Potentiometer and Rheostat
In this article, the working and circuit symbols of potentiometer and rheostat are explained. The types of potentiometers used and their uses are also explained in this post. Rheostat vs Potentiometer is also explained.
I have already explained the working of a variable resistor and its applications in electronic circuits. To know more about it click on the link below.
TAKE A LOOK : VARIABLE RESISTOR – WORKING
TAKE A LOOK : WORKING OF RESISTORS
TAKE A LOOK : METAL OXIDE VARISTOR (MOV)
A potentiometer, also called as POT, is a 3-terminal variable resistor and is used to adjust the resistance in a circuit.
Working of Potentiometer
The working of potentiometer is the same as that of a variable resistor. The construction is also the same. It has a resistive element as the track and a sliding contact called the wiper. The wiper is connected with the help of another sliding contact to another terminal. The position of the wiper depends on the type of POT used. For a panel POT, the wiper is kept in the middle.
The resistive element has a terminal on both the ends and can be linear or logarithmic. It is usually made up of carbon or a mixture of ceramic and metal or even graphite.
There is single turn POTs which changes its entire resistance in one rotation. More accurate POTs called multi-turn POTs are also present. They need about 20 to 30 rotations to change the entire resistance. They are much more accurate than the former. Take a look at the symbol of a POT.
POTs are differentiated according to the track used. There are mainly two of them. They are
This type of a POT is shortly designated with the letter “A” in the component. For example, a POT with 5 kilo ohm resistance will be given as “5k A”. The track of this material will be a resistive material that may be tapered from one end to the other. It may also be a material whose resistivity varies from one end to the other. As like a logarithmic variable resistor this type of POT will have a logarithmic output. Due to this nature, they are always used in audio circuits. They are much more costlier than other types of POTS.
This type of a POT is shortly designated with the letter “B” in the component. For example, a POT with 5 kilo ohm resistance will be given as “5k B”. In this device, the track element has a constant cross-section causing a proportional resistance change between the wiper and one end of the terminal. The device depends on the electrical feature and not the resistive feature. This POT is used for a proportional change like adjusting the centering of a CRO.
Uses of a Potentiometer
- POTs are used for controlling the signal level of a circuit and not the power of the circuit
- Volume control on TV’s and other audio equipments.
- Used in joysticks as a position transducer.
- TRIAC switching applications.
- Voltage divider circuits.
A rheostat is also a variable resistor and is a 2-terminal device. It is commonly used for handling higher currents and voltages. One terminal will be connected to the end of the track and the other to a moveable wiper. When the wiper moves from one end to the other, the resistance changes from zero to maximum.
A rheostat can be made out of a potentiometer. The same mechanism is used except that the terminal that is not used will be connected to the wiper. This helps in reducing the variation in resistance. It also helps in gaining more mechanical strength when connected to a PCB.
The track is usually made from a resistance wire which is wound on a heat resisting cylinder.They are wound together to form the shape of a toroid coil. The slider moves from one phase of the coil to another, thus varying the resistance. The sliders are made in the shape of metal fingers and they move across the tracks through tapping method.
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.