Category: basic Electronics

Basic Electronics – Polarized Capacitors ”; Previous Next Polarized Capacitors are the ones that have specific positive and negative polarities. While using these capacitors in circuits, it should always be taken care that they are connected in perfect polarities. The following image shows the classification of polarized capacitors. Let’s start the discussion with Electrolytic Capacitors. Electrolytic Capacitors The Electrolytic Capacitors are the capacitors which indicate by the name that some electrolyte is used in it. They are polarized capacitors which have anode (+) and cathode (-) with particular polarities. A metal on which insulating oxide layer forms by anodizing is called as an Anode. A solid or non-solid electrolyte which covers the surface of the oxide layer, functions as a cathode. The Electrolytic Capacitors have much higher Capacitance-Voltage (CV) value than the others, due to their larger anode surface and thin dielectric oxide layer. Aluminum Electrolytic Capacitors Aluminum Electrolytic Capacitors are the most common types among the Electrolytic capacitors. In these ones, a pure Aluminum foil with an etched surface acts as an Anode. A thin layer of metal, which has a thickness of few micrometers acts as a diffusion barrier, which is placed between two metals to separate electrically. Hence the diffusion barrier acts as a dielectric. The electrolyte acts as a cathode which covers the rough surface of oxide layer. The following figure shows an image of different sizes of Aluminum Electrolytic Capacitors available. Depending upon the electrolyte there are three types of Aluminum Electrolytic Capacitors. They are − Wet Aluminum Electrolytic capacitors (non-solid) Manganese dioxide Aluminum Electrolytic capacitors (solid) Polymer Aluminum Electrolytic capacitors (solid) The main advantage with these Aluminum Electrolytic capacitors is that, they have low impedance values even at mains frequency and they are cheaper. These are mostly used in Power supply circuits, SMPS (Switched Mode Power Supply) and DC-DC Converters. Tantalum Electrolytic capacitors These are another type of Electrolytic capacitors whose anode is made up of tantalum on which a very thin insulating oxide layer is formed. This layer acts as a dielectric and the electrolyte acts as a cathode which covers the surface of oxide layer. The following figure shows how tantalum capacitors look like. Tantalum provides high permittivity dielectric layer. Tantalum has high capacitance per volume and lower weight. But these ones are costlier than Aluminum Electrolytic capacitors, due to the frequent unavailability of tantalum. Niobium Electrolytic Capacitors A Niobium Electrolytic Capacitor is the other type of Electrolytic Capacitors in which a passivated niobium metal or niobium monoxide is considered as anode and an insulating niobium pentoxide layer is added on to the anode, so that it acts as a dielectric. A solid electrolyte is laid on the surface of the oxide layer which acts as a cathode. The following figure shows how Niobium capacitors look like. The Niobium Capacitors are commonly available as SMD (Surface Mount Devices) chip capacitors. These are easily fitted in a PCB. These capacitors should be operated in perfect polarities. Any kind of reverse voltage or ripple current higher than the specified will eventually destroy the dielectric and the capacitor as well. Super Capacitors The high capacity electrochemical capacitors with capacitance values much higher than the other capacitors, are called as Super Capacitors. These can be categorized as a group that lies between electrolytic capacitors and rechargeable batteries. These are also called as Ultra Capacitors. There are many advantages with these capacitors such as − They have high capacitance value. They can store and deliver charge much faster. They can handle more charge and discharge cycles. These capacitors have many applications such as − They are used in cars, buses, trains, elevators and cranes. They are used in regenerative braking. They are used for memory backup. The types of super capacitors are Double-layered, Pseudo and Hybrid ones. Double-layered Capacitors Double-layered capacitors are electrostatic capacitors. The charge deposition is done in these capacitors according to the principle of Double-layer. All solid substances have negative charge on the surface layer when disposed into a liquid. This is due to the high dielectric coefficient of liquid. All the positive ions come near the surface of the solid material to make a skin. The deposition of positive ions near the solid material get looser with the distance. The charge created at this surface due to the deposition of anions and cations leads to some capacitance value. This double-layer phenomenon is also termed as Helmholtz double layer. The figure below explains the procedure of double-layer phenomenon, when the capacitor is charged and when it is discharged. These capacitors are simply called as Electric Double Layered Capacitors (EDLC). They use carbon electrodes to achieve separation of charge between the surface of conductive electrode and the electrolyte. The carbon acts as dielectric and the other two as anode and cathode. The separation of charge is much smaller than in a conventional capacitor. Pseudo Capacitors These capacitors follow the electrochemical process for the deposition of charge. This is also called as faradaic process. At an electrode, when some chemical substance reduces or oxidizes, some current is generated. During such process, these capacitors store the electric charge by electron transfer between electrode and electrolyte. This is the working principle of Pseudo capacitors. They get charged much faster and store the charge as much as a battery does. They are operated at a faster rate. These are used in tandem with batteries to improve life. These are used in grid applications to handle power fluctuations. Hybrid Capacitors A Hybrid Capacitor is a combination of EDLC and Pseudo Capacitor. In the Hybrid capacitors, activated carbon is used as cathode and the pre-doped carbon material acts as anode. Li ion capacitor is the common example of this type. The following figure shows different types of Hybrid Capacitors. They have high tolerance in a wide range of temperature variations from -55°C to 200°C. Hybrid capacitors are also used in airborne applications. Though cost is high, these capacitors are highly reliable and compact. These are rugged and

Basic Electronics – Transistor Configurations ”; Previous Next A Transistor has 3 terminals, the emitter, the base and the collector. Using these 3 terminals the transistor can be connected in a circuit with one terminal common to both input and output in a 3 different possible configurations. The three types of configurations are Common Base, Common Emitter and Common Collector configurations. In every configuration, the emitter junction is forward biased and the collector junction is reverse biased. Common Base (CB) Configuration The name itself implies that the Base terminal is taken as common terminal for both input and output of the transistor. The common base connection for both NPN and PNP transistors is as shown in the following figure. For the sake of understanding, let us consider NPN transistor in CB configuration. When the emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel the emitter electrons and current flows through the emitter and base to the collector to contribute collector current. The collector voltage VCB is kept constant throughout this. In the CB configuration, the input current is the emitter current IE and the output current is the collector current IC. Current Amplification Factor (α) The ratio of change in collector current ($Delta I_{C}$) to the change in emitter current ($Delta I_{E}$) when collector voltage VCB is kept constant, is called as Current amplification factor. It is denoted by α. $$alpha:=:frac{Delta I_{C}}{Delta I_{E}}::at:constant:V_{CB}$$ Expression for Collector current With the idea above, let us try to draw some expression for collector current. Along with the emitter current flowing, there is some amount of base current IB which flows through the base terminal due to electron hole recombination. As collector-base junction is reverse biased, there is another current which is flown due to minority charge carriers. This is the leakage current which can be understood as Ileakage. This is due to minority charge carriers and hence very small. The emitter current that reaches the collector terminal is $$mathbf{mathit{alpha I_{E}}}$$ Total collector current $$I_{C}:=:alpha I_{E}:+:I_{leakage}$$ If the emitter-base voltage VEB = 0, even then, there flows a small leakage current, which can be termed as ICBO (collector-base current with output open). The collector current therefore can be expressed as $$I_{C}:=:alpha I_{E}:+:I_{CBO}$$ $$I_{E}:=:I_{C}:+:I_{B}$$ $$I_{C}:=:alpha(I_{C}:+:I_{B}):+:I_{CBO}$$ $$I_{C}(1:-:alpha):=:alpha I_{B}:+:I_{CBO}$$ $$I_{C}:=:(frac{alpha}{1:-:alpha}): I_{B}:+:(frac{I_{CBO}}{1:-:alpha})$$ $$I_{C}:=:(frac{alpha}{1:-:alpha}): I_{B}:+:(frac{1}{1:-:alpha})I_{CBO}$$ Hence the above derived is the expression for collector current. The value of collector current depends on base current and leakage current along with the current amplification factor of that transistor in use. Characteristics of CB configuration This configuration provides voltage gain but no current gain. Being VCB constant, with a small increase in the Emitter-base voltage VEB, Emitter current IE gets increased. Emitter Current IE is independent of Collector voltage VCB. Collector Voltage VCB can affect the collector current IC only at low voltages, when VEB is kept constant. The input resistance ri is the ratio of change in emitter-base voltage ($Delta{V_{EB}}$) to the change in emitter current ($Delta{I_{E}}$) at constant collector base voltage VCB. $$eta:=:frac{Delta{V_{EB}}}{Delta{I_{E}}}::at:constant:V_{CB}$$ As the input resistance is of very low value, a small value of VEB is enough to produce a large current flow of emitter current IE. The output resistance ro is the ratio of change in the collector base voltage ($Delta{V_{CB}}$) to the change in collector current ($Delta{I_{C}}$) at constant emitter current IE. $$r_{o}:=:frac{Delta{V_{CB}}}{Delta{I_{C}}}: at: constant:l_{E}$$ As the output resistance is of very high value, a large change in VCB produces a very little change in collector current IC. This Configuration provides good stability against increase in temperature. The CB configuration is used for high frequency applications. Common Emitter (CE) Configuration The name itself implies that the Emitter terminal is taken as common terminal for both input and output of the transistor. The common emitter connection for both NPN and PNP transistors is as shown in the following figure. Just as in CB configuration, the emitter junction is forward biased and the collector junction is reverse biased. The flow of electrons is controlled in the same manner. The input current is the base current IB and the output current is the collector current IC here. Base Current Amplification factor (β) The ratio of change in collector current ($Delta{I_{C}}$) to the change in base current ($Delta{I_{B}}$) is known as Base Current Amplification Factor. It is denoted by β $$beta:=:frac{Delta{I_{C}}}{Delta{I_{B}}}$$ Relation between β and α Let us try to derive the relation between base current amplification factor and emitter current amplification factor. $$beta:=:frac{Delta{I_{C}}}{Delta{I_{B}}}$$ $$alpha:=:frac{Delta{I_{C}}}{Delta{I_{E}}}$$ $$I_{E}:=:I_{B}:+:I_{C}$$ $$Delta I_{E}:=:Delta I_{B}:+:Delta I_{C}$$ $$Delta I_{B}:=:Delta I_{E}:-:Delta I_{C}$$ We can write $$beta:=:frac{Delta{I_{C}}}{Delta I_{E}:-:Delta I_{C}}$$ Dividing by $$ $$beta:=:frac{frac{Delta I_{C}}{Delta I_{E}}}{frac{Delta I_{E}}{Delta I_{E}}:-:frac{Delta I_{C}}{Delta I_{E}}}$$ $$alpha:=:frac{Delta I_{C}}{Delta I_{E}}$$ We have $$alpha:=:frac{Delta I_{C}}{Delta I_{E}}$$ Therefore, $$beta:=:frac{alpha}{1-alpha}$$ From the above equation, it is evident that, as α approaches 1, β reaches infinity. Hence, the current gain in Common Emitter connection is very high. This is the reason this circuit connection is mostly used in all transistor applications. Expression for Collector Current In the Common Emitter configuration, IB is the input current and IC is the output current. We know $$I_{E}:=:I_{B}:+:I_{C}$$ And $$I_{C}:=:alpha I_{E}:+:I_{CBO}$$ $$=:alpha (I_{B}:+:I_{C}):+:I_{CBO}$$ $$I_{C}(1:-:alpha):=:alpha I_{B}:+:I_{CBO}$$ $$I_{C}:=:frac{alpha}{1-alpha}I_{B}:+:frac{1}{1-alpha}:I_{CBO}$$ If base circuit is open, i.e. if IB = 0, The collector emitter current with base open is ICEO $$I_{CEO}:=:frac{1}{1-alpha}:I_{CBO}$$ Substituting the value of this in the previous equation, we get $$I_{C}:=:frac{alpha}{1-alpha}I_{B}:+:I_{CEO}$$ $$I_{C}:=:beta I_{B}:+:I_{CEO}$$ Hence the equation for collector current is obtained. Knee Voltage In CE configuration, by keeping the base current IB constant, if VCE is varied, IC increases nearly to 1v of VCE and stays constant thereafter. This value of VCE up to which collector current IC changes with VCE is called the Knee Voltage. The transistors while operating in CE configuration, they are operated above this knee voltage. Characteristics of CE Configuration This configuration provides good current gain and voltage gain. Keeping VCE constant, with a small increase in VBE the base current IB increases rapidly than in CB configurations. For any value of VCE above knee voltage, IC is approximately equal to βIB. The

Basic Electronics – Junction Diodes ”; Previous Next There are many types of diodes depending upon many factors such as the frequency used, their working and construction, their applications etc. Let us go through few of them. Junction diodes The junction diodes are the normal PN junction diodes but differ in construction. There are three types of junction diodes, as shown in the following figure. Rectifier Diode These diodes are the normal PN junction diodes, which allow current to flow through them in only one direction and stop in the other direction. These diodes are used in rectifier circuits to convert alternating current into direct current. In the above figure, we can see the same rectifier diodes with a metal projection. This is added to the diode to minimize the heat distribution which might affect the diode sometimes. Such a metal projection is called as Heat sink. These help in the improvement of the diode performance and the diodes will be able to withstand high powers, without getting affected. There are circuits such as Half wave rectifier and Full wave rectifier circuits which use these diodes. These circuits are discussed in ELECTRONIC CIRCUITS tutorial. These rectifier circuits are used in Power supply sections of many circuits where alternating input current has to be converted into direct current for that circuit applications. Zener Diode This is a special kind of diode which permits current flow not only in forward direction, but also in reverse direction. A normal diode, when operated in reverse bias, gets damaged if the reverse current above a certain value is passed through it. This “certain value” is called as the Breakdown voltage. The breakdown voltage of a Zener diode is very low. But this diode allows the reverse current to pass through it, once this breakdown voltage is exceeded. That breakdown voltage is called as Zener Voltage. Hence there is a controlled breakdown which does not damage the diode when a reverse current above the Zener voltage passes through a Zener diode. A Zener diode in its reverse bias, exhibits a controlled breakdown voltage and it allows the current flow to keep the value of voltage across that Zener diode close to the Zener breakdown voltage value. This value of Zener breakdown voltage makes any Zener diode to be chosen for certain applications. Avalanche diode is another diode which has the similar characteristics of Zener diode. The avalanche breakdown takes place across the entire PN junction, when the voltage drop is constant and is independent of current. This avalanche diode is used for photodetection. V-I Characteristics of a Zener diode The V-I Characteristics of a Zener diode are common for any diode when operated in forward bias. But the reverse bias operation of a Zener diode makes it very important to consider. Let us have a look at the graph. The point where the bent is shown in the reverse bias operation, is the Zener breakdown voltage, after which the diode allows high reverse currents through it. This Zener voltage is indicated by VZ. This incredible quality of Zener diode made it the most reliable one and have got many applications too. Applications of Zener diode This diode has many applications such as − It is mostly used as a Voltage Regulator. Provides fixed reference voltage in transistor biasing circuits. For peak clipping or limiting in wave shaping circuits. As a Surge protector in many circuits. For meter protection against damage from accidental applications. Switching Diode This is a normal single PN junction diode which is especially designed for switching purposes. This diode can exhibit two states of high and low resistance clearly which can be used alternatively. The junction capacitance of this diode is made very low so as to minimize other effects. The switching speed is made quite high. When the diode has high resistance it works as an open switch and it acts as a closed switch during low resistance. This transition occurs at a faster rate in switching diode, than in any ordinary one. Applications of switching diode These have many applications such as − Used in high-speed rectifying circuits Used in ring modulators Used in radio frequency receivers Used as reverse polarity protectors Used for both General purpose and high speed switching applications Print Page Previous Next Advertisements ”;

Transistor Regions of Operation ”; Previous Next The DC supply is provided for the operation of a transistor. This DC supply is given to the two PN junctions of a transistor which influences the actions of majority carriers in these emitter and collector junctions. The junctions are forward biased and reverse biased based on our requirement. Forward biased is the condition where a positive voltage is applied to the p-type and negative voltage is applied to the n-type material. Reverse biased is the condition where a positive voltage is applied to the n-type and negative voltage is applied to the p-type material. Transistor biasing The supply of suitable external dc voltage is called as biasing. Either forward or reverse biasing is done to the emitter and collector junctions of the transistor. These biasing methods make the transistor circuit to work in four kinds of regions such as Active region, Saturation region, Cutoff region and Inverse active region (seldom used). This is understood by having a look at the following table. EMITTER JUNCTION COLLECTOR JUNCTION REGION OF OPERATION Forward biased Forward biased Saturation region Forward biased Reverse biased Active region Reverse biased Forward biased Inverse active region Reverse biased Reverse biased Cutoff region Among these regions, Inverse active region, which is just the inverse of active region, is not suitable for any applications and hence not used. Active region This is the region in which transistors have many applications. This is also called as linear region. A transistor while in this region, acts better as an Amplifier. This region lies between saturation and cutoff. The transistor operates in active region when the emitter junction is forward biased and collector junction is reverse biased. In the active state, collector current is β times the base current, i.e., $$I_{C}:=:beta I_{B}$$ Where, $I_{C}$ = collector current $beta$ = current amplification factor $I_{B}$ = base current Saturation region This is the region in which transistor tends to behave as a closed switch. The transistor has the effect of its collector and Emitter being shorted. The collector and Emitter currents are maximum in this mode of operation. The figure below shows a transistor working in saturation region. The transistor operates in saturation region when both the emitter and collector junctions are forward biased. As it is understood that, in the saturation region the transistor tends to behave as a closed switch, we can say that, $$I_{C}:=:I_{E}$$ Where $I_{C}$ = collector current and $I_{E}$ = emitter current. Cutoff region This is the region in which transistor tends to behave as an open switch. The transistor has the effect of its collector and base being opened. The collector, emitter and base currents are all zero in this mode of operation. The following figure shows a transistor working in cutoff region. The transistor operates in cutoff region when both the emitter and collector junctions are reverse biased. As in cutoff region, the collector current, emitter current and base currents are nil, we can write as $$I_{C}:=:I_{E}:=:I_{B}:=:0$$ Where $I_{C}$ = collector current, $I_{E}$ = emitter current, and $I_{B}$ = base current. Print Page Previous Next Advertisements ”;

Basic Electronics – Inductors ”; Previous Next Let me introduce you to another important component in the field of Electronics and Electricals, the Inductor. Inductor is a passive two-terminal component that temporarily stores energy in the form of a magnetic field. It is usually called as a coil. The main property of an inductor is that it opposes any change in current. Inductor According to the Faraday’s law of Electromagnetic induction, When the current flowing through an inductor changes, the time-varying magnetic field induces a voltage in the conductor. According to lens law, the direction of induced EMF opposes the change in current that created it. Hence, induced EMF is opposite to the voltage applied across the coil. This is the property of an inductor. The following figure shows how an inductor looks like. An inductor blocks any AC component present in a DC signal. The inductor is sometimes wrapped upon a core, for example a ferrite core. It then looks as in the figure below. The following figure shows an inductor with various parts labelled. Symbols The symbols of various types of inductors are as given below. Storage of Energy One of the Basic properties of electromagnetism is that the current when flows through an inductor, a magnetic field gets created perpendicular to the current flow. This keeps on building up. It gets stabilized at some point, which means that the inductance won’t build up after that. When the current stops flowing, the magnetic field gets decreased. This magnetic energy gets turned into electrical energy. Hence energy gets stored in this temporarily in the form of magnetic field. Working of an Inductor According to the theory of Electromagnetic Induction, any varying electric current, flowing in a conductor, produces a magnetic field around that, which is perpendicular to the current. Also, any varying magnetic field, produces current in the conductor present in that field, whereas the current is perpendicular to the magnetic field. Now, if we consider an inductor which is made up of a conducting coil and when some current passes through the inductor, a magnetic field is created perpendicular to it. The following figure indicates an inductor with magnetic field around it. Now, here we have a varying magnetic field, which creates some current through the conductor. But this current is produced such that it opposes the main current, which has produced the magnetic field. If this current is named as Im which means the current produced due to the magnetic field and the magnetic field is indicated by β, the following figure indicates it. This opposing current gains strength with the varying magnetic field, which gains energy by the input supply frequency. Hence as the input current becomes more and more AC with high frequency, the resulting opposing current also gains its strength in opposite direction to the very cause producing it. Now, this opposing current, tries to stop the high frequency AC to pass through the inductor, which means “blocking of AC”. Print Page Previous Next Advertisements ”;

Basic Electronics – RF Inductors ”; Previous Next RF inductors are the radio frequency inductors, which are used at high resonant frequencies. These can be multilayered coil inductor or a thin film coated ceramic inductor or some wire wound ceramic inductor. The following figure represents few RF inductors. These inductors are characterized by low current rating and high electrical resistance. But as the high frequencies are used here, the wire resistance increases. Also, few effects come into picture because of these high resonant radio frequencies. Let us have a look at them. Skin Effect At high frequencies, the alternating current has a tendency of unequal distribution of current through the conductor. The electric current flows highly at the surface of the conductor than at its center. It gets its energy concentrated in the skin of the conductor, leaving the deep core of the conductor, as shown in the following figure. As the energy gets concentrated at the skin of the conductor, this effect is called as the Skin Effect. Actually this skin effect is caused due to the eddy currents which are produced by the changing Magnetic field, resulting from alternating current. Now-a-days, the conductors carrying higher frequencies are made in the form of tube shape, in order to reduce the weight and cost of the conductors. Proximity Effect Along with the above one, this is another effect, which is observed here. Proximity effect is the one which increases the resistance of the wire at high frequencies. Proximity is the word which says that the effect will be on adjacent wires. The following figure shows the concentration of current on the edges of the adjacent cables. Each turn has some magnetic field which induces eddy currents in the wire that causes the current to be focused on the side of the adjacent wire. With this effect, the effective cross sectional area of the wire gets reduced and its resistance gets increased. Parasitic Capacitance Usually, an inductor internally contains a resistor in series (wire resistance) and a capacitor in shunt (parasitic capacitance). Each turn of winding has slightly different potential, in an inductor. The following figure shows the capacitance effect in an inductor. The two conductors that present in each turn, act as capacitor plates with air as dielectric. A capacitance called as Parasitic Capacitance exists here. In order to avoid this in certain applications, the windings are made far to each other. As the frequency increases, the impedance of the parasitic capacitance decreases and the impedance of inductor increases. Hence the inductor tends to behave like a capacitor. Dielectric losses The current through the conductor of an inductor makes the molecules of the insulators exert energy in the form of heat. The higher the frequency, the greater the heat dissipation will be. Chokes Inductors are also called as chokes. An Inductor blocks AC components and sends DC components through it. Hence as it chokes or stops AC, an inductor can simply be termed as a Choke. A coil of insulated wire is often wound on a magnetic core to form a choke. As the signal frequency increases, the impedance of the choke increases. Due to its reactance, it can limit the amount AC through it. Even though, practically some amount of AC passes through it due to its low electrical resistance. These are mostly used in tube lights and in transformers in electronic applications. Print Page Previous Next Advertisements ”;

Transistor Load Line Analysis ”; Previous Next Till now we have discussed different regions of operation for a transistor. But among all these regions, we have found that the transistor operates well in active region and hence it is also called as linear region. The outputs of the transistor are the collector current and collector voltages. Output Characteristics When the output characteristics of a transistor are considered, the curve looks as below for different input values. In the above figure, the output characteristics are drawn between collector current IC and collector voltage VCE for different values of base current IB. These are considered here for different input values to obtain different output curves. Operating point When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X-axis, which is the cutoff point. When a line is drawn joining these two points, such a line can be called as Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called as Operating point. This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in active region. This can be better understood through the figure below. The load line has to be drawn in order to obtain the Q-point. A transistor acts as a good amplifier when it is in active region and when it is made to operate at Q-point, faithful amplification is achieved. Faithful amplification is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is applied at its input. This is discussed in AMPLIFIERS tutorial. DC Load line When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood as DC condition. Here there will be no amplification as the signal is absent. The circuit will be as shown below. The value of collector emitter voltage at any given time will be $$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$ As VCC and RC are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics. This line is called as D.C. Load line. The figure below shows the DC load line. To obtain the load line, the two end points of the straight line are to be determined. Let those two points be A and B. To obtain A When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as $$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$ $$0:=:V_{CC}:-:I_{C}R_{C}$$ $$I_{C}:=:frac{V_{CC}}{R_{C}}$$ This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure. To obtain B When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as $$V_{CE}:=:V_{CC}:-:I_{C}R_{C}$$ $$=:V_{CC}$$ (As IC = 0) This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure. Hence we got both the saturation and cutoff point determined and learnt that the load line is a straight line. So, a DC load line can be drawn. The importance of this operating point is further understood when an AC signal is given at the input. This will be discussed in AMPLIFIERS tutorial. Print Page Previous Next Advertisements ”;

Basic Electronics – Types of Transformers ”; Previous Next Coming to the classification of transformers, there are many types depending upon the core used, windings used, place and type of usage, voltage levels etc. Single and three phase transformers According to the supply used, the transformers are mainly classified as Single phase and three phase transformers. A normal transformer is a single phase transformer. It has a primary and a secondary winding and it is operated to either decrease or increase the secondary voltage. For a three phase transformer, three primary windings are connected together and three secondary windings are connected together. A single three phase transformer is preferred to three single phase transformers so as to get good efficiency, where it occupies less space at low cost. But due to the transportation problem of heavy equipment, single phase transformers are used in most cases. Another classification of these transformers is Core and Shell type. In Shell type, the windings are positioned on a single leg surrounded by the core. In Core type, they are wounded on different legs. The difference is well known by having a look at the following figure. The classification of transformers can also be done depending upon the type of core material used. These are actually RF transformers, which contain many types such as Air-core transformers, Ferrite core transformers, Transmission line transformers and Balun transformers. Balun transformers are used in RF receiver systems. The main types are the air core and iron core transformers. Air-core Transformer This is a core type transformer in which the windings are wound on a non-magnetic strip. The magnetic flux linkages are made through air as core between the primary and secondary. The following image shows an air-core transformer. Advantages The hysteresis and eddy current losses are low in these Air core transformers. Noise production is low. Disadvantages The reluctance is high in Air core transformers. Mutual inductance is low in Air core compared to Iron-core transformers. Applications Audio frequency transformers. High frequency radio transmissions. Iron Core Transformers This is a core type transformer in which the windings are wound on an iron core. The magnetic flux linkages are made strong and perfect with iron as core material. This is commonly seen in laboratories. The figure below shows an example of iron core transformer. Advantages They have very high magnetic permeability. Iron core transformers has low reluctance. Mutual Inductance is high. These transformers are highly efficient. Disadvantages These are a bit noisy compared to Air core transformers. The hysteresis and eddy current losses are a bit more than Air core transformers. Applications As isolation transformers. High frequency radio transmissions. The transformers are also classified according to the type of core they use. Some transformers use the core immersed in oil. This oil is cooled from outside by various methods. Such transformers are named as Wet core transformers, while the others such as ferrite core transformers, laminated core transformers, toroidal core transformers and cast resin transformers are Dry core transformers. Based on the type of winding technique, we have another transformer which is very popular named as the Auto transformer. Auto Transformer This is type of transformer which is mostly seen in our electrical laboratories. This auto transformer is an improved version of the original transformer. A single winding is taken to which both the sides are connected to power and the ground. Another variable tapping is made by whose movement secondary of the transformer is formed. The following figure shows the circuit of an auto-transformer. As shown in the figure, a single winding provides both primary and secondary in a transformer. Various tapping of secondary winding are drawn to select various voltage levels at the secondary side. The primary winding as shown above is from A to C and the secondary winding is from B to C whereas the variable arm B is varied to get the required voltage levels. A practical auto transformer looks like the figure below. By rotating the shaft above, the secondary voltage is adjusted to different voltage levels. If the voltage applied across the points A and C is V1, then the voltage per turn in this winding will be $$Voltage:per:turn::=::frac{V_{1}}{N_{1}}$$ Now, the voltage across the points B and C will be $$V_{2}::=::frac{V_{1}}{N_{1}}::times::N_{2}$$ $$frac{V_{2}}{V_{1}}::=::frac{N_{2}}{N_{1}}::=::constant:(say:K)$$ This constant is nothing but the turns ratio or voltage ratio of the auto transformer. Print Page Previous Next Advertisements ”;

Basic Electronics – Transformer Efficiency ”; Previous Next When the Primary of a transformer has some voltage induced, then the magnetic flux created in the primary is induced into the secondary due to mutual induction, which produces some voltage into the secondary. The strength of this magnetic field builds up as the current rises from zero to maximum value which is given by $mathbf{frac{dvarphi}{dt}}$. The magnetic lines of flux pass through the secondary winding. The number of turns in the secondary winding determines the voltage induced. Hence the amount of voltage induced will be determined by $$Nfrac{dvarphi}{dt}$$ Where N = number of turns in the secondary winding The frequency of this induced voltage will be same as the frequency of primary voltage. The peak amplitude of the output voltage will be affected if the magnetic losses are high. Induced EMF Let us try to draw some relationship between induced EMF and number of turns in a coil. Let us now assume that both the primary and the secondary coils has a single turn each. If one volt is applied to one turn of the primary with no losses (ideal case) the current flow and magnetic field generated induce the same one volt in the secondary. Hence voltage is same on both sides. But the magnetic flux varies sinusoidally which means, $$phi::=::phi_{max} sin omega t$$ Then the basic relationship between induced EMF and coil winding of N turns is $$EMF:=:turns::times::rate:of:change$$ $$E:=:N frac{dphi}{dt}$$ $$E:=:N:times:omega:times: phi_{max}:times: cos(omega t)$$ $$E_{max}:=:N omega phi_{max}$$ $$E_{rms}:=:frac{N omega}{sqrt{2}}:times:phi_{max}:=:frac{2pi}{sqrt{2}}:times:f:times:N:times:phi_{max}$$ $$E_{rms}:=:4.44:f:N:phi_{max}$$ Where f = flux frequency in Hertz = $frac{omega}{2pi}$ N = number of coil windings ∅ = flux density in webers This is known as Transformer EMF Equation. As alternating flux produces current in the secondary coil, and this alternating flux is produced by alternating voltage, we can say that only an alternating current AC can help a transformer work. Hence a transformer doesn’t work on DC. Losses in Transformers Any Device has few losses in practical applications. The main losses that occur in the transformers are Copper losses, Core losses and Flux leakage. Copper Losses Copper loss is the loss of energy, due to the heat produced by the current flow through the windings of the transformers. These are also called as “I2R losses” or “I squared R losses” as the energy lost per second increases with the square of the current through the winding and is proportional to the electrical resistance of the winding. This can be written in an equation as $$I_{P} R_{P}:+:I_{S} R_{S}$$ Where IP = Primary Current RP = Primary Resistance IS = Secondary Current RS = Secondary Resistance Core Losses Core Losses are also called as Iron Losses. These losses depends upon the core material used. They are of two types namely, Hysteresis and Eddy Current losses. Hysteresis Loss − The AC induced in the form of magnetic flux keeps on fluctuating (like rise and falls) and reversing the direction according to the AC voltage induced. Some energy is lost in the core due to these random fluctuations. Such loss can be termed as Hysteresis loss. Eddy Current Loss − While this whole process goes on, some currents are induced in the core which circulate continuously. These currents produce some loss called as Eddy Current Loss. Actually the varying magnetic field is supposed to induce current only in the secondary winding. But it induces voltages in the nearby conducting materials also, which results in this loss of energy. Flux Leakage − Though the flux linkages are strong enough to produce the required voltage, there will be some flux which gets leaked in practical applications and hence results in the energy loss. Though this is low, this loss is also countable when it comes to high energy applications. Power of a Transformer When an ideal transformer is considered with no losses, the Power of the transformer will be constant, as the product when voltage V multiplied by current I is constant. We can say that the power in the primary equals the power in the secondary as the transformer takes care of that. If the transformer, steps-up the voltage then the current is reduced and if the voltage is stepped-down, the current is increased so as to maintain the output power constant. Hence the primary power equals the secondary power. $$P_{Primary}:=:P_{Secondary}$$ $$V_{P}I_{P}cos phi_{P}:=:V_{S}I_{S}cos phi_{S}$$ Where ∅P = Primary phase angle and ∅S = Secondary phase angle. Efficiency of a transformer The amount or the intensity of Power loss in a transformer, determines the efficiency of the transformer. The efficiency can be understood in terms of power loss between primary and secondary of a transformer. Hence, the ratio of power output of secondary winding to the power input of primary winding can be stated as the Efficiency of the transformer. This can be written as $$Efficiency:=:frac{Power:output}{Power:input}:times:100 %$$ Efficiency is generally denoted by η. The above given equation is valid for an ideal transformer where there will be no losses and the whole energy in the input gets transferred to the output. Hence, if losses are considered and if the efficiency is calculated in practical conditions, the below equation is to be considered. $$Efficiency:=:frac{Power:output}{Power:output:+:Copper:losses:+:Core:losses}:times:100 %$$ Otherwise, it can also be written as $$Efficiency:=:frac{Power:input:-:Losses}{Power:input}:times:100$$ $$1:-:frac{Losses}{Input:Power}:times:100$$ It is to be noted that the input, output and losses are all expressed in terms of power, i.e., in Watts. Example Consider a transformer having input power of 12KW which is rated at 62.5 amps current having equivalent resistance of 0.425ohms. Calculate the efficiency of the transformer. Solution − Given data Input power = 12KW Rated current = 62.5 Amps Equivalent resistance = 0.425 ohms Calculating the loss − The copper loss at rated current is I2R = (62.5)2 (0.425) = 1660W We have $$Efficiency:=:frac{Power:input:-:Losses}{Power:input}:times:100$$ Hence, $$eta:=:frac{12000:-:1660}{12000}:times:100$$ $$eta:=:frac{10340}{12000}:times:100$$ $$eta:=:0.861:times:100:=:86 %$$ Hence the efficiency of the transformer is 86%. Print Page Previous Next Advertisements ”;

Basic Electronics – Fixed Capacitors ”; Previous Next The Capacitors whose value is fixed while manufacturing and cannot be altered later are called as Fixed Capacitors. The main classification of fixed capacitors is done as polarized and non-polarized. Let us have a look at Non-polarized capacitors. Non-Polarized Capacitors These are the capacitors that have no specific polarities, which means that they can be connected in a circuit, either way without bothering about the placement of right lead and left lead. These capacitors are also called as Non-Electrolytic Capacitors. The main classification of Non-Polarized capacitors is done as shown in the following figure. Among the types of capacitors, let us first go through the Ceramic Capacitors. Ceramic Capacitors The common capacitors used among fixed type are Ceramic Capacitors. The Ceramic capacitors are fixed capacitors that have ceramic material as a dielectric. These ceramic capacitors are further classified as class1 and class2 depending upon their applications. For instance, Class1 has high stability and works best for resonant circuit applications, while class2 has high efficiency and gives its best for coupling applications. A hollow tubular or plate like ceramic material such as titanium dioxide and barium titanate is coated with a deposition of silver compound on both walls, so that both sides act as two capacitor plates and ceramic acts as a dielectric. Leads are drawn from these two surfaces and this whole assembly is encapsulated in a moisture-proof coating. The most often used modern ceramic capacitors are Multi-Layer Chip Capacitors (MLCC). These capacitors are made in surface mounted technology and are mostly used due to their small size. These are available in the order of 1ηF to 100µF. Film Capacitors The Film Capacitors are the ones which have a film substance as a dielectric material. Depending upon the type of film used, these are classified as Paper and Metal film capacitors. These film capacitors are both paper dielectric capacitors whereas a paper capacitor uses a waxed paper while a metallic film capacitor uses a metallized paper. The arrangement is almost same as shown below. Paper Capacitors Paper capacitors use Paper as a dielectric material. Two thin tin foil sheets are taken and placed between thin waxed or oiled paper sheets. This paper acts as a dielectric. Now-a-days paper is being replaced by plastic. These sheets are sandwiched and are rolled into a cylindrical shape and encapsulated in a plastic enclosure. Leads are drawn out. The following figure shows an example of Paper Capacitors. Paper capacitors are available in the order of 0.001µF to 2µF and the voltage rating can be as high as 2000volts. These capacitors are useful in high voltage and current applications. Metal Film Capacitors Metal Film capacitors are another type of film capacitors. These are also called as Metal Foil Capacitors or Metallized Paper Capacitors as the dielectric used here is a paper coated with metallic film. Unlike in paper capacitors, a film of Aluminum or Zinc is coated on a paper to form a dielectric in this metallic film capacitors. Instead of Aluminum sheets placing between papers, the paper itself is directly coated here. This reduces the size of the capacitor. The Aluminum coating is preferred over zinc coating so as to avoid destruction of capacitor due to chemical reduction. The Aluminum coated sheets are rolled in the form of a cylinder and leads are taken. This whole thing is encapsulated with wax or plastic resin to protect the capacitor. These capacitors are useful in high voltage and current applications. Other Capacitors These are the miscellaneous capacitors that are named after the dielectric materials used. This group includes Mica Capacitors, Air Capacitors, Vacuum Capacitors and Glass Capacitors etc. Mica Capacitors The Mica Capacitors are made by using thin Mica sheets as dielectric materials. Just like paper capacitors, thin metal sheets are sandwiched with mica sheets in between. Finally the layers of metal sheets are connected at both ends and two leads are formed. Then the whole assembly is enclosed in plastic Bakelite capsule. The following image shows how a Mica capacitor looks like. Mica Capacitors are available in the range of 50pF to 500pF. The Mica capacitors have high working voltage up to 500volts. These are most commonly used capacitors for electronic circuits such as ripple filters, Resonant circuits, Coupling circuits and high power, high current RF broadcast transmitters. Air Capacitors The Air Capacitors are the ones with air as dielectric. The simplest air capacitors are the ones with conducting plates having air in between. This construction is exactly the same as the variable tuning capacitor discussed above. These capacitors can be fixed and variable also but fixed are very rarely used as there are others with superior characteristics. Vacuum Capacitors The Vacuum Capacitors uses high vacuum as dielectric instead of air or some other material. These are also available in fixed and variable modes. The construction of these capacitors is similar to vacuum tubes. They are mostly seen in the form of a glass cylinder which contain inter-meshed concentric cylinders. The following image shows a variable vacuum capacitor. The following image shows how a fixed vacuum capacitor looks like − Variable vacuum capacitors are available at a range of 12pF to 5000pF and they are used for high voltage applications such as 5kV to 60kV. They are used in main equipment such as high power broadcast transmitters, RF amplifiers and large antenna tuners. Glass Capacitors Glass capacitors are very exclusive ones with many advantages and applications. As all of the above types, here glass is the dielectric substance. Along with glass dielectric, Aluminum electrodes are also present in these capacitors. Plastic encapsulation is done after taking out the leads. The leads can be axial leads or tubular leads. There are many advantages of a glass capacitor such as − The temperature coefficient is low. These are Noise-free capacitors. They produce high quality output with low loss. They have the capability of handling high operating temperatures. These capacitors can handle large RF currents. There