CH.2

1. CH.2 Electronic Components Eng.MohammedAlsumady

2. Basics of Electronic Components • An electronic component is any device that handles electricity. Electronic components come in many different shapes and sizes, and perform different electrical functions depending upon the purpose for which they are used. Accordingly, electronic equipments make use of a variety of components.

3. Active Vs Passive Components • There are two types of components: passive components and active components. • Passive Components : A passive device is one that contributes no power gain (amplification) to a circuit or system. It has no control action and does not require any input other than a signal to perform its function. Since passive components always have a gain less than one, they cannot oscillate or amplify a signal. A combination of passive components can multiply a signal by values less than one; they can shift the phase of a signal, reject a signal because it is not made up of the correct frequencies, and control complex circuits, but they cannot multiply by more than one because they basically lack gain. Passive devices include resistors, capacitors and inductors. • Active Components: Active components are devices that are capable of controlling voltages or currents and can create a switching action in the circuit. They can amplify or interpret a signal. They include diodes, transistors and integrated circuits. They are usually semiconductor devices.

4. Discrete vs Integrated Circuits • When a component is packaged with one or two functional elements, it is known as a discrete component. For example, a resistor used to limit the current passing through it functions as a discrete component. On the other hand, an integrated circuit is a combination of several interconnected discrete components packaged in a single case to perform multiple functions. A typical example of an integrated circuit is that of a microprocessor which can be used for a variety of applications.

5. Component Leads • Components can be classified into two types on the basis of the method of their attachment to the circuit board. Through-hole components are those components which have leads that can be inserted through mounting holes in the circuit board. On the other hand, surface mount components are so designed that they can be attached directly on to the surface of the board. • Two types of lead configurations are commonly found in discrete components. The components with axial leads have two leads, each extending from each side of the component like arms. These leads need to be bent for insertion through the holes of a printed circuit board. The other configuration of leads in the components is radial where the leads emanate from the bottom of the components like legs.

6. In the case of integrated circuits, there are a large number of leads which are placed in a row in single line (single in-line package), or in two parallel rows (dual in-line package) These leads can be inserted in the through-holes in the PCB. High density integrated circuits now come in the form of pin-grid arrays that have several rows of round pins extending from the bottom of the component. Leadless components are also available in the surface mount devices in which no metal leads stick out of the component body. They are attached to a circuit board using some type of metallized termination.

7. Classification of components based on the lead configuration (a) through-hole component (b) surface mount component (c) component with axial leads (d) components with radial leads (e) single-in-line package (f) dual-inline package (g) pin grid arrays (h) leadless components.

8. Polarized Capacitors Diodes

9. Component Symbols • Each discrete component has a specific symbol when represented on a schematic diagram. These symbols have been standardized and specified in the Institute of Electrical and Electronics Engineers (IEEE) standard 315 and 315A . The integrated circuits are generally represented by a block in the schematic diagram and each one does not have a specific symbol.

10. Resistors • The most commonly used component in an electronic assembly is the resistor. It is a passive component which exhibits a controlled value of resistance across its two leads. Resistance, by definition, is the opposition to the flow of current offered by a conductor, device or circuit. • Types of Resistors: There are two classes of resistors; fixed resistors and variable resistors. They are also classified according to the material from which they are made. The most commonly used types of resistors are detailed below: • Carbon Resistors: They are made either by mixing finely ground carbon with a resin binder and an insulating filler or by depositing carbon film onto a ceramic rod. Most carbon film resistors have low stray capacitance and inductance, so they are usable at higher frequencies. However, their accuracy is limited to 1per cent. In addition, carbon film resistors tend to drift with temperature and vibration. • Metal Resistors: They are made of metal film on ceramic rod or metal glaze (a mixture of metals and glass) or metal oxide (a mixture of a metal and an insulating oxide). Metal film resistors are more stable under temperature and vibration conditions having tolerances approaching 0.5 per cent. Precision metal film resistors with tolerances below 0.1 per cent are also commercially available.

11. Wire-wound Resistors: They are made by winding resistance wire onto an insulting former. They can be made to very close tolerances. • Thick Film Resistor Networks: Thick film resistor networks comprise precious metals in a glass binding system which have been screened on to a ceramic substrate and fired at high temperatures. These networks provide miniaturization, have rugged construction, are inherently reliable and are not subject to failures. Networks comprising 1 to 50 resistors, 5 to 20 being typical, are commercially available. Single in-line (SIL) packages, DIP (dual-in-line package) and square packages are commonly available. The resistance network is made with many resistors of the same value. One side of each resistor is connected with one side of all the other resistors inside. A common example of this type of arrangement is to control the current in a circuit powering many light emitting diodes (LEDs). Alternatively, some resistor networks have a “4S” printed on the top of the resistor network. The 4S indicates that the package contains four independent resistors that are not wired together inside. The housing has eight leads instead of nine. • The reason for using such a large range of materials in the construction of resistors is simply a trade-off between cost and a particular performance characteristic, be it low noise, high stability or small size.

12. Carbon film resistors From the top of the photograph 1/8W 1/4W 1/2W Metal film resistors From the top of the photograph 1/8W (tolerance ± 1%) 1/4W (tolerance ± 1%) 1W (tolerance ± 5%) 2W (tolerance ± 5%)

15. Characteristics • The main parameters that define a resistor are detailed below: • Resistance: This is the nominal value of resistance between the two leads of the resistor when measured at 25°C. • Tolerance: This is the maximum deviation of value of resistance from the nominal value, usually given as a percentage of the nominal value. For example: for a ± 5% tolerance of a resistor of 500 ohms, the value may vary from 475 to 525 ohms. • Power Rating: This refers to the maximum power that a resistor can dissipate continuously at a temperature of 70°C. This is expressed in watts. Most common resistors are normally 0.25 W and 0.5 W. Modern digital circuits have low current requirements and usually use 0.125 W resistors. Carbon composition and metal resistors are generally available in power ratings of 250 mW, 500 mW, 1 W and 2 W. For dissipating more heat, wire-wound resistors are mostly employed, with the power ratings being up to 25 watts. • Temperature Coefficient: It expresses the extent to which the value of resistance will change with temperature. It is usually expressed in parts per million of the nominal value per degree Celsius (ppm/°C). The temperature coefficient of the most commonly used resistors is in the range of 25 to 500 ppm°C. Carbon composition resistors have poor stability and relatively poor temperature co-efficient, which is of the order of 1200 ppm/°C. Metal film resistors exhibit comparatively low temperature coefficient (±250 ppm/°C) and good stability, both when stored and under operating conditions.

16. The critical temperature in a resistor is the hot spot temperature, which is the sum of the ambient temperature and the temperature rise caused due to the power being dissipated. Due to uniform construction of the resistor, the maximum temperature is in the middle of the resistor body, this temperature, which is known as the hot spot temperature. • Stability or Drift: This is a measure of how much the value of a resistor changes with respect to time because of aging. It is normally measured as a percentage change after 1000 hours of operation at 70°C. The stability of a resistor is defined as the percentage change of resistance value with time. It depends upon the power dissipation and ambient temperature. • Maximum Voltage: It represents the maximum dc voltage, which can be safely applied to a resistor on a continuous basis. For most resistors of value around 100 ohms or more, the maximum dc voltage is 1000 volts. Voltage transients above the rated value may induce permanent changes in resistance values. • Identification: The value of resistance is either printed in numbers or is put in the form of colourcoded bands around the body. In the colour code, each number from 0 to 9 has been assigned a colour.

17. The colour code comes in the form of four-band. The first band closest to the end of the resistor represents the first digit of the resistance value. The second band gives the second digit and the third band gives the number of zeros to be added to the first two digits to get the total value of theresistor. The fourth band indicates the tolerance. If the fourth band is absent, the tolerance is ±20%. • In the five-bandcolour code, the first three bands indicate the value, the fourth band indicates the multiplier factor, and the fifth band, the tolerance of the resistor. • In the six-band colour code, the sixth band indicates the temperature coefficient of variation of resistance in terms of parts per million per degree centigrade (ppm/°C). • When the value and tolerance of the resistor are printed on the resistor body, the values of the tolerances are coded as follows: F = ± 1%, G = ± 2%., j = ± 5%, K = ± 10%, M = ± 20%±

18. Variable Resistors or Potentiometers • Variable resistors basically consist of a track of some type of resistance material with which a movable wiper makes contact. Variable resistors or potentiometers (‘pots’ as they are popularly called) can be divided into three categories depending upon the resistive material used. (a) carbon composition (b) multi-turn cermet (c) wire wound. • Carbon: Carbon potentiometers are made of either moulded carbon composition giving a solid track or a coating of carbon plus insulating filler onto a substrate. • Cermet: Cermet potentiometers employ a thick film resistance coating on a ceramic substrate. • Wire-wound: Nichrome or other resistance wire is wound on to a suitable insulating former for the construction of wire-wound potentiometers.

19. Potentiometers can be categorized into the following types depending upon the number of resistors and the control arrangement used: • Single Potentiometers: Potentiometer control with one resistor. • Tandem Potentiometers: Two identical resistor units controlled by one spindle. • Twin Potentiometers: Two resistor units controlled by two independent concentric spindles. • Multi-turn Potentiometers: Potentiometer with knob or gear wheel for resistance adjustment; they may have up to 40 rotations of spindle. • Potentiometers are typically used for setting bias values of transistors, setting time constants of RC timers, making gain adjustments of amplifiers, and carrying current or voltage in control circuits. Therefore, they are packaged in such a way that they are compatible with PCB mounting applications.

21. Variable resistors can be constructed to follow one of the following laws: • Linear: The resistance of the pot is distributed evenly over its entire length. • Log: The resistance of the pot varies so as to follow the logarithmic law. In these pots, when the wiper is turned, the resistance increases (from zero) very slowly and gradually until about the half way mark. From then onwards, as the wiper shaft is turned further, the resistance will increase much more rapidly in comparison with the first half of the pot-rotor rotation. • Sine-Cosine Potentiometers: As the name implies, the variation of resistance over the track, when the wiper moves, follows the sine-cosine law. The total operative track length over 360 degrees of rotation is divided into four quadrants of 90 degrees each.

22. Light-dependent Resistors (LDRs) • Light-dependent Resistors are made of cadmium sulphide. They contain very few free electrons when kept in complete darkness and therefore, exhibit very high resistance. When subjected to light, the electrons are liberated and the material becomes more conducting. When the light is switched off, the electrons are again recaptured and the material becomes less conducting or an insulator. The typical dark resistance of LDRs is 1 MOhms to 10 MOhms. Its light resistance is 75 to 300 ohms. The LDRs take some finite time to change its state and this time is called the recovery time. The typical recovery rate is 200kOhms/sec.

23. Thermistors • Thermistors are resistors with a high temperature co-efficient of resistance. Thermistors with negative temperature co-efficient (fall in resistance value with an increase in temperature) are the most popular. They are oxides of certain metals like manganese, cobalt and nickel. Thermistors are available in a wide variety of shapes and forms suitable for use in different applications. They are available in the form of disks, beads or cylindrical rods. Thermistors have inherently non-linear resistance–temperature characteristics. However, with a proper selection of series and parallel resistors, it is possible to get a nearly linear response of resistance change with temperature over a limited range. • Thermistors with a positive thermo-resistive co-efficient are called posistors. They are made from barium titanate ceramic and are characterized by an extremely large resistance change in a small temperature span. Thermistors have various applications such as excess current limiters, temperature sensors, protection devices against over-heating in all kinds of appliances such as electric motors, washing Machines.

24. Capacitors • A capacitor, like a resistor, is also a passive component, which can be used to store electrical charge. Capacitors find widespread applications in the electrical and electronics fields in the form of: • Ripple filters in power supplies; • Tuning resonant circuits, oscillator circuits; • Timing elements in multi-vibrators, delay circuits; • Coupling in amplifiers; • De-coupling in power supplies and amplifiers; and • Spark suppression on contacts on thermostats and relays. • A capacitor (also called a ‘condenser’) consists of two facing conductive plates called electrodes, which are separated by a dielectric or insulator. The dielectric can be made of paper, mica, ceramic, plastic film or foil. To make a practical capacitor, a lead is connected to each plate or electrode. The charge Q which can be stored in a capacitor, when connected to a voltage V across it, is given by: Q = CV

25. Capacitance is measured in farads. A capacitor has a capacitance of one farad when one coulomb charges it to one volt. The farad is too large a unit. The usual sub-units used are microfarad and the picofarad. The value of a capacitor is indicated on the body of the capacitor, either in words or in a colour code. • The value of a capacitor is also sometimes written on the body in the form of numbers. Values beginning with decimals are usually measured in microfarads (µF), while all other values are assigned to be in picofarads (pF). Four-digit values are also indicated in picofarads but without a multiplier. • Some capacitors are coded with a three-digit number which is similar to the colour band system, with a value and multiplier numbers. For example, 203 means that 2 and 0 are attached to 3 zeros and the value of the capacitor would be 20,000 pF or .02 µF. • The tolerance letter codes indicate: F = ±1%, G = ±2%, J = ±5%, K = ±10%, M = ±20%

26. Types of Capacitors • Capacitors are categorized into various types depending upon the dielectric medium used in their construction. The size of the capacitor, its tolerance and the working voltage also depend upon the dielectric used. • Paper Capacitors: Paper capacitors make use of thin sheets of paper wound with thin aluminium foils. In order to increase the dielectric strength and to prevent moisture absorption, the paper is impregnated with oils or waxes. The capacitor is normally encapsulated in resin. Paper capacitors tend to be large in size due to the thickness of paper and foil. The thickness is reduced in metallized capacitors by directly depositing the aluminium on the dielectric. • Typical range : 10 nF to 10 µF • Typical dc voltage : 500 V(max.) • Tolerance : ± 10%

27. Mica Capacitors: A mica capacitor is made by directly metallizing the thin sheets of mica with silver and stacking together several such sheets to make the complete capacitor. The assembly is encapsulated in resin or moulded in plastic. • Typical range : 5 pF to 10 nF • Typical dc voltage : 50 to 500 V • Tolerance : ± 0.5% • Ceramic Capacitors: Ceramic capacitors generally employ barium titanate as the dielectric medium. However, low-loss ceramic capacitors use steatite, which is a natural mineral. A thin plate of ceramic is metallized on both sides and the connecting leads are soldered to it. The body is coated with several layers of lacquer. Modern ceramic capacitors of the monolithic type are made of alternate layers of thin ceramic dielectric and electrodes, which are fired and compressed to form a monolithic block. These capacitors have a comparatively small size. • Typical range : (a) Low loss (steatite) 5 pF to 10 nF • (b) Barium titanate 5 pF to 1 mF • Typical voltage range for both: 60 V to 10 kV • Tolerance : ± 10% to ± 20%

28. Plastic Capacitors: The construction of plastic capacitors is very similar to that of paper capacitors. They are of both foil and metallized types. Polystyrene film or foil capacitors are very popular in applications requiring high stability, low tolerances and low temperature co-efficient. However, they are bulky in size. For less critical applications, metallized polyethylene film capacitors are used. They are commonly referred to as ‘polyester capacitors’. • Electrolytic Capacitors: High value capacitors are usually of electrolytic type. They are made of a metal foil with a surface that has an anodic formation of metal oxide film. The anodized foil is in an electrolytic solution. The oxide film is the dielectric between the metal and the solution. The high value of capacity of electrolytic capacitors in a small space is due to the presence of a very thin dielectric layer. Electrolytic capacitors are of the following types: • Aluminum • Tantalum • Electrolytic capacitors exhibit a very wide range of tolerances, typically ranging from –20 to +50%. They are usually polarized. Care must be taken not to reverse the voltage applied across it. If a reverse voltage is applied, the dielectric will be removed from the anode and a large current will flow as oxide is formed on the cathode. Sometimes the gases released from the capacitor may build up and cause the capacitor to explode and damage other parts of the circuit.

29. Packages • Capacitors are available in a large variety of packages, shapes and dimensions. The most common packages are axial, disc, rectangular, tubular, etc.

30. Performance of Capacitors • The important parameters which characterize a capacitor are: • Capacitance: This is the nominal value of a capacitor, measured in Farads (or its sub-multiples) at 25 °C. • Tolerance: This refers to the deviation of the actual value of a capacitor from its nominal value. Different types of capacitors have different values of tolerance. • Working Voltage: This is the maximum voltage which can be applied continuously across the capacitor. This is indicated as ac or dc. The maximum voltage that causes permanent damage in the dielectric is referred to as breakdown voltage. This is generally twice the working voltage. • Temperature Coefficient: It indicates the change in the value of capacitance with temperature and is expressed as parts per million per degree Celsius (ppm/C). • DC Leakage: The amount of current which flows through a charged capacitor because of losses due to the conductivity of the dielectric. • Parasitic Effects: The capacitor impedance is a function of frequency: at low frequencies, the capacitor blocks signals and at high frequencies, the capacitor passes signals. Depending on the circuit configuration, the capacitor can pass the signal to the next stage or it can shunt it to ground. All capacitors have a self-resonant frequency. Aluminium electrolytic capacitors have a very low self-resonant frequency, so they are not effective in high frequency applications above a few hundred kHz. Tantalum capacitors have a mid-range self-resonant frequency. Thus, they are found in applications up to several MHz and beyond that, ceramic and mica capacitors are preferred because they have self-resonant frequencies ranging into hundreds of MHz. Very low frequency and timing applications require highly stable capacitors. The dielectric of these types are made from paper, polypropylene, polystyrene and polyester. They exhibit low leakage current and low dielectric absorption.

31. The dissipation factor (DF) is mathematically defined as R/X where R is the resistance in the capacitor and X is the reactance of the capacitor. The higher the R, the higher would be the DF and poorer the capacitor. From the formula, DF = R/X, it is clear that DF is an inverse function of X. As X goes down, DF goes up and vice versa. DF varies proportionately with frequency, which shows that DF is a function of the test frequency. • The quality factor Q serves as a measure of the purity of a reactance, i.e. how close it is to being a pure reactance i.e. having no resistance. This represents as the ratio of the energy stored in a component to the energy dissipated by the component. Q is a dimensionless unit and is expressed as Q = X/R. However, Q is commonly applied to inductors; for capacitors the term more often used to express purity is dissipation factor (DF). This quantity is simply the reciprocal of Q.

32. Variable Capacitors • Variable capacitors are constructed by using any one of the dielectrics like ceramic, mica, polystyrene or teflon. Basically, a variable capacitor has a stator and a rotor. The area of the stator is fixed and turning the rotor from 0° to 180° varies the amount of plate surface exposed, thereby varying the value of the capacitor. In most variable capacitors, the change in capacitance is linear throughout the rotation of the rotor. • Variable capacitors are available in the following two configurations: • Button type: This has a variable rotor. • Tubular type: This has an adjustable core. • It may be noted that adjustments made with a variable capacitor by using a metal screwdriver will alter when the screwdriver is lifted from the turning screw. This is because placing the metal screwdriver on this screw changes the effective area of the metal plated surface of either the stator or, more often, the rotor. In such a case, the use of a non-metallic screwdriver is recommended.

34. Inductors • Inductance is the characteristic of a device which resists change in the current through the device. Inductors work on the principle that when a current flows in a coil of wire, a magnetic field is produced, which collapses when the current is stopped. The collapsing magnetic field produces an electromotive force which tries to maintain the current. When the coil current is switched, the induced EMF would be produced in such a direction, so as to oppose the build-up of the current. • The unit of inductance is Henry. An inductance of one Henry will induce a counter emf (electromotive force) of one volt when the current through it is changing at the rate of one ampere per second. Inductances of several Henries are used in power supplies as smoothing chokes, whereas smaller values (in the milli-or micro-Henry ranges) are used in audio and radio frequency circuits.

35. The inductors are also sometimes called coils. The Inductors are available in many sizes and shapes. The value of an inductor may be printed on the component body or it may be printed with color bands , much in the same way as a resistor. For example, if the first and second bands of an inductor are red (value 2) and the third band is orange (value 3), the value of the inductor is 22,000 µH. A fourth silver band will indicate its tolerance as ±10%.

36. The primary use of an inductor is filtering. There are two very different types of filter inductors: the high current inductors wound around a large core are used in power supply filters, and the low current air core inductors are used in signal filters. The basic components of an inductor are the former (or bobbin), winding wire and the core material. Bobbins are normally made of moulded plastic and carry the wire and the core. Winding is usually copper wire whose diameter is calculated to keep the temperature rise under full load to an acceptable level. The core material can be laminated steel, powdered iron or ferrite. The shape of the core is also variable. • The toroidal coil consists of copper wire wrapped around a cylindrical core. It is possible to make it so that the magnetic flux which occurs within the coil doesn’t leak out, the coil efficiency is good, and that the magnetic flux has little influence on other components. Toroids are the most efficient cores in ferrites, but they are difficult to wind. • Air core coils are frequently used at very high frequencies.

37. High current inductors require cores to keep the losses within acceptable limits and to achieve high performance. The cores are big and heavy, so they have large weight and size. Switching power supplies require extensive inductors or transformers to control the switching noise and filter the output voltage waveform. • Low current inductors are used for filters in signal processing circuits. An inductive/capacitive filter has sharper slopes than a resistive/capacitive filter, and is thus a more effective filter in some applications. In general, inductors are rarely seen outside power circuits. • The range of inductor style and shape is considerably larger than for either capacitors or resistors. This is because many organizations need to wind their own inductors to meet their specific demands, which could be for RF coils, audio filters, power supply chokes, etc. • Variable inductors in which the inductance value can be adjusted are also available. The ferrite core of the inductor is made like a screw. The core can be made to move in and out of the inductor by turning it with a screwdriver. A special plastic screwdriver is better to use for adjustment of the coils.

39. Diodes • A diode is an active component through which the current flows more easily in one direction than in the other. It is made from semiconductor material. The main functions of the diode in a circuit are to act as a switching device, a detector or a rectifier. • Since the diode is a two-element device, its symbol shows the two electrodes. The cathode and anode ends of metal encased diodes can be identified on the body. The arrow head of the symbol points in the direction of conventional current flow. In case of glass encased diodes, the cathode end is indicated by a stripe, a series of stripes or a dot. For most silicon or germanium diodes with a series of stripes. • Conventional diodes normally show a low value of forward resistance and a very high value of reverse resistance. The variation in resistance is due to the non-linear voltage/current characteristics of the diode.

40. Signal diodes are general purpose diodes, which find applications involving low currents and a wide range of voltages, sometimes extending upto 50 kV. Switching diodes change their state from conducting to non-conducting state and vice versa in a very short time when the voltage is reversed. Rectifiers are similar to signal diodes, but are more suitable for high currents. • Low and medium power diodes are usually available in axial packages whereas high power diodes are available in a large variety of packages of a vast range of shapes and sizes. Very high power diodes have a thread for mounting on to a PCB or a heat sink. Diode arrays or networks, containing up to 48 devices are also available in packages similar to integrated circuits. • Devices containing four diodes in one package are called ‘diode bridges’. Diode bridges with large current capacities require a heat sink. Typically, they are screwed to a piece of metal or the chassis of the equipment in which they are used. The heat sink allows the device to radiate excessive heat.

41. Special Types of Diodes • there are many other types of diodes which have special characteristics. Following is a description and characteristics of some of the special types of diodes. • Zener Diode: at some specific value of reverse voltage, a very rapid increase occurs in reverse current. This potential is called breakdown avalanche or the zener voltage and may be as low as 1 volt or as high as several hundred volts, depending upon the construction of the diode. • A zener diode has very high resistance at bias potentials below the zener voltage. This resistance could be several Megohms. At zener voltage, the zener diode suddenly shows a very low resistance, say between 5 and 100Ω. A zener diode behaves as a constant voltage source in the zener region of operation, as its internal resistance is very low. The current through the zener diode is then limited only by a series resistance . The value of series resistance is such that the maximum rated power rating of the zener diode is not exceeded. In order to help in distinguishing the zener diode from a general purpose diode, the former is usually labeled with its specified breakdown voltage. Since this voltage is required in the circuit design, the value is generally indicated on the diode. For example, some common values are 6.8 V, 7.2 V, 9.6 V etc.

42. Varactor Diode A varactor diode is a silicon diode that works as a variable capacitor in response to a range of reverse voltage values. Varactors are available with nominal capacitance values ranging from 1 to 500 pF, and with maximum rated operating voltages extending from 10 to 100 volts. They mostly find applications in automatic frequency control circuits. In a typical case, a varactor shows 10 pF capacitance at reverse voltage of 5 volts and 5 pF at 30 volts.

43. Varistor • A varistor is a semiconductor device that has a voltage-dependent non-linear resistance which drops as the applied voltage is increased. A forward biased germanium diode shows such types of characteristics and is often used in varistor applications. • Symmetrical varistor arrangements are used in meter protection circuits wherein the diodes bypass the current around the meter regardless of the direction of current flow. If the meter is accidentally overloaded, varistors do not permit destructive voltages to develop across the meter.

44. Light Emitting Diodes (LED) • A light emitting diode is basically a pn junction that emits light when forward biased. LEDs are available in various types and mounted with various colored lenses like red, yellow and green. They are used mostly in displays employing seven segments that are individually energized to form alphanumeric characters. • LED displays are encountered in test equipment, calculators and digital thermometers whereas LED arrays are used for specific applications such as light sources, punched tape readers, position readers, etc. Electrically, LEDs behave like ordinary diodes except that their forward voltage drop is higher. For example, the typical values are; IR (infra-red): 1.2 V, Red: 1.85 V, Yellow: 2 V, Green: 2.15 V. Further, the actual voltages may vary depending upon the actual technology used in the LED.

46. Photodiode • A photodiode is a solid state device, similar to a conventional diode, except that when light falls on it ( pn junction), it causes the device to conduct. It is practically an open circuit in darkness, but conducts a substantial amount of current when exposed to light

47. Tunnel Diode (TD) A tunnel diode is a pn junction which exhibits a negative resistance interval. Negative resistance values range from 1 to 200 ohms for various types of tunnel diodes.

48. Transistors • Bipolar Transistors: • The most commonly used semiconductor device is the transistor having the characteristic to control voltage and current gain in an electronic circuit. These properties enable the transistor to provide amplification, rectification, switching, detection and almost any desired function in the circuit. It is the basic device of all solid state electronics, both as a single component or as an element of integrated circuit. • All transistors have leakage current across their reverse-biased base-collector diodes. For silicon transistors, this current is more than several nanoamperes. In germanium transistors, the leakage current may even be several microamperes. Leakage current increases with temperature and doubles about every 10 °C.

49. More than 500 packages of transistors are listed in the component manufacturers’ catalogues. However, only about 100 types are in common use. Metallic packages (TO-3, TO-5 and TO-18) have been in use for a long time. However, they have been mostly replaced in low and medium power applications by cheap plastic packages due to the low cost of the latter. For high power applications, however, metallic packages, both stud or bolt type, are still common, though flat type packages are being replaced by plastic versions, with metallic tabs to improve heat dissipation.

50. Power Transistors • The junctions of the power transistors have comparatively larger areas than small signal transistors and have the following characteristics: • Forward resistance values are generally lower than those for small signal silicon transistors. • Similarly, they have lower reverse resistance values. • Power transistors are usually mounted on the heat sinks or heat radiators. They are sometimes mounted on the chassis using silicone grease to increase heat transfer.