Potentiometer
It is a three-terminal adjustable resistive element; by dividing the voltage applied across its two ends it produces an analogue reference value at the middle terminal for the driver. In DC motor drivers, settings such as speed, ramp time and current threshold are typically given with trimmer pots of 5–10 kΩ. The SPEED and RMP pots on the PT500 serve this purpose.
The two end legs of the potentiometer are connected to the driver's reference supply line (typically 5 V) and to chassis; the middle leg gives a voltage varying between 0 V and the supply according to the wiper position. The driver reads this voltage with an analogue-to-digital converter and turns it into an output ratio.
- On-board trimmer: It is for service settings that are adjusted once and left; on the PT500 the A1, A2, HIZ and RMP pots are of this type.
- Panel pot: It is the speed setting the operator uses continuously; on the KS250 it can be connected to the IN5 analogue input.
There are two limits in choosing the value: a pot of very low resistance (below 1 kΩ) draws needless current from the reference source, while a pot of very high resistance (above 50 kΩ) gives an unstable reading because of the input's own leakage current and interference. In practice 5–10 kΩ is a good balance.
If the pot cable is to be run a long distance, keep it at least 50 mm away from the motor cable and preferably use shielded cable; otherwise the speed reference jitters with the switching noise.
Context where this term is used: DC Motor Driver Selection Guide