Potentiometers Overview

A potentiometer takes an input voltage and outputs a variable amount of it to a circuit, which is determined by the position of a slider on a resistive element. It is also known by the names variable resistor and rheostat, with these names being dependent upon the usage. The potentiometer can be used as a voltage regulator, as described above, a means of introducing a variable amount of resistance into a circuit or as a means of adjusting the power in a circuit. The last case, it is filling the role of a rheostat.

Other names that are used for these devices include “pot” or, more specifically, slider pot, trimmer pot or thumbwheel pot. These terms describe the mechanical action that alters the resistance, power or voltage flowing from the device, with a slide pot being controlled by a slider, a thumb mot being turned by a wheel and a trimmer pot not being easily adjustable and intended to be set to a given value and to stay there the majority of the time.

The internal structure of a potentiometer is simple, but very effective for what the device does.

 

Inside a Potentiometer

A potentiometer has three leads that protrude from the device, along with an adjusting mechanism, as described above.

Inside the device, the two outermost terminals are connected to the part of the device that serves as the resistor. It is usually a stop of material such as plastic, carbon, graphite or a more specialized ceramic material called cermet. This part of the potentiometer is sometimes known as the track of the device, but this term is not very frequently used.

The third terminal is connected to a part called the wiper. The wiper makes contact with the resistive strip. By moving the thumbwheel, slider or screw on a trimmer, the wiper is moved across the resistive element, which allows the voltage across the device to be altered.

The divided voltage is produced at the third terminal. If the voltage across the device is 16V and the potentiometer is turned all the way to the positive contact, the third contact will output the full 16V potential. If the potentiometer is turned 50% of the way between the positive and negative leads, the third terminal will have a potential of 8V. Turning the potentiometer so that it is aligned with the negative terminal results in a third-lead potential of 0V. These figures depend upon the potentiometer having a linear taper, which is not the case with all potentiometers.

In the case of trimmer pots, the adjustability of the potentiometer is usually only designed to be utilized during construction or adjustment. A small screwdriver is typically used to move the slider along the track, avoiding accidental adjustment during use.

There are potentiometer designs that can vary voltage over a series of full terms. In some cases, the restive element in these potentiometers may be arranged in a helix shape to accommodate this. These are used in applications where the adjustments need to be very precise.

There are other applications for these devices, such as polarity shifters. In a polarity shifter, the potentiometer is attached to two different sources of DC power. The positive and negative polarities are attached +/- on one side of the potentiometer and -/+ on the other. When the potentiometer is turned to midway along the strip, zero voltage is delivered over the third terminal.

 

Rheostats

Rheostat is a term that is falling out of usage, but that is common enough that it is still encountered. A rheostat is used to alter power, but it is so similar to a potentiometer that the term potentiometer can be used to describe these devices, as well, and often is.

Rheostats are commonly seen in applications such as dimming lights or lowering the speed of variable motors. They are two-terminal variable resistors, though a standard three-terminal potentiometer component is commonly employed in applications where the power level is below 1W.

Rheostats have more specialized designs in applications where they have to regulate a significant amount of power. In such cases, they may have a wire-wrapped insulator at their core. When the adjustment device is turned, the slider is moved across the wires, making contact with different turns as the adjustor is turned.

There are other designs, as well, such as devices where the slider is actually made up of several different fingers that contact multiple strands of wire. These are used in very high power applications.

 

Digipots/Digital Potentiometers

Digital potentiometers can be used in place of the common analog potentiometers and have advantages for certain applications. These devices can be adjusted using digital input rather than by manually turning a slider. The devices share a feature in common with computer memory: volatility. Some digital potentiometers are volatile, which means that, when they are depowered, the device will go back to a specific position, usually its minimum position. Other digital potentiometers are non-volatile and, if depowered, they will return to the state to which they were last adjusted before the power was removed.

These are commonly used for adjusting equipment, where precise values are involved. The digital potentiometers also have good resistance against shock and vibration, which may alter the slider position of an analog potentiometer.

An analog potentiometer can be adjusted by anyone with access to it. Conversely, a digital potentiometer can be secured so that it cannot be adjusted by anyone except those who have access to the equipment and knowledge to program the device.

 

Membrane Potentiometers

Membrane potentiometers are very precise devices that are used in touch screen devices and in many other applications. The potentiometer uses a membrane made out of materials such as Kapton, FR4 and PET. The device adjusts when the membrane is deformed by the slider. An ideal device would have an infinite resolution.

Despite their advanced capacities, membrane potentiometers are actually simple devices that have very long life cycles. In a touch screen application, for instance, a conductive coating underneath the glass and a resistive coating beneath that provide the main function of the device. As a finger is drawn across the screen, the voltage on the top layer and the voltage at the edges are used to provide the coordinate information, which is processed through an ADC to provide the data for the device.

These potentiometers are simple, but do need to be adjusted from time to time to ensure proper function. Many touch screen devices are capacitive devices, which do not require that the user apply pressure to the screen and which require no adjustment to give consistently accurate position information.

 

What Are Potentiometers Used For?

The most visible application of potentiometers is as volume controls on audio equipment. They are also used widely in light dimmers, though very high-power lighting arrays use them less frequently than was once the case.

Potentiometers are typically used to regulate power of less than 1W. Digital controls are very commonly used today in applications where potentiometers were once the norm. In radio equipment, for example, volume knobs were very common until the 1990s, when they started to be replaced with digital push-button devices that move the volume up or down a fixed amount became more common. Rotary versions of digital controls are also used, providing the convenience of a thumb knob with the inexpensive and reliable nature of digital components.

In audio equipment, these devices can be used for applications other than adjusting the volume. They may be found serving the roles of adjusting frequency, loudness, the power sent to different speakers and other properties important to audio devices.

Potentiometers are also used as motion control devices. This is common when working with motors, allowing properties such as the speed to be adjusted quickly and easily by changing the power to the device.

In their roles as trimmers, they are used to adjust the electrical properties of many different devices, including oscillators and other circuitry. The device is set to the desired value when the equipment is manufactured. It can be adjusted by a tech if needed, but is usually not touched by the user once the value is set. A fixative may be used to set the component at the desired value permanently once it is adjusted.

A potentiometer can also be used to measure voltages. In these applications, the voltage to be measured is applied to the input of the potentiometer, which is set at a known value. The resistance is then changed so that it is equal to a standard cell voltage. This allows standard algebra to be used to calculate the unknown voltage using the resistance value of the potentiometer and the standard cell voltage.

 

Choosing Potentiometers

Like all electronic components, potentiometers come in specialized designs intended for specific applications and in designs that can be used under operating conditions that are not particularly taxing. For example, in industrial applications where the equipment that the potentiometer is used with may get very hot, there are models available that can tolerate temperatures up to 150°C. Durable components such as these may be necessary for some builds.

Potentiometers can also be chosen based on the number of turns that they offer. Simple volume controls may require a potentiometer with only one turn, while equipment that requires very precise adjustment many require the models that have 5, 10 or even 15 turns.

The mounting type, shaft diameter and termination style have to be taken into account so that specific builds can be accommodated. These specs have no influence on the electrical properties of the device, but are necessary to ensure that the device can be easily accessed or, in the case of trimmers, so that it cannot be easily accessed and adjusted, preventing accidental damage to equipment or danger to personnel.

The style of potentiometer’s adjustment type also has to be considered. In some applications, this may merely be a design consideration. A volume control for audio equipment, for instance, can work well in a slider or thumb pot design. For other applications, however, a specific design may increase the operability of a device. A motor speed control, for instance, may best be constructed with a thumb wheel with a larger knob that offers very precise tactile feedback as to the amount of turns the device has been rotated, making such a choice more appropriate than a slider for some applications.

 

Other Considerations

Potentiometers can have linear or log tapers. With a linear taper, the voltage changes at a constant ration relative though the rotation of the control. A potentiometer can also have a log taper, in which case the device changes potential logarithmically. These are commonly used on modern audio equipment, owing to the fact that human perception of volume does not move literally, making linear controls sometimes seem to produce uneven gains in volume across the rotation of the dial.

Some potentiometers also have a switch integrated with them, which allows a device to be completely depowered or turned on and then adjusted via the same potentiometer being adjusted.

 

Replacing Potentiometers

Potentiometers have very long lifecycles but, with wear and tear, they will eventually need to be replaced. There are enough standardized sizes of these devices that replacing them generally isn’t an issue, but there are problems that builders have to take into account.

The potentiometer should be as close as possible to an exact match for the original. In some cases, the potentiometer may not need to be replaced, but the knob might wear out, the slider might not function smoothly or a mounting nut might be lost. These can usually be replaced cheaply without swapping out the entire device.

If the potentiometer is operating at high wattages, overheating can be an issue. It’s imperative that the proper heat tolerances are chosen for these devices, which requires that the voltage drop and the current are both calculated accurately. When a rheostat is being used, the current level will increase as the resistance decreases, and this has to be taken into account, as well. The current and voltage drop can be used to calculate the wattage that will be dissipated from the device.

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