COMPONENT INFORMATION – POTENTIOMETERS

INTRODUCTION

A potentiometer, also commonly called a pot, is an electronic device with a resistance that can be changed by the turning of a dial or the sliding of a lever. Depending on how it is connected to the circuits, it can be used either as a voltage divider or as a variable resistor. More on this later.

POTS IN THE REAL WORLD

Pots are best known for being the volume dial on your radio. They are also used in dimmer switches in your home.

IMAGES

Here are a few images of common pots:

And a few pots that are breadboard friendly:

HOW POTS WORK

Understanding how a pot works is pretty simple.

For the pot animation on the left, the black material resists the flow of current through it. The left pin is connected to one end of the black material while the right pin is connected to the other end of that same material. Because the distance between these two pins is always the same, the resistance between the two is always the same. The middle pin is connected through the rotary dial to the moving half circle. The half circle makes contact with the black material at its top peak (near the black dot). So the amount of black material between the middle pin and the side pins changes as one turns the dial. This means that the resistance between the middle pin and the side pins changes as one turns the dial. When the dial is turned counter-clockwise to the end, the center pin is in contact with the left pin meaning there is no resistance. However, it also means that the center pin is as far as possible from the right pin, meaning they experience the maximum resistance.

The idea with a slider pot is exactly the same. As the central level which is connected to the central pin moves, its distance from the other two pins changes.

CIRCUIT DIAGRAM SYMBOLS

There are two different symbols that are used for pots. Thankfully they are fairly easy to understand.

Please note that the arrow does not specify direction of current.

SPECIFICATIONS

The most important information is the maximum resistance of the potentiometer. This can be easily measured using an ohm-meter by connection both prongs of the meter to the outside legs of the pot.

Otherwise, you are most likely interested in its form and whether or not it can be conveniently used with a breadboard.

COMMON USES

There are two common approaches to using a potentiometer – as a variable resistor and as a voltage divider.

VARIABLE RESISTOR

We can use a pot as a variable resistor simply by leaving one of the outside legs open (floating). The pot now only makes use of two legs (like normal resistors) and the resistance between these two varies with the dial turning. Here is a diagram of a pot used as a variable resistor:

VOLTAGE DIVIDER

We can also use pot as a voltage divider. For a load that has a resistance that is considerably higher than the pot’s max resistance, turning the pot’s dial can provide the load with a voltage ranging from 0V up to nearly100% of the source’s voltage. Here’s the circuit (similar):

ANALYSIS - VARIABLE RESISTOR vs VOLTAGE DIVIDER

Circuit Type	Load Resistance	Pot – Max Resistance	R1	R2	V1	V2	VL
Variable Resistor	10K	1K	500	500	1/21^st of source	N/A	20/21st of source
Voltage Divider	10K	1K	500	500	~50% of the source	~50% of the source	~50% of the source

Uncertain Issue: Mr. Campeau questions if the above voltage divider would see most of the current go through the branch that has no load. Efficiency issue?

TRIMPOTS

Potentiometers that are designed for breadboards tend to have a dial that requires a screwdriver to turn them. These pots are made more to be “set it and forget it” than to me used to change the circuit. They are designed as trimmers (of voltage) that can be set once. So they are called trimpots. We will use these in class because they are so convenient to use with breadboards.