Linear and Logarithmic Potentiometers
Potentiometers are rarely used to control large amounts of power. Mostly, they are used to control up to around a watt of power.
Test it out
Tap the diagram for more information and click on the buttons to change a component and see what happens!
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Example calculations
In order to work with Linear and Logarithmic Potentiometers, electrical engineers need to understand and work with a variety of calculations. Here are just a few:
Ohms Law:
$V = IR$
Voltage Output:
$V_{OUT} = V_{IN}(\frac{R_2}{R_1 + R_2})$
Potential Gradient of Potentiometer Wire:
$K = (\frac{\varepsilon}{R + r})\frac{r}{L}\ \ therefore \ \ V = (\frac{\varepsilon}{R + r})\frac{rl}{L}$
 Voltage Supply Terminal $V_{CC}$ Wiper Terminal $W$ Ground Terminal $GND$ Voltage $V$ Voltage Input $V_{IN}$ Voltage Output $V_OUT$ Current $I$ Resistor $R$ Length of Wire $L$
A potentiometer is a type of variable resistor with a movable contact terminal that allows it to divide resistance and voltage, and transfer different amounts to different parts of the circuit. They are often used in light and volume control as dimmer switches for lights, brightness and contrast controls on TVs and faders for audio equipment.
Linear vs Logarithmic Potentiometers
Change in resistance across the resistance track is determined by the type of potentiometer used. They can be linear or logarithmic. Both types of potentiometers will adjust and control voltage and output by controlling resistance, but their physical positions on the dial or resistance track have different approaches and outcomes. The way the resistance changes is called the taper, or law.
With a linear potentiometer, the change in resistance taper is consistent across the length of the resistance track. This means that if you were to put the dial at the half way point, you would get half the output. It also means that no matter where you started the dial from, if you were to move it up or down by a certain amount, you would get a consistent change in resistance.
Logarithmic potentiometers are set up differently. This type of taper has a built in bias, and the resistance track is not consistent the whole way through. For example, at the lower end of the dial, the resistance might be slower to change, while at the top of the range, the resistance might change quickly.
This means that if you were to turn the knob from 2 to 4 on a volume scale for a radio, you would experience less of a change in resistance and volume than you would if you were changing it from 12 to 14. It also means that setting your dial at the half way position does not necessarily mean you are getting half the resistance, voltage or output.
Practice Questions
Test your new knowledge on potentiometers by answering these questions.
1. A Potentiometer is used to:
2. What is the difference between a linear and logarithmic potentiometer?
3. A linear potentiometer of 100 Ω has been used in a circuit that receives a known input voltage of 10V, with the wiper set to the middle position. What is the output voltage?
Formula:
$V_{OUT} = V_{IN}(\frac{R_2}{R_1 + R_2})$