Electronics Components: How to Vary Resistance with a Potentiometer
Many electronic circuits call for a resistance that can be varied by the user. For example, most audio amplifiers include a volume control that lets the user turn the volume up or down, and you can create a simple light dimmer by varying the resistance in series with a lamp.
A variable resistor is called a potentiometer, or pot for short. A potentiometer is a resistor with three terminals. Two of the terminals are permanently fixed on each end of the resistor, but the middle terminal is connected to a wiper that slides in contact with the entire surface of the resistor. Thus, the amount of resistance between this center terminal and either of the two side terminals varies as the wiper moves.
The resistive track and slider (properly called the wiper) are enclosed within the metal can, and the three terminals are beneath it. The rod that protrudes from the top of the metal can is connected to the wiper so that when the user turns the rod, the wiper moves across the resistor to vary the resistance.
The inside of the potentiometer is made of a semicircular piece of resistive material such as carbon. The two outer terminals are connected to either end of the resistor. The wiper, to which the third terminal is connected, is mounted so that it can rotate across the resistor. When the wiper moves, the resistance between the center terminal and the other two terminals changes.
|The symbol used for a potentiometer in schematic diagrams is an
arrow pointing at the center tap of the resistor. It is meant to
reflect that the value of the resistance at this terminal varies
when the wiper moves.
Potentiometers are rated by their total resistance. The resistance between the center terminal and the two other terminals always adds up to the total resistance rating of the potentiometer.
Here are a few other rambling thoughts to keep in mind about potentiometers:
Potentiometers come in a wide variety of shapes and sizes. With a little hunting around in stores or on the Internet, you should be able to find the perfect potentiometer for every need.
Some potentiometers are very small and can be adjusted only by the use of a tiny screwdriver. This type of pot is called a trim pot, designed to make occasional fine-tuning adjustments to your circuits.
Some potentiometers have switches incorporated into them so that when you turn the knob all the way to one side or pull the knob out, the switch operates to either open or close the circuit.
When the wiper reaches one end of the resistor or the other, the resistance between the center terminal and the terminal on that end is essentially zero. Keep this point in mind when you’re designing circuits. To avoid circuit paths with no resistance, it’s common to put a small resistor in series with a potentiometer.
In some potentiometers, the resistance varies evenly as you turn the dial. For example, if the total resistance is 10 kΩ, the resistance at the halfway mark is 5 kΩ, and the resistance at the one-quarter mark is 2.5 kΩ. This type of potentiometer is called a linear taper potentiometer because the resistance change is linear.
Many potentiometers, however, aren’t linear. For example, potentiometers designed for audio applications usually have a logarithmic taper, which means that the resistance doesn’t vary evenly as you move the dial.
Some variable resistors have only two terminals: one on an end of the resistor itself, the other attached to the wiper. This type of variable resistor is properly called a rheostat, but most people use the term potentiometer or pot to refer to both two- and three-terminal variable resistors.