Variable Resistors (Potentiometers) – B1P2

Let’s learn how to use variable resistors. As implied by the name, a variable resistor is a type of resistor whose value can be adjusted. They are commonly referred to as potentiometers, and the type contained in the kit is called a trimmer-potentiometer or trim-pot.

Potentiometer symbols

The commonly used symbols for potentiometers are shown above and it has three terminals.

Potentiometer terminals and value

The potentiometer value is generally printed on the component and it is 103 for this one. The last digit tells you how many zeros need to be added to give you the resistance value in Ohms. 103 is equal to 10,000 Ohms or 10K Ohms. The resistance between the two outermost terminals is fixed and it is 10K ohms. The resistance between the central terminal and the end terminals is variable. The central terminal is also called a slider arm or wiper.

Equivalent potentiometer representation

The potentiometer can be represented by two resistors as shown with both the resistors adding up to a total value of 10K ohms.

Equivalent potentiometer representation

If the slider is moved closer to terminal A, it will reduce the resistance in the top half and increase that in the bottom half. The total will still be 10K ohms.

Equivalent potentiometer representation

Conversely, if we move the slider closer to terminal B, it will increase the resistance in the top half and decrease that in the bottom half. We can use a potentiometer to vary the LED brightness or control the audio volume for instance.

Basic potentiometer circuit - schematic

This is the circuit diagram for the potentiometer circuit. It is very similar to the previous circuit with the main change being that the LED is now connected to the slider arm of the potentiometer.

Basic potentiometer circuit - breadboard layout

Let’s use the breadboard layout to build the circuit. Keep in mind that the middle terminal is the slider terminal which is connected to the LED anode.

Let’s switch ON the battery box and vary the LED brightness by rotating the potentiometer. The LED will be the brightest when the slider is in the topmost position, and the brightness will reduce as we move the slider to the lowermost position. We know from the previous circuit that the current flowing through the LED should be 3mA, by adding a potentiometer, we can either allow all of the 3mA to flow through the LED, or we can reduce it and turn OFF the LED.

This is the basic concept behind potentiometers, and before we move on to the next circuit, let’s discuss something called a voltage divider.

Voltage divider representation

Here we have the equivalent circuit for a potentiometer. If both the resistors have a value of 5K Ohms and if we apply 3V to the end terminals then the voltage at the common terminal will be 1.5V. By changing the slider position, we can change the output voltage from 0V to the entire supply voltage of 3V. What this means is that we can use two resistors to divide an input voltage to some other output voltage – this is called a voltage divider circuit and it is commonly used in electronics.

Voltage divider equation

If this resistor is R1 and this one is R2, then the output voltage is given by the following equation. We can use this to either determine the output voltage or the value of any one resistor.

Basic potentiometer circuit wiring

We will look at transistors in the next post and I will see you there.