A rheostat is an electrical component with variable resistance. Basically, it is a variable or adjustable resistor. As the name suggests a resistor resists or inhibits the flow of electric current through a circuit. The amount of current through a circuit is determined by the voltage applied and the total resistance, current is proportional to voltage and inverse proportional to resistance. Since most devices are powered by a fixed voltage source, regulation of current is achieved by a rheostat. ‘Rheo’ is a Greek word meaning ‘current controlling’, I found it good to let you know this.
Construction and Operation
This is one of the simplest components in electrical engineering. It’s made of a resistive element with three terminals in contact with it. Two terminals are fixed while one is movable and is commonly referred to as sliding contact or wiper. A rheostat is made by connecting one fixed contact and a movable one. You can guess, how resistance is achieved, right? Yes, it is by sliding or rotating the movable terminal along with the resistive element. But how is that, you ask? Resistance is directly proportional to the length of the resistive element. The closer the terminal is, the smaller the resistance and good news are, the vice versa holds true as well.
Rheostat can be confused with the potentiometer, because of their great resemblance. The potentiometer has the three terminals connected, two fixed and the moving one. Unlike rheostat, it only handles small voltage and currents. The analog device was a huge success as it will control the current flowing through the load effectively. One major disadvantage it has is the power loss in form of heat where the resistive part carries current. The longer the resistive element where current flows the greater the losses, suffice to say this heat need to be controlled.
There two types based on
There are two types namely; linear – it has a cylindrical-shaped resistive element and a linearly sliding wiper that makes the movable terminal.
Type of resistive element
There are three main types
Carbon rod- it works with low voltage and current, the rod forms the resistive element and a wiper sliding linearly varies the resistance.
Resistive wire-wound– an insulating ceramic core is wound with a resistive wire such as nichrome. The moving terminal makes contact with the wire and one fixed contact end of the wire is connected with the circuit in series.
Electrolytic – an electrolyte makes the resistive element, the plates make the contact. The level of electrolytic liquid covering one plate is adjusted to vary the resistance. The device can operate at a higher voltage than the other two.
Application of Rheostat
- Dimming the illuminance of the light bulb- varying the resistance varies the current and this adjusts the brightness.
- Meter speed control- varying current varies the speed effectively controlling the machine speed.
- Heater and oven temperature control- this regulates current is proportional to heat generated. This, therefore, control the desired heat.
- Audio volume control- the sliding tab you see in audio mixers, the rotary knob in a radio or TV is made of rheostats. Current being proportional to loudness.
Although semiconductor technology has caused migration from analog to the digital world, rheostat was and still is a very important component in the control of electrical parameters. The control of the current is a perfect way of regulating the operation of the powered equipment. As a do-yourself project, buy a rheostat or potentiometer and try adjusting the brightness of an incandescent bulb to appreciate the much I have been saying.