The Potentiometer

Following figure is a simplified drawing of the construction of a potentiometer, or variable resistor. A resistive strip, similar to that found on film-type fixed resistors, is bent into a nearly complete circle, and terminals are connected to either end. This forms a fixed resistance. To obtain the variable resistance, a sliding contact is attached to a rotatable shaft and bearing, and is connected to a third terminal. The resistance between the middle terminal and either of the end terminals can vary from zero up to the resistance of the whole strip.

A simplified functional drawing of a rotary potentiometer (A), and the schematic symbol (B).

Some potentiometers use a straight strip of resistive material, and the control moves up and down or from side to side. This type of variable resistor, called a slide potentiometer, is used in hi-fi audio graphic equalizers, as the volume controls in some hi-fi audio amplifiers, and in other applications when a linear scale is preferable to a circular scale. Potentiometers are manufactured to handle low levels of current, at low voltage.

Linear-Taper Potentiometer

One type of potentiometer uses a strip of resistive material whose density is constant all the way around. This results in a linear taper. The resistance between the center terminal and either end terminal changes at a steady rate as the control shaft is turned.

Suppose a linear-taper potentiometer has a value of zero to 280 Ù. In most units the shaft can be rotated through about 280°, or a little more than three-quarters of a circle. The resistance between the center and one end terminal will increase right along with the number of angular degrees that the shaft is turned. The resistance between the center and the other end terminal will be equal to 280 minus the number of degrees the shaft is turned. The resistance is a linear function of the angular shaft position.

Linear-taper potentiometers are commonly used in electronic test instruments and in various consumer electronic devices. Following figure is a graph of relative resistance versus relative angular shaft displacement for a linear-taper potentiometer.

Resistance as a function of angular displacement for a linear-taper potentiometer.

Audio-Taper Potentiometer

In some applications, linear taper potentiometers don’t work well. The volume control of a radio receiver or hi-fi audio amplifier is a good example. Humans perceive sound intensity according to the logarithm of the actual sound power. If you use a linear-taper potentiometer as the volume control for a radio or other sound system, the sound volume will vary too slowly in some parts of the control range, and too fast in other parts of the control range.

To compensate for the way in which people perceive sound level, an audio-taper potentiometer is used. In this device, the resistance between the center and end terminal increases as a nonlinear function of the angular shaft position. The device is sometimes called a logarithmic-taper potentiometer or log-taper potentiometer because the nonlinear function is logarithmic. This precisely compensates for the way the human ear-and-brain “machine” responds to sounds of variable intensity.

Audio-taper potentiometers are manufactured so that as you turn the shaft, the sound intensity seems to increase in a smooth, natural way. Following figure is a graph of relative resistance versus relative angular shaft displacement for an audio-taper potentiometer.


The Rheostat

A variable resistor can be made from a wirewound element, rather than a solid strip of material. This is called a rheostat. It can have either a rotary control or a sliding control. This depends on whether the resistive wire is wound around a donut-shaped form (toroid ) or a cylindrical form (solenoid ). Rheostats have inductance as well as resistance. They share the advantages and disadvantages of fixed wirewound resistors.

A rheostat is not continuously adjustable, as is a potentiometer. This is because the movable contact slides along from turn to turn of the wire coil. The smallest possible increment is the resistance in one turn of the coil.

Rheostats are used in high-voltage, high-power applications. A good example is in a variablevoltage power supply. This kind of supply uses a transformer that steps up the voltage from the 117-V utility mains, and diodes to change the ac to dc. The rheostat can be placed between the utility outlet and the transformer following figure. This results in a variable voltage at the power-supply output.

Connection of a rheostat in a variable voltage power supply.