Electronic gadgets of about one generation back (prior to the prolific use of SMD), used rotary mechanical potentiometers for setting different parameters such as volume, tone, brightness, contrast, etc. For adjusting circuit parameters within the gadget, a smaller variation called the trim pot was a common sight. These are outdated now, but those who still own and use these gadgets often wonder how mechanical potentiometers function.
The most common example of a mechanical potentiometer is the rotary volume (also tone, bass, midrange and treble) control in an audio amplifier. Unless your amplifier has a remote control, chances are that for increasing or decreasing the loudness, you turn a knob labeled as “Volume” control. When you want to reduce the sound output from the speakers, you rotate the knob counterclockwise. The sound output increasing when the knob is turned clockwise. On the remote control, the knob is replaced by two buttons, one marked “+” for increasing and another marked “–“ for decreasing the sound output. Another example is the fan speed control, used mostly prior to the electronic versions.
Therefore, a mechanical potentiometer is a device to control a gadget’s complete range of operation. In its most common use, a potentiometer acts as a voltage divider. This three terminal device has a central pin that allows the resistance to be varied. This is called the wiper – named for the way it makes the mechanical connection to the fixed resistor between the other two terminals. If you connect the wiper electrically to any one of the other terminals, you transform it into a variable resistor or rheostat.
That the device acts like a voltage divider is easily verified from the schematic. If the voltage across the fixed resistor is the difference of voltages Va and Vb, the wiper will show a voltage Vw anywhere between Va and Vb depending on its position. Mathematically, (Va ~ Vb) = (Va~Vw) + (Vw-Vb). The device serves to cancel out tolerances of other components, when used in-circuit as a trim pot; thus allowing the required voltage setting to be achieved.
In construction, a mechanical potentiometer has a fixed resistor in the form of a circular track, stopping just short of a full circle (usually about 270-degrees). The track is printed on a ceramic or a glass-fiber base. The two ends of the resistance form terminals that may either be PCB solderable or suitable for wire termination. Depending on the requirement, the resistance may be of any value between a few ohms to several hundreds of ohms. The entire arrangement is encased in a housing, which also serves to accommodate the wiper assembly.
The wiper assembly consists of a phosphor-bronze spring that lightly bridges the surface of the resistance track and a circular metal track that forms the third terminal. The spring and track arrangement is attached to a metal shaft, but electrically isolated from it. The spring turns as you rotate the shaft, allowing the wiper to change its voltage.
The trim pot has a similar arrangement, lacking only the metal shaft for rotating the wiper. Instead, the phosphor-bronze spring doubles as the shaft, suitable for adjustment with a trimmer screwdriver. Potentiometers may also be of multiple turns to increase their resolution.