How do Electronic Potentiometers work?

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.