Variable Resistor – Overview and Explanation

An Overview on Variable Resistors – Construction, Working and Different Applications

What is a Resistor Overview

• In an electric circuit, the resistor is a passive, two terminal component that obstructs the flow of current and dissipates power once current flows through it. It is represented in electric circuits by the symbol in Figure 1. The ratio of voltage to current is called the resistance and expressed by Ohm’s Law as shown in Figure 2.

Figure 1: Resistor Symbol

Figure 2: Ohm’s Law

• Ohm’s law states that current flowing through a resistor in an electric circuit is directly proportional to the applied voltage when the temperature remains constant. Therefore, it is logical to assume that normal resistors have two terminals and they have constant resistance since their resistance can not be changed.

Definition of a Variable Resistor

• As the name suggests, the variable resistor is a passive, three terminal device that can adjust its resistance via third terminal located between two terminals so that the obstruction to the flow of current goes up and down. Therefore, variable resistor circuit symbol has an arrow which represents resistance variation. The electrical symbol of variable resistor shown in Figure 3.

Figure 3: Variable Resistor Symbol

Figure 4: Variable Resistor Symbol

• The resistance of a variable resistor can be changed between zero to a certain maximum value with its third terminal. When the circuit diagram of the variable resistor in Figure 4 is carefully examined, you can see constant resistance exist between the terminals 1 and 3. Terminal 2 (in the middle) is the only terminal which has the ability to move. Therefore, in order to change resistance you must use anyone of the side terminals with the moving terminal.

Operation Principles of Variable Resistors

• Variable resistors are widely used in electric circuits to adjust the value of current or voltage, since the resistance of variable resistors can be set to a certain value. Variable resistors allow you to adjust the value of voltage by changing the resistance and keeping current constant . To adjust the input voltage, a voltage source is connected to the terminals 1 and 3 as shown in Figure 5. The output voltage between terminals 1 and 2 can be calculated by the voltage division formula shown in Figure 6.

Figure 5: Usage of Variable Resistor as Voltage Divider

Figure 6: Voltage Division Formula

Construction of a Variable Resistor

• Although there are different types of variable resistors, their working principle is the same. When the inside of a variable resistor is examined such as Figure 7, there is a fixed resistance called the resistive track which is between terminals 1 and 3. Terminal 2 is connected to the knob and the slider (wiper) has a direct contact with the knob. The resistance between terminals 1 and 2 or 2 and 3 can be changed by adjusting the knob in the middle as represented by red circle in Figure 7.

Figure 7: Variable Resistor

Types of Variable Resistors

• There are different types of variables resistors which all have almost the same working principle that was illustrated in the previous sections. However, terminal configuration and resistance value of a variable resistor can be adjusted with respect to various environmental parameters. These different types of variable resistors include:

Potentiometers

• As mentioned in previous sections, variable resistors are often used to control voltage or current. Potentiometers is one of the most popular types of variable resistors. They are preferred in applications where voltage control is required. There are mainly two groups of potentiometers known as mechanical and digital. Mechanical potentiometers such as linear and rotary potentiometers have accuracy problems in vibrational circumstances. Digital potentiometers are commonly used due to sensitivity problem of mechanical potentiometers. One of the most fundamental uses of digital potentiometers is to come up with resistance drift problem that occurs in challenging environmental conditions. Since digital potentiometers can be adjusted by communication protocols such as I2C, they are also quite useful in cases where mechanical resistance adjustment isn’t possible.

Rheostats

• The construction of a rheostat is similar to that of a potentiometer. However, the rheostat’s moving terminal is short circuited with one of the side terminals as shown in Figure 8. Rheostats are preferred in applications where resistance adjustment or current limitation required.

Figure 8: Rheostat Symbol

Photoresistors

• Photoresistors also known as light dependent resistors (LDRs) are a common type of variable resistor. Their resistance changes with respect to the intensity of incoming light due to the photoelectric effect. Photoresistors can be preferred in environments where intensity of the light varies.

Force Sensitive Resistor

• As the name suggests, the resistance of a force sensitive resistor changes with respect to the applied force level. They are generally used in robotic applications such as inside of the grippers of a robot.

Thermistors

• The resistance of a thermistor changes with respect to temperature. There are two types of thermistors known as negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors . The resistance of PTC thermistors is directly proportional to the temperature, whereas the resistance of NTC thermistors is inversely proportional to the temperature. Thermistors are preferred in different industrial applications where detection of the temperature variation is critical.

Humistors

• As the name suggests, the resistance of the humistor changes with respect to the variation of the humidity. Humistors are used in many internet of things (IOT) devices to detect environmental changes.

Applications of Variable Resistors

• Variable resistors are found in many of the devices/electronics we have in our homes. Some of these include radios, speakers, microphones, TVs, oscillators, smart home control devices etc. Potentiometers are generally used in home electronic appliances where speed or volume level control needed.
• Rheostats are used where current or resistance levels should be adjusted. A common example is the dimming of lights. In summary, variable resistors are popular in applications where voltage control or current adjustment required.

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Tantalum Capacitors – Overview and Explanation

Tantalum Capacitors

Capacitor Overview

• In an electric circuit, the capacitor is a passive, two terminal device that can statically store electric energy between it’s terminals by using a technique called charge separation. The charge separation phenomenon occurs in capacitors due to the dielectric material placed between its metal plates. The capacitance (C) , the fundamental effect of capacitor, is measured in farads (F) which is named after the English scientist Michael Faraday.  The capacitance of a capacitor is equal to one farad (F) when one coulomb (Q) of electric charge changes the electric potential between the plates by one volt (V). Capacitance is expressed by the equation in Figure 1.

Figure 1: Capacitance Equation

• It is important to note that, there are also both fixed and variable capacitors similar to the case with resistors. Capacitors that have fixed values can be grouped into two categories with respect to their polarity as non-polarized and polarized capacitors. Electric symbols of the three most commonly used capacitor types are represented in Figure 2.

Figure 2: Capacitor Symbols

Definition of the Tantalum Capacitor

• As mentioned in the previous section, fixed and variable capacitors have different electric symbols and operation principles. Capacitors having fixed value of capacitance can be categorized into two groups as polarized and non-polarized capacitors. Polarized capacitors have asymmetrical construction and always must be operated with a higher voltage on the positive terminal (anode) than on the negative terminal (cathode). Polarized capacitors can also be grouped into two categories as electrolytic and super capacitors. An electrolytic capacitor’s positive (anode) plate is made of a special metal that forms an oxide layer. Then the oxide layer operates as the dielectric of the electrolytic capacitor through anodization. A liquid, gel or solid electrolyte covers the surface of oxide layer and behaves as the negative (cathode) plate of the electrolytic capacitor. The three families of electrolytic capacitors aluminum, tantalum and niobium are shown in Figure 3.

Figure 3: Types of Capacitors

• Without any doubt, the tantalum electrolytic capacitor is one of the most prevalent types of capacitors due to its much higher charge capacity than film or ceramic capacitors. Tantalum electrolytic capacitors have also less leakage and higher frequency response than aluminum electrolytic capacitors. Therefore, tantalum electrolytic capacitors are preferred in various electronic applications where small size and higher operation frequencies are needed. The niobium electrolytic capacitor is also an alternative to tantalum electrolytic capacitors and started to drawing more and more attention at the huge capacitor market due to its safety features such as higher flame retardation.

Construction of a Tantalum Capacitor

• Tantalum (Ta) is a bright, silver-gray metal element with the atomic number 73. When the cross-sectional view of the tantalum capacitor is examined such as a typical SMD tantalum electrolytic chip capacitor with solid electrolyte shown in Figure 4, it consists of tantalum powder pressed and sintered into a pallet as the positive (anode) terminal of the chip capacitor. The positive (anode) terminal is covered by an insulating oxide layer that forms the dielectric and the solid manganese dioxide electrolyte acts as the negative (cathode) terminal.
• Tantalum capacitors have high capacitance per volume and weight, because of their thin and relatively high permittivity dielectric layer which distinguishes itself from other electrolytic capacitors. Therefore, the large capacitance of tantalum electrolytic capacitors makes them suitable for passing or bypassing low-frequency signals, and storing large amounts of electric energy.

Figure 4: SMD tantalum electrolytic chip capacitor with solid electrolyte

Applications of Tantalum Capacitors

• Capacitors appear in all types of electronic devices from TVs, radios, computer equipment, Wi-Fi routers to mobile phones. Obviously, any piece of electronic equipment has several capacitors inside and those capacitors come in all shapes and sizes. Capacitors are produced using various methods of construction and dielectric materials that considerably affect their performances. Since some types of capacitors work better in some applications than others, understanding the differences between the different types of capacitors is extremely important.
• Tantalum capacitors are one of the most popular capacitor types in the microelectronics industry. More than 80% of all tantalum electrolytic capacitors are manufactured in surface mount device (SMD) style as tantalum electrolytic chip capacitors. However, niobium based electrolytic capacitors are also getting popular in electronics industry due to abundance of the niobium element in nature.  Nowadays, the energy conservation requirements for capacitors is increasing due to the demand for low power applications. Since the equivalent series resistance (ESR) of a tantalum electrolytic capacitor is relatively low, they are still very popular in electronic application where energy saving is vital.

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An Overview on Rheostats

Rheostat Overview – Construction and Working, Different Applications

Rheostats are resistors which are adjustable, and are used when applications require current adjustment or different resistance in an electrical circuit. Rheostats can adjust generator features, low lights, and start or stable the speed of electrical motors.

There are two things that affect the amount of current flowing into an electric circuit: the magnitude of the applied voltage and the overall resistance of that circuit. If the circuit resistance is lowered, the electric current flow that is passing through the circuit increases. Conversely, the electric current is limited if the circuit resistance is increased.

There is direct relation between the length of the wire to the resistance of the circuit. Increasing the length of the wire increases the resistance between in the circuit. Rheostats allows resistance to be changed, which in turn, either increases or decreases the current through the circuit. The need to add different resistors for varying resistances automatically disappears since a single rheostat can incorporate different resistances necessary for the circuit depending upon its range.

Construction and Working

A rheostat is a wire wound variable resistor which has two connection points: one is a moving point and the other one is a fixed point. Similar to a potentiometer, some rheostats can have three connection points (A, B, and C) as shown in Figure 1, but still, only two of them are used. In such cases, there are two fixed points (A and C), only one of which is used, and the second connection point is the moving one (B).

Rheostats also have to bear large currents when compared with potentiometers. Therefore, rheostats are made up of wire wound resistors. They are mainly constructed by wrapping the Nichrome wire across a ceramic core. Such a core behaves as an insulator to the heat energy and does not permit it to flow through the rheostat.

Figure 1 Rheostat internal Structure (Linear)

Figure 1 explains the working principles of a rheostat. As mentioned above, rheostats work on the principle that the resistance of a certain track or wire depends upon its length. Let’s assume that we are using the fixed connection point A and the moving connection point B of the rheostat illustrated in Figure 1. The rheostat will offer minimum resistance to the circuit if the slider lies closer to point A, since the resistive coil length is at a minimum. Therefore, a large current can flow through the circuit in this case.

Similarly, the rheostat will offer maximum resistance if the slider lies closer to point C, as the resistive coil length is at its maximum. Hence, a tiny amount of current would flow through the circuit and a large part of the current would be opposed by the rheostat.

Now, let’s say that we are using the fixed connection point C and the moving connection point B. In this case, when the slider is made to lie near point C, the rheostat would offer minimum resistance and maximum current flow through the circuit. Likewise, when the slider is moved near to point A, the rheostat would offer maximum resistance and minimum current flow through the circuit.

Finally, it is important to know the maximum and minimum resistances necessary for your circuit. Rheostats have maximum and minimum resistance ratings, and thus, they cannot offer resistance outside their inherited range.

Now you may be wondering if there is a high point up to which the resistance may be decreased or elevated inside the rheostat. For all rheostats, they come with a resistance rating, for instance if 50kω is the rating of the rheostat, the minimum resistance it will deliver is zero and the maximum will be near 50kω.

Different Applications

Rheostats are used in situations where high voltage is needed to transmit electricity. They either work as a variable resistor or as a potential divider. An example of rheostats working as a variable resistor is in dimmers. Fan dimmers and light dimmers frequently use rheostats to control the variation of speed and the intensity of light respectively.

Rheostats are used to change the intensity of light when the lights are dim. The flow of electric current decreases through the bulb. If rheostats resistance is increased the light brightness is lessened. Similarly, The flow of electric current increases through the bulb. If rheostats resistance is increased, the light brightness is increased.

When the rheostat amplifies its resistance, the electric current through the bulb is reduced and the light dims. This same process would slow a ceiling or portable wall fan. Radios are equipped with rheostats to manage volume. Motor speeds can also be controlled through rheostats. They can also be used to control temperature in an oven, a heater or an apartment.

Rheostats also work as potential dividers. Wheatstone bridge applies the same potential dividing principle. Different types of resistive sensors use the potential dividing technique, strain gauges, light dependent resistors and thermistors. Rheostats can be used to measure the resistance of a sensor through a microcontroller. Rheostats can accomplish high voltage readings as well as accurate logic level shifting.

Rheostats are still a primary and common component to control the discharging of current in an electrical circuit. However, solid state devices like triacs and silicon-controlled rectifiers (SCR) have taken the place of rheostats. Rheostats are less efficient than a triac and are less reliable because of presence of mechanical components.

They are mostly when circuits need to be tuned or calibrated. High voltage power lines also use rheostats as potential dividers. Low current and high voltage induce minimal losses in power transmission. This helps supply electricity to millions of homes around the globe.