Circuit designing and assembling is an interesting hobby. A large number of electronic components are now available in the market. For a beginner, it is very difficult to identify the components through its value and more difficult task is identification of pin outs. To get an idea about the components we have to search for the data sheets and books. To overcome such a time consuming task, a ready to use guide is given here. It will guide you to select the appropriate components and you can easily design a circuit without much effort.
As you know the resistor is an inevitable part of a circuit. It is meant for reducing the current and voltage in the circuit parts. Resistors are identified using the standard colour code chart. A simple trick can be used to identify the resistor value range. The third colour band on the body of the resistor represents the multiplier value. So by identifying the third colour, it easy to know the value in range.
A capacitor is a body which can store an electrical charge. It consists of 2 conducting plates facing each other and separated by an insulating material. This insulating material is also called dielectric material. When a charge is stored in one plate, an equal and opposite charge is inducted on the other plate and thus a potential difference is set up between the plates.
The unit of measurement for capacitance is Farad but this unit is much too large for practical work. It is usually measured in microfarads (uF) or picofarads (pF). The formula of calculating capacitance is
C = [(0.224 KA) (n-1)]/d
C = capacitance in pF
K = dielectric constant of material between plates
A = area of one side of the plates square inches
d = separation of plate in inches
n = number of plates
The potential difference V developed when a charge Q is stored depends directly on the value of Q and inversely with the capacitance C of the cap.
V = Q/C
Capacitors are used for:
DC Voltage Rating
The DC working voltage of a cap. is the maximum voltage which may be applied continuously on the electrodes of the cap. at the upper limits of the working temperature range. The peak value of an alternating voltage should not exceed this rating and have to be derated according the the FMEA as recommended in FMEA.
The dielectric of a practical cap. introduces power losses which can be represented by a small resistance connected in series with the cap. The insulating resistance is often greater than 3,000 Meg ohm.
Types of Capacitors
There are many different types of cap. that are used for different types of applications. They are electrolytic cap., ceramic cap., tantalum cap., polyester cap., polystyrene cap. and safety cap.(namely X and Y types of cap.).
Electrolytic cap. have leads that are marked with + or – signs. They have polarity and must be connected with the correct polarity. The values of the capacitance and voltage rating are printed with on its body. The voltage rating can range from 5V up to 440V DC. Generally this type of capacitor is used as smoothing cap. in power supply regulation. The bigger the value of the cap. is, the less ripple the DC supply that has been rectified will be.
Electrolytic capacitors have value printed on its body. Pins can be easily identified. Large pin is positive. Moreover a black band is printed near the negative terminal to identify the polarity. Do not change the polarity. Capacitor will explode. In Disc capacitors, only a number is printed on its body so it is very difficult to determine its value in Pf, KPF, UF, N etc. In some capacitor, its value is printed in UF eg.0.1 in some others EIA code is used e.g. 104. The following tricks can solve the problem.
Polyester capacitors have five colour bands similar to the resistor colour code value.
Transistors may be in plastic or metal can packages. Value of the transistor is printed on its body. Pin numbers are generally assigned as 1, 2 and 3 from the facing side. General purpose NPN transistors have pins 1 – Collector, 2 – Base, 3 – Emitter. That is CBE. In PNP types the pins are reversed. 1 – Emitter, 2 – Base and 3 – Collector. That is EBC. Metal can transistors have a small projection in the rim of the body. The pin close to it is the emitter. Pin opposite to the emitter is Collector and the middle pin is base. Pin assignment of some common transistors is given below.
Remote operation of TV is based on Infrared transmission and reception. Infrared is a form invisible light with a wavelength of 950 nano meters. Human eye cannot sense this wavelength so that infrared rays cannot be detected visually. The remote handset is used to emit modulated infrared light using an infrared diode. This diode emits pulsed infrared waves in a coded form at a frequency of 38 kHz By pressing each button in the remote handset, it is possible to emit infrared rays at a particular coded form. This helps to control the functions of the TV precisely.
Inside the TV there is an Infrared receiver circuit. It consists of an infrared sensor – TSOP 1738. This sensor is sensitive only to pulsed infrared rays at the 38 kHz frequency and not other forms of light. The infrared sensor has a black covering which filter visible light and allow only the infrared to enter. It has an FET bas signal amplifier. The modulated infrared rays from the handset are received by the sensor and it amplifies the signal. These signals are used for controlling different functions of the TV.
Batteries can be divided into two categories. The primary type is intended for one time use only and is disposed after the charge has dropped to a level that cannot be used. Primary type should not be discharged as heat will be generated within sealed cells. It will also damage the equipment as a consequent of fluid leakage. The storage or secondary type can be recharged many times and is reusable. The rating of its capacity is ampere hours (Ah) which is a product of current drain and time.
Carbon zinc is the most common primary cell in which the chemical oxidation converts the zinc into salts and electricity. When there is no current flowing, the oxidtion stops. If keep for a long period of time, the stored batteries will degrade and dry out where it will no longer able to supply the desired current. The time taken for the degradation without being used is called shelf life. It has a nominal voltage of 1.5V.
Alkaline types have longer capacity at low temperatures. Lithium type have nominal voltage of 3V/cell and has the best capacity, discharge, shelf life and temperature characteristics. Its setback is the high cost.
Silver Oxide and Mercury has voltages of 1.5 V and 1.4 V respectively and are used where constant voltage is desired at low currents over a long period of time. Their main used and applications are in hearing aids.
The most common type is nickel-cadmium (Ni-Cd) type with a nominal voltage of 1.2V/cell. If used carefully, it can be rechargeable up to 500 times compared to alkaline type which is 50 times or so. The most widely used storage type is the lead-acid type in automobile. The Lead Acid battery is made up of plates, lead, and lead oxide with a 35% sulfuric acid and 65% water solution.
Gas escaping from it may be explosive and always keep flame away. It should not be subjected to unnecessary heat, vibration or physical shock. Frequent inspections for leak are recommended. The electrolyte is chemically active and conductive and may ruin electrical equipment if leaks occurred. Its acidity may be neutralized with sodium bicarbonate or baking soda.
In order to ensure that all the cells in NiCd reach a fully charged condition, it should be charged by a constant current of 0.1 C current levels. It is around 50 mA for a AA size cells. Charging should be terminated after 15 hours at the slow rate. A built in circuit that will stop charging when 1.43V/cell is reached will enhance the life of the battery.
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In small signal application of which the current requirement is less than 100mA, 1N4148 is a typical choice. It has a forward voltage drop of 0.7V and is made from Silicon type.
In rectifier circuit applications, the typical ones used are 1N4001 to 1N4007 for current rating of 1A and 1N5401-1N5408 for current rating up to 3A. The table below shows the devices and their maximum reverse voltage ratings.
Diodes as Switches
They can be used in series switching or shunt switching in place of relays or mechanical switches. They can be used in applications from DC up to audio frequencies. Its recovery time must be taken into account when chosen for the frequency of operation. The higher the operating frequency is, the faster the switching speed is required. In audio and DC applications, normal power supply rectifier types can be used.
Diodes as Voltage References
Zener diodes can be used as voltage regulators.
When used as voltage regulators in power supplies, they provide a near constant DC output voltage even though there are changes in load impedance or the input voltage. They use the reverse breakdown voltage characteristics of the devices to maintain a fixed voltage across them. One example of the circuit as voltage reference is as shown below. The various zener ratings ranges from 2.4 V to 200 V. Its power ratings range from 0.25W to 50W.
Diodes as back EMF Protection
When relay coil is switched off by a transistor, the inductance of the coil will create a back EMF that may be high enough to damage the transistor. In most circuits, one can see a diode connected across the relay coil to conduct when this happens. In this way, the relay coil is protected from the high voltage that is induced by the switching off of the coil. In normal operation, it will not conduct. Without it, no current could flow and the coil will create a high voltage pulse to keep the coil current flowing.
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The primary component in optoelectronics is the LEDs. Light emitting diode is a diode with PN junction of crystal material that produces luminescence around the junction when forward bias current is applied. The junctions of this light emitting diode are made from Gallium Arsenide (GaAs), Gallium Phosphide(GaP) or a combination of both(GaAsP).
The available colors are red, white, yellow, green and blue. Some are housed in plastic affixed to the base header of a transistor package. Others are contained in plastic packages that have a dome shaped head at the light emitting end. Two wires protrude from the opposite end for applying forward bias to the device. These days, surface mount types are commonly used.
The forward bias current of a typical LED ranges between 10 and 20 mA for maximum brilliance. A 1 Kohm resistor in series with a 12 V DC source will caused it to operate at 12 mA. In order to ensure the lifetime of it is preserved, do not exceed the maximum rating of the current. The voltage drop across it is typically 1.8 to 2.0 V DC.
Incident Flux Density
This is defined as the amount of radiation per unit area expressed as lumens/cm2 or watts/cm2. This is the measurement of the amount of flux received by a detector measuring its output.
Emitted Flux Density
This is defined as radiation per unit area and is used to describe light reflected from a surface. This measure of reflectance determines the total radiant luminous emittance.
This is the flux density that will appear at a distant surface and is expressed as lumens/steradian or watts/steradian.
This is a measure of photometric brightness and is obtained by dividing the luminous intensity at a given point by projected area of the source at the same point.
LED is glowing
As the name implies light emitting diodes exploit the property of the pn junction to emit photons when it is biased. LEDs are specially made to emit light and there was a revolution in the LED industry during the past few years. LEDs form an inevitable part in the modern electronics as simple indicators to optical communication devices. The history of LED date backs to 1907 when Captain Henry Joseph observed the property of electro-luminescence in Silicon Carbide.
The first LED was born in 1962. It was developed by Holonyak worked at General Electric (GE). It was a GaAsP device. The first commercial version of LED came on 1960s. LED industry made a boom during 1970s with the introduction of Gallium Aluminium Arsenide (GaAlAs). These LEDs are high bright types and are ten times brighter than the diffused varieties. Blue and White LEDs born in 1990 and used Indium Gallium Nitride (InGaN) as the semiconductor. White LED contains a blue chip with white inorganic Phosphor. When blue light strikes the phosphor, it emits white light.
Secrets behind LEDs
Brightness is an important aspect of LED. Human eye has maximum sensitivity to light near 550 nm region of yellow – green part of the spectrum. That is why a Green LED looks brighter than a Red LED even though both use same current. Three parameters of LED are responsible for its performance.
Forward current, forward voltage, Viewing angle and Speed of response are the factors affecting the brightness and performance of LEDs. Forward current ( IF) is the current flowing through the LED when it is forward biases and it should be restricted to 10 to 30 milli amperes other wise LED will die. Viewing angle is the off – axis angle at which the luminous intensity fall to half its axial value.
This is why the LED becomes brighter in full on condition. High bright LEDs have narrow viewing angle so that light is focused into a beam. Forward voltage ( VF ) is the voltage drop across the LED when it conducts. The forward voltage drop range from 1.8 V to 2.6 Volts in ordinary LEDs and in Blue and White it will go up to 5 volts. Speed of response denotes how fast an LED switch on and off. This is an important factor if LEDs are used in communication systems.
Why a ballast resistor accompany each LED?
LED always has a series resistor along with it. This is” Ballast resistor”, the life saving device of LED. It controls the forward current to the LED to a safer limit and protects it from burning. Value of the resistor if the factor that determines the forward current and hence the brightness. The simple equation Vs – Vf / If solves the problem of resistor value. Vs represents input voltage, Vf the forward voltage and If the allowable current through the LED. The resulting value will be in Ohms. It is better to restrict the current to a safer limit of 20 mA.
LED along with the limiting resistor R4 is the power on status indicator. A significant voltage drop (about 2 volts) occurs across the LED when it passes forward current. The forward voltage drops of various LEDs are shown in the table:
A typical LED can pass 30 –40 mA current without destroying the device. Normal current that gives sufficient brightness to a standard Red LED is 20 mA. But this may be 40 mA for Blue and White LEDs. Current limiting resistor R4 protects LED from excess current that is flowing through it. The value of R4 should be carefully selected to prevent damage to LED and also to get sufficient brightness at 20 mA current. The current limiting resistor can be selected using the formula
R = V / I
Where R is the value of resistor in ohms, V is the supply voltage and I is the allowable current in Amps. For a typical Red LED, the voltage drop is 1.8 volts. So if the supply voltage is 12 V ( Vs ) , voltage drop across the LED is 1.8 V ( Vf ) and the allowable current is 20 mA ( If ) then the value of R4 will be
Vs – Vf / If = 12 – 1.8 / 20 mA = 10.2 / 0.02 A = 510 Ohms.
A suitable available value of resistor is 470 Ohms. But is advisable to use 1 K resistor to increase the life of the LED even though there will be a slight reduction in the brightness. Since the LED takes 1.8 volts , the output voltage will be around 10 volts. So if the circuit requires 12 volts, it is necessary to increase the value of Zener slightly. The table is a ready reckoner for selecting limiting resistor for various versions of LEDs at different voltages.
Usually an LED makes its presence through its beautiful colour light. But there are LEDs performing their functions without emitting visible light. Infrared diode is such a kind of LED. Infra red actually is normal light with a particular colour. Human eye is not sensitive to its light because its wave length is 950 nm which is below the visible spectrum. Many sources like sun, bulbs, even the human body emit infra red rays. So it is necessary to modulate the emission from IR diode to use it in electronic application to prevent spurious triggering. Modulation makes the signal from IR LED stand out above the noise. Infra red diodes have a package that is opaque to visible light but transparent to infra red. The massive use of IR LEDs at TV / VCR remote controls and safety alarm systems brought IR diodes at very low cost at the market.
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A laser diode is similar to LED but it produces a narrow beam of high intensity. A laser is a device in which a number of atoms vibrate in such a fashion that all the emitted radiation of a single wave length is in phase with each other. Laser light is monochromatic and can be focused into a narrow pencil beam. The beam of typical laser diode is 4 mm x 0.6 mm widening only to 120 mm at a distance of 15 m.
Laser diode can be switched on and off at higher frequencies even as high as 1 GHz. So it is highly useful in telecommunication systems. Since the laser generates heat on hitting the body tissues, it is used in surgery to heal lesions in highly sensitive parts like retina, brain etc. Laser diodes form important components in CD players to read the recorded memory.
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