Rheostat – Definition, Working, Construction, Types, Uses



What is a rheostat?

A rheostat is a variable resistor that is used for current control. They can vary the resistance without interruption in a circuit. The construction is very similar to a potentiometers construction. It only requires two connections even when there are three terminals (as in a potentiometer). The first connection is made to one end of the resistive element and the other (sliding contact) connection to the wiper.  Rheostats must hold a large current as compared to potentiometers. Most of them are therefore designed as wire wound resistors. Resistive wire is wound around a ceramic core that is insulated and the wiper slides over the windings.

Rheostats have often been used as power control devices, such as light intensity (dimmer), motor speed, heaters, and ovens. They are no longer used for this purpose today. This is because of their relatively low productivity. In power control systems they are replaced by switching electronics. These are often used as a variable resistance in circuit tuning and calibration. Only during production or circuit tuning (preset resistor) is adjusted in these situations. Trimpots are commonly used in such situations, wired as a rheostat. Yet there are also dedicated 2 terminal resistors  preset.

Rheostat definition-

A rheostat is a variable resistor used to control a circuit’s current flow.

Rheostat Symbol-

Rheostat

Working principle of a Rheostat-

Let’s refresh our basic electrical circuits to understand the significance of rheostat and its operating theory.

An electrical circuit’s three key parameters are: the voltage applied to the circuit, the current through the circuit, and the circuit’s resistance.

Now, we know that these parameters depend on each other. That is, we can either change the voltage applied or adjust the circuit’s resistance to change the current.

In a loop, when we use a rheostat, what we are essentially doing is adjusting the circuit’s resistance to change the current. Since the current and resistance are inversely proportional, we will increase the resistance of the rheostat if a current decrease is necessary. Likewise, if a current increase is required, we will simply reduce the rheostat’s resistance.

Now you may wonder if there is a maximum limit that can decrease or increase the resistance in a rheostat. The answer is yes, that’s it. For each rheostat comes with a resistance rating, such as if a rheostat has a 50 K rating, the minimum resistance it can give is 0 and the maximum resistance is 50k.

So how do we change the resistance of the rheostat?

To do this, rewind the resistance fundamentals. We discussed the parameters of the material’s resistance. The main three factors that depend on a material’s resistance are its length, cross-section area, and type.

Here, the effective length is changed with a sliding touch in this unit. As described above, a rheostat has a fixed and a moving terminal. The effective length is that on the resistive path between the fixed terminal and the sliding terminal’s position.  The effective length varies as the slider travels, thereby increasing the rheostat’s resistance.

As the resistance is directly proportional to the length, the resistance increases as the effective length increases. Similarly, the resistance offered by the rheostat decreases as the effective length decreases.

Construction of Rheostat-

Rheostat construction is closely related to the construction of potentiometers. It has only two connections, even when there are three potentialometer-like terminals. Such resistors must hold a significant current relative to potentiometers. Consequently, they are often designed as wire wound resistors.

Below is the construction of the rheostat. It has three terminals denoted as A, B and C. Nevertheless, whether A&B terminals or B&C terminals, we just use two terminals. The two terminals such as A & C are fixed in this structure, which are connected to the track known as a resistive element. And terminal B is the uneven terminal and  it is connected to the sliding wiper otherwise slider.

Rheostat

When the sliding wiper travels over the resistive lane, it changes the resistance of the rheostat. The resistive element of the rheostat can be constructed using a wire loop otherwise a lean carbon film.

These are often made of wire-wound. So these are also often referred to as adjustable wire wound resistors. In general, these are constructed in the region of an isolating ceramic core by winding the wire-like Nichrome. So this works toward the heat like the insulating material. The ceramic core will therefore not let it heat through.

Types of Rheostats-

1. Linear Rheostat

This type of rheostat have a linear restive path. The sliding terminal glides on this path. Nevertheless, there are two fixed terminals, only one of the two is used. The other terminal is connected with the slider.

These are mostly used in laboratories. Mostly wire-wound resistive path is used along a surface formed by a linear cylinder.

A typical linear rheostat is shown in the following figure.

Rheostat

2. Rotary Rheostat

A rotary rheostat has a rotary resistive path with complete justice to its name. Most of these are used in power applications. These rheostats have a wiper mounted on the shaft. Wiper is nothing but the sliding touch that can rotate over 3⁄4 of a circle for a rotary rheostat.

For both forms of rheostats, the purpose and operating theory are the same.

The following figure shows a rotary rheostat.

Rheostat

3. Preset Rheostat

They are used as trimmers or preset rheostats when rheostats are used in a printed circuit board. Trimmers are nothing more than a small rheostat, mostly used in calibration circuits. There are two terminal trimmers available, although three terminal potentiometer trimmer is used as a two terminal rheostat in most cases.

The following figure shows a trimmer.

Rheostat

Rheostat vs Potentiometer-

The variable resistors bind both Potentiometer as well as Rheostat. Technically, however, these represent the two different configurations and similar components are offered.

Construction of these components is the same.

A rheostat is a 2-terminal device, while a 3-terminal device is a potentiometer.

We use two terminals for the operation in rheostats, whereas we use three terminals for the operation in the potentiometer.

A rheostat can not be used as a potentiometer, while a potentiometer can be used as a rheostat.

Rheostats are used to change the current, while potentiometers are often used to change the voltage.

Uses and Applications of Rheostat-

These are typically used where high voltage otherwise high current is required.

Rheostats are primarily used to adjust the light intensity. If we amplify the resistance of the rheostat, it will reduce the flow of electrical current through the light bulb. Thus the bulb’s intensity is decreasing. Similarly, if we reduce the resistance of the rheostat, the electrical current  flow will increase throughout the bulb. Eventually, there will be a rise in light intensity.

They are often used as power control tools such as control of light intensity, control of motor speed, heaters, and ovens.

Because of their low efficiency, these are not usually used in power control applications. Therefore these are replaced by switching electronics In power control applications.

They are often used in circuits that need tuning due to uneven resistance as well as calibration. Rheostats are modified in these situations while the circuit is otherwise tuned by manufacturing.

Also Read – Potentiometer – Working principle, Types, Applications

Also Read – Resistor Colour Code

Also Read – Ohm’s Law – Definition, Formula, Applications

Also Read – Measurement of resistance

Also read – Capacitor – Types, Applications,Symbol,Unit

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