Solid State devices

• Resistors
• Inductors
• Capacitors
• Diodes
• Rectifiers
• Components in series and shunt
• Response of Inductor to DC and AC
• Response of Capacitor to DC and AC

What is an Electrical Resistor?

A resistor is a passive component in a circuit which provides resistance to the flow of current.

In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines

There are many different types of resistors. These resistors vary in their construction, power dissipation capacities, and tolerance to various parameters (such as temperature and light). The types of resistors include:

• Carbon Composition Resistor
• Thermistor
• Wire Wound Resistor
• Metal Film Resistor
• Carbon Film Resistor
• Variable Resistor
• Varistor
• Light Dependent Resistor

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• Carbon Composition Resistor
• A carbon composition resistor (also known as a carbon resistor) is a commonly used resistor.
• Carbon resistors are mainly made of carbon clay composition covered with a plastic case. The lead of the resistor is made of tinned copper.
• The main advantages of these resistors are that they are readily available, low cost, and they are very durable.
• The main disadvantage of carbon composition resistors is that they are very temperature sensitive. The tolerance range in resistance of carbon composition resistor is of ± 5 to ± 20 %.
• This type of resistor has a tendency to produce some electric noise due to the passage of electrical current from one carbon particle to others.
• Carbon resistors are provided with a different colored band on their cylindrical body. These color bands are code for the resistance values of resistors along with their tolerance range.

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Thermistor

• The word Thermistor means a thermal resistor. Its resistance value changes with the change in the temperature.
• Most thermistors have a negative temperature coefficient which means its resistance will fall down when the temperature increases.
• These are normally made of semiconductor materials. Resistance up to a few megaohms can be obtained from thermistors.
• They are used to detect small temperature changes, when there is a temperature change, however small, there will be a large change in the value of the resistance.

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Wire Wound Resistor

In wire wound resistor a wire of manganin or constantan is wound around a cylinder of insulating material. The temperature coefficient of resistance of manganin and constantan is almost zero. So, resistance variation with temperature of these resistors is negligible.

• The wounded wire is covered with an insulating cover such as baked enamel. This cover of insulating heat resistible material resists the effect of ambient temperature variation.
• Different sizes and ratings of wire wound resistors can easily be achieved by using different lengths and diameters of the wire. The range of resistance values varies from 1 Ω to 1 MΩ.
• The typical tolerance limit of these resistors varies from 0.01 % to 1 %. They can be used for high power applications of 5 to 200 W dissipation ratings.
• The cost of these resistors is much higher than carbon resistors. Normally a wire wound resistor is used where a carbon composition resistor cannot meet the purpose because of its limitations.
• The main disadvantage of this resistor is the inductance that arises because of its coil-like structure. At high frequency, the behavior of the circuit may be changed due to its reaction.
• This problem can be solved if one half of the wire is wound in one direction and another half in the opposite direction so that the inductance due to these two halves cancels each other hence the net inductive effect of the resistor becomes nil.

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Metal Film Resistor and Carbon Film Resistor

• The resistor is constructed by means of depositing a thin film of a conductive material such as pure carbon or metal on to an insulating core.
• The desired value of resistance of metal film resistor or carbon film resistor can easily be obtained by either trimming the layer of the thickness or by cutting helical grooves of suitable pitch along its length.
• Metallic contact cap is fitted at both ends of the resistor. The caps are in contact with the conductive film or helical grooves. The lead wire is welded to the end caps.
• Metal Film Resistor or Carbon Film Resistor can be made up to a value of 10,000 MΩ and the size of this type of resistor is much smaller than a wire wound resistor.
• Carbon film resistors give lower tolerances and smaller values of electrical resistance than those available with a metal film. However, the carbon film posses a mildly negative temperature coefficient of resistance which is very useful for certain electronic circuits.

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Variable Resistor

The variable resistor means its resistance value can be adjusted (similar to a potentiometer). There are a rotating shaft and a wiping contact.

• Basically, there is a resistive semicircular bar or coil and by wiping the contact we change the effective length of the resistive element and hence the resistance gets changed. One example of such resistors is a rheostat.
• The variable resistor or rheostat can also be a linear sliding type where the sliding contact moves on the resistive element linearly for adjustment of the effective resistance of the resistor.

Non Linear Resistor or Varistor

They are also known as varistors. They are popular for having the non-linear V-I characteristics curve. That is its resistance is not uniform and it does not obey Ohms law.

• They are made of materials such as silicon carbides, zinc oxide.
• There are three types of varistors:
1. Silicon carbide disc type varistor
2. Silicon carbide rod type varistor
3. Zinc oxide type varistor

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• Light Dependent Resistor
• A Light Dependent Resistor (or LDR) will vary in resistance depending on the intensity of light falling on it. This is made of cadmium sulfide which contains a small number of electrons when it is not illuminated.
• When a light ray falls on it, electrons get ejected and hence the conductivity of it increases. Hence, it offers low resistance when light falls on it and offers high resistance in the dark.

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Inductor

An inductor (also known as an electrical inductor) is defined as a two-terminal passive electrical element that stores energy in the form of a magnetic field when electric current flows through it. It is also called a coil, chokes, or reactor.

• An inductor is simply a coil of wire. It usually consists of a coil of conducting material, typically insulated copper, wrapped into an iron core either of plastic or ferromagnetic material; thus, it is called an iron-core inductor, which is used to increase the magnetic field and thus the inductor’s inductance.
• According’s to Faraday’s law of electromagnetic induction, when an electric current flowing through an inductor or coil changes, the time-varying magnetic field produces an e.m.f (electromotive force) or voltage in it. The induced voltage or e.m.f. across an inductor is directly proportional to the rate of change of the electric current flowing through the inductor.
• Inductance (L) is a property of an inductor that opposes any change in magnitude or direction of current flowing through it. The larger an inductor’s inductance, the greater the capacity to store electrical energy in the form of the magnetic field.

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The inductor in a circuit opposes changes in current flow through it by inducing a voltage across it which is proportional to the rate of change of current flow.

• As shown, a lamp, a coil of wire (inductor), and a switch are connected to a battery. If we remove the inductor from the circuit, the lamp lights up normally. With the inductor, the circuit behaves completely differently.
• The inductor or coil has much lower resistance compared to the lamp, thus when the switch is closed most of the current should starts flows through the coil as it provides a low-resistance path to the current. hence, we expect that lamp to glow very dimly.
• But due to inductor behavior in the circuit, when we close the switch, the lamp glows brightly and then gets dimmer and when we open the switch, the bulb glows very brightly and then quickly goes out.
• The reason is that, when voltage or potential difference is applied across an inductor, the electric current flowing through an inductor produces a magnetic field. This magnetic field again creates an induced electric current in the inductor but of opposite polarity, according’s to Lenz’s law.
• This induced current due to the magnetic field of the inductor tries to oppose any change, an increase or a decrease, in the current. Once the magnetic field is built, the current can flow normally.
• Now, when the switch is open, the magnetic field around the inductor keeps current flowing in the inductor until the magnetic field collapses. This current keeps the lamp glowing for a certain amount of time even though the switch is open.
• In other words, the inductor can store energy in the form of a magnetic field and it tries to oppose any change in the current flowing through it. Thus, the overall result of this is that the current through an inductor cannot change instantaneously.

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• Inductor Equation
• Voltage Across an Inductor
• The voltage across an inductor is directly proportional to the rate of change of the electric current flowing through the inductor. Mathematically, the voltage across the inductor can be expressed as,

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The voltage across an inductor is due to the energy stored in the magnetic field of the inductor.

If d.c. current flows through the inductor      becomes zero as d.c. current is constant with respect to time. Hence, the voltage across the inductor becomes zero. Thus, as far as d.c. quantities are considered, in steady-state, the inductor acts as a short circuit.

Types of Inductor �

• Iron core Inductors: This type uses ferromagnetic materials such as ferrite or iron in manufacturing the inductor for increasing the inductance. Due to the high magnetic permeability of these materials, inductance can be increased in response of increasing the magnetic field. At high frequencies it suffers from core loses, energy loses, that happens in ferromagnetic cores.
• Air Core Inductor: Air cored inductor is the type where no solid core exists inside the coils. In addition, the coils that wound on nonmagnetic materials such as ceramic and plastic are also consideredas air cored. This type does not use magnetic materials in its construction. The main advantage of this form of inductors is that, at high magnetic field strength, they have a minimal signal loss. On the otherhand, they need a bigger number of turns to get the same inductance that the solid cored inductors woulproduce. They are free of core losses because they are not depending on a solid core.
• Toroidal Core Inductor: Toroidal Inductor constructs of a circular ring formed magnetic core that characterized by it is magnetic with high permeability material like iron powder, for which the wire wounded to get inductor. It works pretty well in AC electronic circuits' application. The advantage of this type is that, due to its symmetry, it has a minimum loss in magnetic flux; therefore it radiates less electromagnetic interference near circuits or devices. Electromagnetic interference is very important in electronics that require high frequency and low power.
• Laminated Core Inductor: This form gets typified by its stacks made with thin steel sheets, on top of each other designed to be parallel to the magnetic field covered with insulating paint on the surface; commonly on oxide finish. It aims to block the eddy currents between steel sheets of stacks so the current keeps flowing through its sheet and minimizing loop area for which it leads to great decrease in the loss of energy.

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Applications of Inductors: In general there are a lot of applications. Here are used in radio frequency for suppressing noise. Signals isolation and for high power applications, energy storage, sensors, transformers, filters and motors