Potentiometer Centre Terminal (Wiper)
The wiper is the centre terminal that slides along the resistive track of the potentiometer and outputs the voltage corresponding to its position. A reference voltage is applied to the end terminals, and the wiper reads the ratio between the two. The signal terminal is always the wiper; the two outer terminals are the supply and the reference ground.
The order of connection also matters: which of the two outer terminals is connected to the supply determines in which direction the value increases as the potentiometer is turned. If the direction comes out reversed, the outer terminals are swapped.
- Broken wiper: when contact at the centre terminal is lost the input floats and the reading jumps randomly. This shows up as sudden, unexplained movement at the actuator.
- Wear: contact degrades over time on the most-used section of the carbon track; the symptom is a jump in the reading at a particular angle.
- Contamination and moisture give the same symptom; that is why the potentiometer is located inside an enclosure and on the cabin side.
On the GS740-3P the yellow wire connects to the wiper and the green wire to the GND terminal of the potentiometer. If a channel does not work, this is the first measuring point: while the potentiometer is turned, check whether the voltage at the yellow terminal changes smoothly between 0 V and 3.3 V.
Context in which this term is used: Electronic speed and throttle control on diesel engines