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What are Potentiometers (and What do they do)?

What are Potentiometers (and What do they do)?

Potentiometer Definitions

Potentiometers  are: “Potential – Meters”

Monitoring the “potential” across a resistor

Potentiometer Definitions

Potentiometers  are used in two key areas;

1- Position sensing; providing the feedback of actual position of device into a control system 

2- Control device  providing the ‘demand’ signal for a control system .

Potentiometer Terms and Definitions

Potentiometers as position sensors- converting motion (rotary or liner movement) into an electrical output

PCR  Printed Circuit Resistor

PCB -Printed Circuit Board (typically FR4)

Voltage Divider – the Ratiometric property of all potentiometers divides the applied (input) voltage

Conductive Plastic / CP /  Resistive ink / ink – a blend of carbon with epoxy to create a hard wearable material that has conductive properties.  

Example:  a passenger car interior ventilation system would have a control device on the instrument panel (knob) where one controls the vents position.  A second position sensor would be located on the actual vent to provide true position of the vent into a closed loop motion control system.

Collector Track- a conductive trace used to ‘collect’ the signal off of the resistor track.

Contact –a conductive ‘link’ that moves across the resistor track and collector track.  Typically a precision stamping fabricated  in precious metal.  The contact  connects the resistor track to the collector, creating the signals 

Contacting technology – a reference to the feature that the contact  is touching the conductive plastic surface and moves across this conductive surface.  Contacting technologies are considered mature in comparison to…

Non Contacting technology where the sensing element does not contact the target while generating an output, typically Hall effect, Optical or Magneto Resistive

Contacting technologies are very low cost requiring only a PCB with conductive ink, a rotor / slider and a contact.

Potentiometers are “absolute” vs.  Incremental sensors

Absolute position sensors know their true position all the time, even with power interruptions.  When power comes back on, they ‘wake’ with know position

vs.

Incremental position devices (typically encoders) that ‘increment’ from one known position to the next, basically counting pulses to determine position to one full rotation.  Incremental sensors lose their true position at power off.

Linearity & Hysteresis / When does Linearity and Hysteresis matter?

Where the application needs to define exact positions (discrete settings) along the potentiometers travel.

Not necessary in Trimmers as they are used in applications requiring quantitive adjustment (eg Up or Down; exactly where is irrelevant)

In Control applications this is normally associated with the use of either internal or external detents to feedback a click confirmation of position to the user.

It is always important in Sensor applications.

The 3 types of linearity are:

Relative: Where the linearity is measured from the beginning of the resistive element to the end.

Absolute: This is the most demanding of the linearities It takes in to account the associated mechanical characteristics, eg the trueness of the resistive element mounting.

Independent: Similar to Relative but only includes a certain portion (specified by the customer) of the resistive element, eg. ±1.5 % through 180º

Potentiometer Circuit configurations

As a variable resistor

Varies the current flowing from A to B the majority flowing through the wiper

As a potential divider

Taps off a percentage of the applied voltage. The current from A to B is more or less constant the majority flowing through the resistor NOT the wiper

Potentiometers

used as a CONTROL (man/machine [user] interface) convert physical movement in to electrical information.

Actuators

do the reverse; they convert electrical information in to physical movement. They employ potentiometers as POSITION SENSORS

 

Two-way potentiometer.
Lift potentiometer.
Boost potentiometer.
Hydraulic Lift Potentiometer.