ENGINEERING FIELD GUIDE
How Does a String Potentiometer Work? 3-Wire Wiring Guide
A retracting cable turns motion into a position signal. Understand the passive divider before connecting the DAQ.
THREE WIRES. ONE POSITION SIGNAL.
Passive voltage-divider wiring reference
A passive string potentiometer uses three electrical connections. The wiper voltage follows string travel as a proportion of the excitation voltage.
Ideal relation: Vout ≈ Vexc × x / L. At 50% travel and 5V excitation, an ideal zero-offset output is 2.5V.
Illustrative colors only. Confirm the exact model pinout, maximum excitation and power rating. Do not apply 24V unless the specified unit permits it. Conditioned voltage and two-wire current-loop models need their own wiring diagram.
What is a string pot?
A string pot is a string potentiometer: a measuring cable turns a spring-loaded spool coupled to a rotary potentiometer. The wiper moves along the resistive element as the cable extends. Cable return force maintains tension; it is not a load-bearing tether.
How many wires are used with a potentiometer type sensor?
A potentiometer used as a voltage divider needs three electrical connections: excitation, wiper signal and common. A cable shield may be an additional conductor. A four-pin connector does not, by itself, identify the output type or its pin assignments.
Excitation, power and loading
For a passive element, dissipation is approximately P = Vexc² / R. A 5kΩ element at 5V dissipates 0.005W; at 24V it dissipates about 0.115W. The safe limit comes from the exact element rating and temperature derating, not the resistance alone. Use a DAQ input impedance high enough to limit loading error.
Ratiometric does not mean regulated
A passive divider’s full-scale output tracks its excitation. Supplying 5V may give an approximately 0–5V span only within the specified electrical travel and endpoint limits. A regulated 0–10V output requires a suitable conditioned model. A two-wire 4–20mA loop also requires its own supply and load calculation.
Commissioning checklist
- Check the model drawing and terminal labels with the supply disconnected.
- Confirm working stroke, reserve travel, cable alignment and safe return speed.
- Apply the rated excitation and confirm common/reference connections.
- Set zero and span using known physical positions; compare an intermediate point.
- Check noise, sample rate and travel rate in the actual DAQ setup.
Display resolution and sensor performance
Conductive-plastic elements can provide a continuous analog signal. The useful measurement resolution still depends on noise, ADC resolution, mechanics, loading and calibration. Wirewound potentiometers use a different resistive construction; do not treat them as interchangeable without review.
Frequently asked
Can I use Arduino, NI CompactDAQ, MoTeC, AiM or Haltech?
Review the exact input module or ECU channel: excitation source, maximum input voltage, input impedance, analog reference and calibration. A brand name alone does not establish compatibility.
Can a string pot measure suspension damper movement directly?
Yes, a suitable installation can measure movement. Check the mounting geometry and motion ratio, avoid side loading, and review peak speed and retraction force. Damper travel and wheel travel may differ.
What affects string potentiometer price?
Stroke, housing, output conditioning, environmental requirements, connectors, calibration and quantity affect the quote. Compare the complete configuration and delivered scope.
Manufacturer reference documents
TE Connectivity SP2-50 datasheet and UniMeasure LX-PA datasheet illustrate why ratings and wiring must be checked by exact model. These documents do not certify an SPS configuration.
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