as the dial goes from 0 to 90. A red mark on the main dial indicates the transition zone when an incremental count exists between the two digits shown in the window. A locking lever could hold a knob setting.
Photo courtesy of TT electronics plc.
By the way, you can buy 15-turn potentiometers, too, thus the 0-14 count on my knob unit and the one shown above.
I used several 10-turn pots in an electrochemical potentiostat during grad-school research. Based on measurements from experiments, I had to carefully set a working voltage to strip electrons from organic molecules. The pots worked well and I got good results. But after that experience, I can’t recall ever designing a circuit that needed such good linearity and such fine resistance settings. Have you designed equipment that required multi-turn potentiometers? –Jon Titus