A very common misconception students have about potentiometers is the relationship between resistance and direction of wiper motion. For instance, it is common to hear a student say something like this, “Turning the potentiometer so the wiper moves up will increase the resistance of the potentiometer.”
Explain why it does not really make sense to say something like this.
Moving a potentiometer wiper changes two resistances in complementary directions: one resistance will increase as the other will decrease.
Ask your students to identify which resistance (which two connection points on the potentiometer) increases and which decreases, and how they know this from the “internal views” of the potentiometers. This is a very important thing for your students to learn.
Year after year of teaching has revealed that a great many students have difficulty grasping this concept. This is especially true when they become accustomed to using a potentiometer as a rheostat and not as a voltage divider. The more you can make them think carefully about the operation of a potentiometer, the better!
Potentiometers are manufactured in two different “tapers”: linear and audio. Linear taper potentiometers provide a direct, linear relationship between wiper position and resistance division, so that equal changes in wiper position result in equal changes in resistance. Audio taper potentiometers provide a non-linear (logarithmic, to be exact) relationship between wiper position and resistance division, so that the same amount of wiper motion at one end of its range gives a much greater change in resistance than at the other end of its range.
Suppose you have a potentiometer, but do not know whether it has a linear or audio taper. How could you determine this, using a meter?
A linear taper potentiometer will exhibit resistance measurements between the wiper and the other two terminals, proportional to the wiper position.
Discuss with your students the purpose of an audio taper potentiometer: to provide logarithmically proportioned increases in audio power for volume control applications. This is necessary for a “proportional” response when turning the volume knob on an audio amplifier, since human hearing is not linear, but logarithmic in its detection of loudness. In order to generate a sound that the human ear perceives as twice as loud, ten times as much sound power is necessary.
A challenging question to ask your students is which way an audio taper potentiometer should be connected as a voltage divider in an audio amplifier circuit. Being that audio taper potentiometers are non-symmetrical, it truly matters which way they are connected!
Published under the terms and conditions of the Creative Commons Attribution License
In Partnership with PEI-Genesis
by Jake Hertz
by Robert Keim
by Jake Hertz