Chapter 4 : Potentiometers and Rheostats

Potentiometer

• A Potentiometer is adjustable voltage divider.
• It is a three terminal device with a wiper or a slider in between.
• The wiper and slider is moved to increase and decrease the value of resistance
• Its Symbol is

• It is  use to control voltage
• Potentiometers have less power and are commonly used to control audio and television brightness.
• They also function as position transducers in joysticks and other devices that use the same mechanism.
• There are manually adjustable potentiometers, which can be divided in rotary or linear movement types.
• Types of Potentiometer

Rheostat

• A rheostat is a two-terminal variable resistor used to control current flow.
• It functions as an adjustable resistor between the source of electricity and a device that uses adjustable current.
• Rheostats are commonly used to set lighting levels without needing to change the lights.
• Rheostats use the principles of Ohm’s Law, which states that the current is inversely proportional to resistance for a given voltage.
• This enables an object, like a dimmer switch, to produce a lower output level. It is important to remember that rheostats are not polarized to they can work in reverse.

 

 

 

Difference Between Potentiometer and Rheostat

•  A potentiometer is a three terminal variable resistor, but a rheostat is a two terminal variable resistor.
• A potentiometer can be used as a rheostat but a rheostat cannot be used as a potentiometer.
•  Potentiometers are often used to vary voltage and rheostats are used to vary current.

Working

 

 

 

Exercise Time 

So lets try ……. Few examples

1.5 k	3.3 k	4.5 k
· Voltage Across A and C · Volatgae Across A and B o Min o max	· Voltage Across A and C · Volatgae Across A and B o Min o max	· Voltage Across A and C · Volatgae Across A and B o Min o max

 

Example

Find the voltage range v1 and v2 .

 

Now the important thing to notice here is that V1 is across a potentiometer , so the voltage across it will vary as we will move pot up and down . So we need to find Vmin (R min) and Vmax (R max) to find the range .

Solving for V min (pot set to minimum)

• Rmin=0

Total resistance of a circuit is now = R1+R2+R3

=5k+0+10k

=15k

Finding V1 (using VDR)

= (0/15k)*24= 0 Volts

Solving for V max (pot set to maximum)

Rmax=10K

Total resistance of a circuit is now = R1+R2+R3

=5k+10k+10k

=25k

Finding V1 (using VDR)

= (10/25)*24= 9.6 Volts