Introduction

In this module, you will learn about precision potentiometers. Bourns offers a broad range of products, including single-turn and multiturn precision potentiometers. These products are designed for electromechanical position sensing in applications where accuracy is an important consideration.

The objective of this module is to provide you with basic information on the wide range of products. The basic information is intended to increase your confidence in our precision potentiometer products and improve your ability to obtain orders.

When you’ve completed this module:

  • You will feel more confident about discussing precision potentiometers and the various options
  • You will be able to help your customer select the Bourns precision potentiometer that will best fit the customer’s application

You will have a better understanding of:

  • The construction and function of a precision potentiometer
  • The different types of precision potentiometers
  • The market segments and applications

What is a Precision Potentiometer?

In simplest terms, a precision potentiometer is an electromechanical variable resistor typically used to sense change in position. It is typically used in a machine-to-machine interface (MMI) application and is thereby designed with high quality, durable materials for reliability.

Precision potentiometers are designed for:

  • High accuracy
  • Stability
  • Long rotational life
Figure 1 - Model 3500
Figure 1 - Model 3500

 

Precision potentiometers can have a wide price range, from about five dollars to several hundred dollars, depending on the level of performance accuracy, rotational life requirement, and any value-added options required for the application.

Top of Page

Precision Potentiometer Available Functions

A precision potentiometer is typically used as a voltage divider in a circuit. The variation in voltage output is used to sense change in position of the shaft providing electromechanical feedback to the system. A precision potentiometer provides a precise relationship between the shaft position and the electrical output.

Although precision potentiometers are typically used in MMI applications, they can also be used in human-to-machine interface (HMI) applications. In the case of HMI applications, they would typically be used in conjunction with a turns-counting dial.

There are five performance parameters that are specifically important for precision potentiometers:

  • Resolution
  • Output smoothness
  • Linearity
  • Noise (wirewound) or CRV (non-wirewound)
  • Rotational life
Figure 2 - Model 6657
Figure 2 - Model 6657

 

Top of Page

Product Construction - Multiturn Precision Potentiometers

The multiturn and single-turn precision potentiometers have the same basic assembly parts as a panel control:

  • Housing
  • Resistive element
  • Wiper
  • Terminals
  • Shaft

The following section will discuss the piece parts, assembly and features of multiturn potentiometers. Highlights of each model family will also be reviewed. Multiturn precision potentiometers are assembled with wirewound or hybrid (wirewound with a conductive polymer coating) elements.

The housing for a multiturn potentiometer has helical grooves inside to hold the element in place as shown in Figure 3. This aspect demands that the housing have a high degree of dimensional stability when subjected to temperature changes and moisture. The element is fixed in place by cementing the ends to the housing.

Figure 3 - Cutaway view of a wirewound multiturn potentiometer
Figure 3 - Cutaway view of a wirewound multiturn potentiometer

 

Multiturn precision potentiometers use two kinds of resistive elements:

  • Wirewound
  • Hybritron® (wirewound element with a conductive polymer coating)

In both cases, the resistance wire is wound on an insulated copper mandrel. The mandrel is then formed into a helix and cut to the proper diameter and number of turns (3, 5 or 10 turns).

In hybrid elements, a conductive polymer resistance material is deposited on the inside surface of the wirewound element, filling the space between the wires. The result is a smoother output and better resolution as the wiper moves across the element. This type of element is known as the Hybritron® element.

Hybritron® elements exhibit the temperature coefficient and resistance stability of a wirewound element, while displaying the long operational life and essentially infinite resolution typical of conductive polymer printed elements.

Typically, there are three terminals on each precision potentiometer. Two are attached to the ends of the resistive element and one is attached to the collector ring, which translates the position of the wiper along the resistive element. The collector ring is at the back of the housing. The shaft-rotor assembly consists of the shaft, the collector bar/contact fingers, the slider block, the rotor block and the wiper.

A voltage is applied across the element (TR) terminals with the wiper terminal functioning as a variable voltage tap. Turning the shaft rotates the rotor block assembly. The wiper and guides track along the helical coiled element as the shaft rotates, moving the slider block along the rotor block. As the wiper contact moves along the resistive element, voltage is tapped off of the element and transmitted to the collector bar. The voltage is passed through the contact fingers to the collector ring and out through the wiper terminal. Figure 4 shows the assembly view of a 10-turn wirewound potentiometer and identifies the major parts.

Figure 4 - 10-turn wirewound potentiometer assembly view
Figure 4 - 10-turn wirewound
potentiometer assembly view

Typically, precision potentiometers are mounted onto the end product in either of two methods depending on the application.

Figure 5 - Potentiometer with servo mount lid
Figure 5 - Potentiometer with servo mount lid

 

Servo mount is used in applications where the potentiometer shaft is coupled to a motor, or other mechanically driven applications. Figure 5 illustrates a servo mount type potentiometer.

Figure 6 - Potentiometer with bushing
Figure 6 - Potentiometer with bushing

Bushing mount is used in applications where the potentiometer shaft is mounted through a front panel or a bracket and is to be adjusted manually or with a gear at low RPMs. Figure 6 illustrates a bushing mount type potentiometer.

Figure 7 - Potentiometer with bushing
Figure 7 - Potentiometer with PCB mounting features

Printed circuit mount is a third and less commonly used mounting method. With this mounting method, the potentiometer is directly mounted on the PCB and soldered in place by the solder terminals. Model 3590 offers printed circuit terminals and housing features to stabilize the potentiometer and facilitate direct mounting on a PCB.

How To Order

Each individual product data sheet will have a "How To Order" table to assist you in selecting a part number. Consider the following illustration:

How to order
Click for larger image

The part number selected in the boxes indicates a 3549 10-turn wirewound, dual cup, no anti-rotation lug, nickel plated 1/4 ” diameter shaft with 3/8 ” diameter bushing, 10 kΩ resistance value on both elements, and 0.20 % independent linearity.

Product Construction – Single-Turn Precision Potentiometers

A single-turn precision potentiometer has a similar construction to a panel control. Typical features which differentiate a precision potentiometer include:

  • Wipers which are generally fabricated with precious metal
  • Ball bearings for better ergonomic feel and longer life
  • Single-turn precision potentiometers are assembled with conductive polymer elements

Most single-turn precision potentiometers are continuous rotation, although a mechanical stop is available as a special order. The housing for a single-turn precision potentiometer can be fabricated from machined metal or molded plastic. Plastic housings are lower in cost when compared to metal housings. Metal housings may give the product a more rugged package, however, either housing will give the end-user a highly reliable product.

Single-turn precision resistive elements are fabricated using thickfilm silkscreen technology. The resistive ink may be conductive polymer or cermet (a combination of fine ceramic or glass particles with precious metals). Conductive polymer ink is used to achieve long rotational life. The termination print is achieved with low resistance metallic inks. A typical resistive element is illustrated in Figure 8.

Figure 8 - Single-turn resistive element
Figure 8 - Single-turn resistive element

 

The outer terminals are connected to the termination print pads by one of several methods. The most common methods are solder or swage type joints.

Turret style terminals may be utilized in single-turn precision models. Plastic molded housings typically have the terminals molded into the rear of the housing. Metal housings typically have the terminals mounted through the sidewall of the housing. Some models with a metal housing will have a rear plastic cover which doubles as the element. In this case, the terminals will be molded into the cover.

The shaft-rotor assembly for single-turn thick film element models is simpler as compared to the wirewound models. Models with a plastic shaft normally have a single piece molded shaft/rotor, whereas, models with a metal shaft will have the shaft swaged or molded to a plastic rotor. A contact spring is mounted to the rotor, which makes mechanical and electrical contact with the resistive element.

A voltage is applied across the element (TR) terminals with the wiper terminal functioning as a variable voltage tap.

Turning the shaft rotates the rotor/contact spring assembly. The contact spring makes contact with the element and moves along the circular printed pattern producing a change in resistance. As the wiper contact moves along the resistive element, voltage is tapped off of the element and transmitted through the contact fingers to the collector ring and out through the wiper terminal.

Single-turn precision potentiometers are continuous-turn devices. There are options available in each of these models for a mechanical stop should it be required by your customer's application. Figure 9 illustrates an assembly view of a single-turn precision potentiometer.

Figure 9 - Assembly view of 
single-turn precision potentiometer
Figure 9 - Assembly view of
single-turn precision potentiometer
Features & Benefits

Some of the features and benefits of precision potentiometers are listed in Table 1.

Features Benefits
• Bushing mount
• Servo mount
• PCB mount
Flexibility in mounting options for MMI applications
• Conductive polymer element
• Wirewound or Hybritron® element
Flexibility to meet performance requirements
• Turrets
• Solder lug
• PCB mount
Terminal options
• Sizes from 1/2 " to 2 " Products to accommodate open space and minimum space requirements
• Single Turn
•Multiturn: 3, 5, 10 turns
Suitability to the application
• Multi-ganging option (some models) Provides multiple pots with a single control
• Metal and plastic housings Durability vs. economical options
• Ball bearing option (some models) Great for applications with side load
• 1 watt to 5 watts power rating Covers a broad range of applications
Rotational Life:
• Single Turn – 1M to 25M cycles
• Multiturn – 300k to 5M shaft revolutions
Provides long life and reliability
• Seal to IP65 (Model 3590) Provides protection from moisture ingression
Table 1: Precision potentiometer features and benefits
Modification Capabilities and Value-Added Solutions

A wide range of modifications and value-added enhancements are available for many precision potentiometer models. Bourns' capability to provide custom solutions include:

  • Modifications
    • Customized shafts and bushings
    • Voltage/current taps
    • Custom linearity
    • Torque
    • Mechanical stops
    • Clutch
    • Special electrical angles
    • Custom marking
    • Terminal configurations
  • Value-added solutions
    • Mounting brackets
    • Cable harness/lead wires
    • Connectors
    • Special packaging
    • Additional hardware
    • Special test requirements
Figure 10 - Example of a modification
Figure 10 - Example of a modification
Figure 11 - Example of a value-added solution
Figure 11 - Example of a value-added solution

For information regarding modifications or value-added solutions, please contact Bourns Customer Service.

Market Segments and Applications

Panel control potentiometers are used in many market segments including:

  • Test and measurement equipment
  • Medical equipment
  • Industrial and factory automation
  • Aerospace/avionics
  • Telecommunications
  • Military

Common applications for precision potentiometers include:

  • Diagnostic equipment
  • Patient monitoring equipment
  • Blood/chemical analyzers
  • Dialysis equipment
  • Emergency medical equipment
  • EKG and EEG monitors
  • Neonatal incubators
  • Patient mobility equipment
  • Respirators
  • Physical therapy equipment
  • Surgical equipment
  • Insulin/infusion pumps
  • Motorized hospital beds
  • Motorized dental chairs
  • Radio/flight controls
  • Cockpit and passenger controls
  • Wing/flap sensors
  • Landing gear sensors
  • Motorized airplane seats
  • Climate controls
  • Electrostatic testers
  • Power generators
  • Scanners
  • Calibration equipment
  • High voltage testers
  • Robotic/motor controls
  • Environmental chambers
  • Satellite positioning systems
Competition

Precision potentiometer competitors include:

Useful Terms

Cermet
A mixture of metal particles, precious metal oxides, glass powders, and a liquid vehicle, screened onto a substrate (commonly ceramic) and fired at temperatures that flow glass.

Conductive Polymer
A binder or resin carrying carbon powder and/or other conductive material screened onto an insulating substrate.

Contact Resistance Variation (CRV)
The variation in resistance between the wiper and a non-wirewound resistive element when the wiper is energized with a specified current and moved over the electrical travel in either direction at a specified speed (does not include the effects of roll-on or roll-off).

Cycle (Potentiometer)
One traversal of the wiper over at least 90 % of the electrical travel in both directions.

End Resistance
The resistance measured between the wiper terminal and end terminal with the shaft positioned at the corresponding end of mechanical travel.

Equivalent Noise Resistance (ENR)
The variation in resistance between the wiper and a wirewound resistive element when the wiper is energized with a specified current and moved over the electrical travel in either direction at a specified speed (does not include the effects of roll-on or roll-off).

Hybritron®
A hybrid element consisting of a wirewound element with a conductive polymer coating; this element will exhibit the stability characteristics of a wirewound element with the long operational life of a CP element.

Independent Linearity
The maximum deviation of the actual function characteristic from a straight reference line with its slope and position chosen to minimize deviations.

Load
The ability of a component to dissipate rated power for a specified length of time under specific operating conditions while remaining within allowable specifications.

Absolute Minimum resistance
The resistance measured between the wiper terminal and any other terminal with the shaft positioned to give the minimum value.

Noise
Any spurious variation in the electrical output not present in the input.

Output Smoothness
A measurement of any instantaneous variation in the electrical output not present in the input.

Potentiometer
A variable resistive device used in applications that typically require frequent or even constant adjustment.

Resolution
A measure of the sensitivity to which the output ratio of the potentiometer may be set.

Power Rating
The maximum power that a component can dissipate under specified conditions while meeting specified performance requirements.

Resolution
A measure of the sensitivity to which the output ratio of the potentiometer may be set.

Rheostat
A variable resistor configured so that all the current flows through the wiper.

Roll-on/Roll-off
The abrupt output voltage or resistance change observed as the shaft is displaced from the end of mechanical travel into the electrical travel.

Rotational Life
The ability of a component to withstand a specified number of cycles under specific operating conditions while remaining within allowable specifications.

Solderability
The ability of the terminals to be wetted by a coating of solder under specified conditions.

Tap
An electrical connection fixed to an intermediate point on the resistive element.

Taper
The output curve of resistance measured between one end of the element and the wiper.

Temperature Coefficient of Resistance (TC)
The unit of change in resistance per °C °temperature change from a reference temperature to a specified test temperature, expressed in PPM/°C.

Total Resistance (TR)
The DC resistance between the end terminals with the shaft positioned to give a maximum resistance value.

Voltage Divider
A potentiometer configured in a circuit such that the resistive element is substituted for two resistors in series with the wiper providing an adjustable output voltage.

Wirewound
A resistive element made from multiple turns of wire around an insulated mandrel or cord.

Industry Standards & References

Applicable industry standards and references include:

  • VRCI-P-200 Industry Standard For Wirewound And Non-Wirewound Precision Potentiometers
  • Bourns Potentiometer Handbook (Bourns website Library section)