Rotary Potentiometer

Summary

Introduction

The word potentiometer comes from potential, which refers to voltage potential, or difference. A potentiometer (sometimes called a pot) is a device for measuring voltage difference. In fact, a potentiometer is simply a resistor with a variable resistance value. Typically it is composed of a resistive material through which a current is passed. Unlike a static resistor, however, the point at which the current exits the resistive material is variable. Most often this is done by using a brush, which makes electrical contact with the resistive material, and which can be moved using a handle. By moving the brush, the voltage potential being measured can be modified.

The voltage of a potentiometer may not necessarily vary linearly. In other words, if you turn the knob at a constant speed, a linear potentiometer will respond with a voltage exactly proportional to your distance around the circular range. However, if the potentiometer has a logarithmic response curve, it will vary as a logarithmic proportion.

The reason for the existence of logarithmic potentiometers is because there are many variables, especially in the world of audio and psycho-acoustics, which are logarithmic by nature. For example, we hear “loudness” of audible sound in terms of decibels (dB). Decibels are a logarithmic value. Therefore, to hear an apparent linear change in loudness, we must adjust volume levels logarithmically. Every notch on the volume knob of your stereo system is actually an increase in exponential power. Another example is pitch, where every time you increase pitch by an octave, you are actually doubling the frequency. For controlling these types of variables, a logarithmic potentiometer should be used.

An alternative shape for the rotary potentiometer is the linear potentiometer (slider), where the word “linear”, in this case, refers to its physical shape of a straight line.

Variations

For audio applications, it can be useful to use the same value to modify two or more signals. For this purpose, it is possible to get stereo pots, which apply the same resistance to two seperate circuits.

Devices

Panasonic ECG EVL-HFAA06A53

Sources

Digikey.ca CDN$ 2.57

Description: 13mm. 5 KΩ
Datasheet: aok0000ce3.pdf
Resources:
Notes:
Variants:

Alternatives

Another method for reading rotary position is by using a rotary encoder. The main difference between a rotary potentiometer and an encoder is that the potentiometer can measure absolute position, while the encoder can measure only relative position. The advantage of a rotary encoder is that it can turn in the same direction indefinitely, while a potentiometer will typically only turn once around. Also, an encoder requires digital logic (hardware or software) to read position, while the potentiometer can be directly used in an analog circuit.

Media

Images

A Rotary Potentiometer

External links & references

sensors/rotary_potentiometer.txt · Last modified: 2009/01/26 11:23 by joe
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