Rheostat | All you need to know | Complete Guide

Rheostat

What is a rheostat? || Rheostat Definition

To define a rheostat, we need to know what is Resistor or Resistance. Resistors are electrical devices that are to controls the flow of current.  A formal rheostat’s definition will be –

Rheostat is an element of an electrical circuit whose resistance value can be changed whenever needed, that means a variable resistor.”

It is a three-terminal device, of which two are usable. There is a slider as a moving terminal, and only one is usable out of two fixed terminals. A typical rheostat also consists of a resistive material and a slider.

What does a rheostat do?

The basic principle of this device is simple. In electrical circuits, whenever we need to change the resistance value, a rheostat comes into action. If we need to increase the flow of current – we will increase the resistance of the device. When we need to decrease the current flow in the circuit, we will raise the resistance value.

How does a rheostat work?

Rheostat works on the property of resistance. The resistance of a material (let’s say wire) depends linearly with the length and inversely with the cross-section area.

R∝L/A

R=𝝆L/A,

𝝆is the resistivity of the material

Thus, if we keep the cross-section area constant, increasing the length will increase the resistance. As shown in the figure, the slider- is moved through the resistive element for linear rheostats. It moves either from input to output or vice-versa. The effective length changes accordingly. While moving the wiper towards the output port, the effective length decreases, causing a fall in resistance, increasing the current.

What is a rheostat symbol?

Institute of Electrical and Electronics Engineers(IEEE) and the International Electrotechnical Commission(IEC), has defined two different rheostat’s symbols.

Rheostat Switch

Rheostats control current of a circuit by controlling the resistance of the circuit. A rheostat, thus, can be used as as switch to vary the resistance as well as the current of the circuit. That is why a rheostat is used as switch.

What is a rheostat used? || Applications of Rheostats

A rheostat has its application in an electrical circuit.  When there’s a need to control the flow of current with a change of time. Based on the property of current controlling, some of the purpose of rheostat is given below.

• Dimmer Circuits: A dimmer circuit changes the intensity of the light. Using a rheostat will serve the purpose. It has also found application in microscopes for this purpose. While studying a specimen using a light microscope, it is necessary to observe any sample using various light intensity to get an enhanced and different view. For that microscope, a microscope rheostat- is used.
• Speed control: A rheostat switch – 12-volt rheostat switch, is used to control a DC motor’s speed working on specific power and current value. In an additional note, we cannot increase a DC motor’s speed as much as we wish. There are certain limits provided for the DC motor.
• Heaters and ovens: Rheostats generate an excessive amount of heat while providing resistances. That is the main idea behind heaters. Sometimes they are used to provide extra heats that a reptile needs when they are inhibited, outside normal conditions. They are also known as reptile rheostats.
• A high voltage circuit generally needs a rheostat. It is also – used for audiocontrol and many other applications.

Why is a rheostat connected in series?

To connect rheostat in the circuit, we must place it in series, not in parallel. The flow of current is much in a lesser resistive path. Thus, when it finds an option between a less resistive path and a more resistive path, it always chooses the lesser one.

Now, a rheostat is a device that has some variable resistance value. If we connect it to the parallel path, that path gains some more resistance than the other way available. When current flows in the circuit, electrons will never choose the parallel-path instead- they will flow straight through the series path. So, the rheostat will not function at all. It needs the current flow to work as a rheostat.

Type of Rheostat

Though there are several types of rheostat available, three main types are –

A. Linear Rheostats

B. Rotary Rheostats

C. Preset Rheostats

A. Linear Rheostats: This type of rheostat consists of a cylindrical resistive element. The slider is moved linearly along the resistive element. It has two fixed terminals; one- is used, and another connects the slider. This type of rheostats is mostly used  – in laboratories and experiment purposes.

B. Rotary Rheostats: This type of rheostat has a resistive element, which is circular. To use it, one needs to move the slider in a rotary way. They find applications in power electronics and also, they are used widely because of their smaller size than linear types. As the wiper needs to rotate to change the value, that’s why it is called a rotary rheostat.

C. Preset Rheostats: When it is needed to implement a rheostat in a PCB (Printed Circuit Board), one should use preset rheostats or trimmers. It provides fine-tuning, that’s why they have found application in calibration circuits. This types of rheostats are suitable for industrial uses.

Write some of the differences between rheostat and potentiometer?

There is a misconception that a rheostats and potentiometer are the same things, but there are some differences. Let us discuss some of them –

Some frequently asked questions on Rheostats

1. How are rheostats rated?

Rheostats are rated – in Amperes and Watts. There is also a resistance value. For example – 50W – 0.15 A, 100k Ohm. It means the maximum amount of current that can be measured is 0.15A. The resistance offered by the rheostat will be in the range of 0 to 100k Ohm.

2. How to select a rheostat based on rating?

While selecting a rheostat, the current rating is more important than the power rating.

It is the current that limits what power the device will generate provided for any resistance value. One should select rheostats with a current rating more than or equal to the actual need of current in the circuit.

3. What are the differences between resistors and rheostats?

A resistor is a passive electronic component that reduces the flow of current by providing resistance. On the other hand, rheostats are – variable resistors, which gives different values of resistances as per the need.

4. What is the function of a rheostat in the circuit?

• A. It decreases the current
• B. It increases the current
• C. It limits the current
• D. It makes the current constant in the circuit
• E. All of the above

The correct option will be E. All of the above. Using Ohm’s law, we can find the answer to the question. As the ohm law states, V = IR, where V is the applied voltage, I is current, and R is the resistance. Rheostat provides variable resistances value; thus, it can increase, decrease, and limits the current. Keeping the resistance value constant will keep the current steady. So, all the options are correct.

5. Can rheostats be used as a potentiometer?

The answer is no, but there is some way to do this. A rheostat is a two-terminal device, while a potentiometer is a three-terminal device, so it seems impossible. But if a rheostat has inbuilt three terminals, the unused terminal can be joined to the circuit to use it as a potentiometer.

6. Can potentiometers be used as a rheostat?

Yes, a potentiometer is usable as a rheostat. A potentiometer controls voltage in a circuit. A potentiometer has three terminals. One terminal should connect the wiper while another should leave unconnected.

7. What are the drawbacks of using a rheostat?

There are few drawbacks of using this device. Some of them are –

A. The main drawback of this device is that it produces excessive heat causing loss of power.

B. It is bigger & does not fit into modern devices. That’s why rheostats are not used – in modern technologies. Though in rotors and various laboratory experiments, they are irreplaceable. Some of the replacements of rheostats are – triacs, SRCs, etc.

8. What type of taper a rheostat has?

Rheostat has linear types of taper. A taper is a relationship between resistance and sliding position. It is one of the most important parts of the device.

9. What is the use of a rheostat in a Wheatstone Bridge?

A Wheatstone bridge is used in labs to measure mean value resistances. Rheostats find its application in a Wheatstone Bridge to determine the value of unknown resistance in imbalanced conditions. The maximum resistance a rheostat can offer is the maximum resistance that a mounted Wheatstone Bridge can measure.

10. Why is choke of a coil preferred over a rheostat in AC circuits?

A rheostat is a resistive element. It provides resistance and produces an excessive amount of heat. So it causes a loss of electricity. On the other hand, a choke coil is an inductive element by nature. It maintains the same power but changes voltage according to Faraday’s Law. That is why a choke coil is preferred more.

11. Does a Rheostat change voltage?

No, a rheostat doesn’t change the voltage of the circuit. One of the conditions for working of a Rheostat is to keep the voltage constant. As the Ohm’s law states- V= IR, where V is the voltage, I is current, R is resistance. Using a rheostat, we change the current. One of the conditions for working of a Rheostat is to keep the voltage constant. Then only it can change the current of the circuit.

12. Does a rheostat’s terminal have polarity?

A rheostat is a three-terminal device of which two are fixed, and one is a moving terminal. The terminals have no polarity. So, any terminal can be connected.

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