A rheostat is popularly known as a variable resistor that is used to control the electric current flow by increasing or decreasing the resistance. The resistance can be varied in a circuit without any sort of interference with the help of rheostats. They are built in a very similar way to potentiometers and use only two connections despite the presence of three terminals. They are popularly used for controlling generator characteristics, dim lights, or to start and control the speed of motors. The resistance element can be carbon, metal wire or ribbon, or a conducting fluid, which all depends upon the application they will be utilized for.
Construction of a Rheostat
As already mentioned, Rheostats are constructed in a very similar way to potentiometers. Just like potentiometers, they consist of three terminals but only use two connections. The three terminals can be termed as terminal A, terminal B, and terminal C. the two terminals that are used for the two-wire connections could be either A and B or B and C. Terminal A and C are the two terminals that are fixed and connected to both sides of the resistive element which is known as track whereas the terminal B is a variable terminal that is connected to the slider.
When the wiper moves along the resistive path it changes the resistance of the rheostat. The resistive element could be made out of a coil, thin carbon film, or wire. Rheostats are also known as wire wound resistors because they are mostly wire-wound.
Generally, rheostats are built by winding the Nichrome wire around an insulated ceramic core. The ceramic core Is heat resistant and hence, does not allow any heat to pass through it.
Types of Rheostats
There are different types of rheostats which vary depending upon their applications.
Rotatory Rheostats
For power control applications, the rotary type is the most suitable. Most of the time you will find them with open construction but there are also other options available in a closed construction. Similar to potentiometers that have multi-gang types available. They are utilized to control many applications in parallel and to increase the adjustment rate or power rating. Some rheostats can be equipped with mechanical stops to put limitations on the maximum or minimum resistance. For special applications, some of them are available with tapered windings.
Slide Rheostats
Slide rheostats are commonly used in educational and laboratory environments. These types are built of resistive wire wound present on an insulating cylinder. Sliding contact is used to increase and decrease the resistance however it is required.
Preset Resistor
Trimmers are utilized as variable resistors on many printed circuit boards. There are effective preset resistors with two terminals available. However, the 3-terminal potentiometer is more widely used and is operated by wiring it as a rheostat.
Applications of Rheostats
Rheostats are commonly used in applications where high current or voltage is needed.
- They are used to change the intensity of the lights. If we reduce the resistance of the rheostat the flow of electric current will also decrease and the light bulb will become dim. This means the brightness of the bulb decreases due to reduced current flow. Similarly, if you intend to increase the brightness of the light bulb then the resistance of the rheostat must be decreased so that more electric current can flow through the light bulbs hence making them brighter.
- Another common use of rheostats is to turn up or down the volume of radios and the speed of electric motors.
- They used as etalon resistance in laboratories
- They make the resistance measurement easy by using the bridge method.
- Rheostats used as the electric buttons of the modern electric stoves to control the temperature setting of the stove. Besides the electric stove, they used in all kitchen appliances with heating elements that need to adjusted.
How to Choose Rheostats?
When choosing rheostats for a certain application it is important to consider certain things like current and power in watts. The current is often of more significance in most cases to select the right rheostat.
Other than power or current the style and the size of the Rheostats is an important factor as well. Most of the rheostats feature a round or straight shank that allows you to connect a handle to the rheostat. Some of them also come with screwdriver slots that help you to adjust the rheostat.
The type of the rheostat also matters when purchasing it for a specific application as it can vary majorly depending on its application.
