Nowadays, most electronic gadgets change their settings such as volume, bass, treble, brightness, contrast, sharpness etc., through “up/down” or “+/-” buttons in contrast to the rotary mechanical controls earlier. These are the electronic or digital potentiometers in action.

While the principle of operation remains the same whether it is a mechanical or an electronic potentiometer, the functionality of the two is quite different. While the mechanical potentiometer offers a continuous variation because it is an analog device, the electronic version is a digital type, offering discrete variations. The difference is due to the way the voltage dividing functionality is implemented in the two versions. As opposed to a single resistance in a mechanical potentiometer, the electronic version has several resistors, also called the resistor ladder network, switched into the circuit with multiple switches.

The voltage at the center terminal of the electronic potentiometer, therefore, depends on the resistance presently connected to the circuit with one of the switches. At first glance, it would seem to be a simple matter to increase the number of resistors and switches to reduce the granularity. However, electronic or digital switching requires each switch to have a unique digital address, usually in the binary format, defined by bits (0 or 1). Higher number of switches means larger number of bits, which increases the complexity of the electronic circuit controlling the switches. The switches, instead of being individual mechanical types, are usually semiconductor types and are integrated into a circuit along with their controller and the resistor ladder network.

This results in a neat little package, which can be soldered in-circuit and requires no panel or knob for its control. Usually, the control is via a micro-controller, which reads the increase or decrease instruction from the UP/DOWN buttons on the remote. It then generates a suitable binary word and sends it to the specific electronic potentiometer chip controlling the volume or any other function you want to change. Depending on the binary word, a specific switch turns on, connecting the required resistance to the central terminal of the potentiometer.

Just like their analog counterpart, electronic or digital potentiometers are also available in a variety of tapers such as linear, log, reverse-log, etc. Taper defines how the wiper voltage changes as it is moved from one end of the potentiometer to the other. For a linear taper potentiometer, both analog and digital, the wiper voltage bears a linear relationship to its physical position. For a potentiometer following the logarithmic or the reverse logarithmic taper law, the wiper voltage is nonlinear.

Since an electronic potentiometer is implemented in a chip, manufacturers adopt different ways of achieving the same results. While resistor ladder network is the most popular, the ladder implementation itself may differ for achieving the desired taper. Some manufacturers use operational amplifiers as buffers before or after the network, some use an anti-thump circuit to reduce the plop noise that is heard when one switch opens and another closes.

One major difference between the analog and the digital versions of potentiometers is the power handling capacity. Typically, the digital versions, in chip form, are unable to handle more than a few mill watts. In comparison, analog potentiometers capable of handling a few watts are quite common.

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.