Pot Taper (Linear / Logarithmic)
It is the characteristic that defines how the resistance change is distributed against the rotation angle of the potentiometer. In the linear type (code A or B, depending on the manufacturer) half rotation gives half the resistance; in the logarithmic type only about 10–20% of the total resistance is exposed in the first half of the rotation. In motor speed adjustment the linear type is preferred.
In speed adjustment the aim is for every degree of the lever to contribute equally to the speed. A linear taper provides this: 50% rotation means roughly 50% reference, and therefore roughly half speed. A logarithmic taper, on the other hand, is designed for volume control, and if used on a motor driver there is almost no movement in the first half of the lever, while the speed suddenly shoots up in the last quarter.
- Linear: It is the correct choice for the speed reference, ramp time and current threshold settings alike.
- Logarithmic: It spoils the feel of adjustment in a motor application; consider it only if a special distribution of sensitivity is wanted.
Trimmer pots often also come in multi-turn types (10 turns, 3600 degrees) instead of a single turn (270 degrees). Where fine adjustment within a narrow range is needed, such as the AntiSwitch current threshold, a multi-turn pot lets you hit the same physical precision ten times more easily.
Since the letter code on the pot body varies from manufacturer to manufacturer, if you are unsure, measure the resistance at half rotation with an ohmmeter and verify it.
Context where this term is used: DC Motor Driver Selection Guide