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over-under voltage cut-off with ON-Time delay PROJECT REPORT

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Here is an inexpensive auto cut-off circuit, which is fabricated using transistor and other discrete components. It can be used to protect loads such as refrigerator, T.V., and VCR from undesirable over and under line voltages, as well as surges caused due to sudden failure/resumption of main power supply. This circuit can be used directly as a standalone circuit between the mains supply and the load, or it may be inserted between an existing automatic/manual stabilizer and the load.
The over/under voltage cut-off with ON-Time delay provides various types of protection
1) Over-voltage protection.
2) Under-voltage protection.
3) Protection against transients.
4) Protection to load from frequent turning ON & OFF by providing time delay.
The on-time delay circuit not only protects the load from switching surges but also from quick changeover (off and on) effect of over/under-voltage relay, in case the mains voltage starts fluctuating in the vicinity of under or over voltage preset points. When the mains supply goes out of preset (over or under voltage) limits, the relay/load is turned off immediately and it is turned on only when A.C. mains voltage settles within the presets limits for a period equal to the on time delay period. The on-time delay period is preset able for 5 seconds to 2 minutes duration using presets VR3 and VR4. For refrigerators the delay should be preset for about 2 minutes duration to protect the compressor motor from frequently turning on and off.
In this circuit the on-time and off-time delay depends on charging and discharging time of capacitor C1. Here the discharge time of capacitors C1 is quite less to suit our requirement. We want that on switching Ëœoff â„¢ of the supply to the load, the circuit should immediately be ready to provide the required on-time delay when A.C. mains resumes after a brief interrupted for a short period due to over-/under voltage cut-off operation. This circuit is also useful against frequent power supply interruption resulting from loose electrical connections; be it at the pole or switch or relay contact, or due to any other reason.
Fig 1.1 Power Circuit
Here supply for the over- and under- voltage sampling part of the circuit [marked +12 v(b)] and that required for the rest of the circuit [marked +12 v(a)] are derived separately from lower half and upper half respectively of centre-tapped secondary of step-down transformer X1 as shown in fig. 1.if we use common 12 volt DC to this circuit would fall below preset low cut-off voltage and thus affects the proper operation of the sampling circuit. The value of filtering capacitor C4 is so chosen that a fall in mains voltage may quickly activate under-voltage sensing circuit, should the mains voltage reach the low cut-off limit.
In the sampling of the circuit, wired around transistor T1, presets VR1 and VR2 are used for presetting over or under voltage cut off limits respectively. The limits are set according to load voltage requirement as per as manufactureâ„¢s specifications.
Once the limits have been set, zener D1 will conduct if upper limit has been exceeded resulting in cut-off of transistor T2. The same condition can also result when mains voltage falls below the under-voltage setting as zener D2 stops conducting. Thus in either case transistor T2 is cut-off or transistor T3 is forward biased via resistor R3. This causes LED 1 to be Ëœonâ„¢. Simultaneously, capacitor C2 quickly discharges via diode D5 and transistor T3. As collector of transistor T3 is pulled low, transistor T4 and T5 are both cut-off, as also transistor T5. Thus, LED2 and LED3 are off and the relay is De-energized.
Fig 1.2 Schematic diagram of over/under voltage cut off with time delay
Now, when the mains voltage comes within the acceptable range, transistor T2 conducts to cut-off transistor T5 gets forward biased and LED2 becomes Ëœonâ„¢. However transistors T4 and T5 are still Ëœoffâ„¢, since base of T4 via zener D4 is connected to capacitor C1, which was in discharged condition. Thus LED3 and relay RL1 or load remain Ëœoffâ„¢.
Capacitor C1 starts charging slowly towards +12(v) rail via resistors R6 and R7 and presets VR3 and VR4. When the potential across capacitor C1 reaches 5.8V (after a delay termed as on-time delay) to breakdown zener D4 transistor T4, as also transistor T5,gets forward biased to switch Ëœonâ„¢ LED3 and relay RL1 or load, while LED2 goes Ëœoffâ„¢. Should the mains supply go out of preset limits before completion of the on-time delay, capacitor C1 will immediately discharge because of conduction of transistor T3. And the cycle will repeat until mains supply stabilizes within preset limits for the on-time delay period.
The on time delay is selected by adjusting preset VR3 and VR4, and resistor R6. Zener diode D3 is used to obtain regulated 9.1 volts for timing capacitor C1, so that preset on-time delay is more or less independent of variation in input DC voltage to this circuit (which would vary according to mains A.C. voltage). To switch Ëœoffâ„¢ the relay/load rapidly during undesired mains condition, The timing capacitor C1 is discharged rapidly to provide complete control over turning Ëœonâ„¢ or Ëœoffâ„¢ of relay RL1(or the load). The functioning of the LEDs and relay, depending on the circuit condition, is summarized in table 1.
2.1 Transistor:
Fig 2.1 Assorted discrete transistors.
A transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but most are found in integrated circuits.
2.1.1 Simplified operation :
The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. A transistor can control its output in proportion to the input signal, that is, can act as an amplifier. Or, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.
The two types of transistors have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.
The image to the right represents a typical bipolar transistor in a circuit. Charge will flow between emitter and collector terminals depending on the current in the base. Since internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The size of this voltage depends on the material the transistor is made from, and is referred to as VBE.
2.1.2 Transistor as a switch:
Fig 2.2 BJT used as an electronic switch, in grounded-emitter configuration.
Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.
In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:
VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)
VRC + VCE = VCC, the supply voltage shown as 6V
If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off,[12] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.
2.1.3 Advantages:
The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
¢ Small size and minimal weight, allowing the development of miniaturized electronic devices.
¢ Highly automated manufacturing processes, resulting in low per-unit cost.
¢ Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
¢ No warm-up period for cathode heaters required after power application.
¢ Lower power dissipation and generally greater energy efficiency.
¢ Higher reliability and greater physical ruggedness.
¢ Extremely long life. Some transistorized devices have been in service for more than 30 years.
¢ Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
¢ Insensitivity to mechanical shock and vibration, thus avoiding the problem of micro phonics in audio applications.
2.1.4 Types:
Fig 2.3
Thus, a particular transistor may be described as silicon, surface mount, BJT, NPN, low power, high frequency switch.
The 'BC' letters in a common transistor name like BC547B means
Prefix class Usage:
BC Small signal transistor ("all round")
BF High frequency, many MHz
BD Withstands higher current and power
BA Germanium
Table No. 1.1 Code and applications of transistor.
Fig 2.2.1
A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current through it in accordance with Ohm's law:
V = IR
Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).
The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance depends upon the materials constituting the resistor as well as its physical dimensions; it's determined by design.
2.2.1 Units:
The ohm (symbol: O) is a SI-driven unit of electrical resistance, named after Georg Simon Ohm. Commonly used multiples and submultiples in electrical and electronic usage are the milliohm (1x10-3), kilo ohm (1x103), and mega ohm (1x106).
2.2.2 Theory of operation:
Ohm's law:
The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:
V = IR
Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance ®.
2.2.3 Construction:
A single in line (SIL) resistor package with 8 individual, 47 ohm resistors. One end of each resistor is connected to a separate pin and the other ends are all connected together to the remaining (common) pin - pin 1, at the end identified by the white dot.
A. Carbon composition:
Carbon composition resistors consist of a solid cylindrical resistive element with embedded wire leads or metal end caps to which the lead wires are attached. The body of the resistor is protected with paint or plastic. Early 20th-century carbon composition resistors had uninsulated bodies; the lead wires were wrapped around the ends of the resistance element rod and soldered. The completed resistor was painted for color coding of its value.
The resistive element is made from a mixture of finely ground (powdered) carbon and an insulating material (usually ceramic). A resin holds the mixture together. The resistance is determined by the ratio of the fill material (the powdered ceramic) to the carbon. Higher concentrations of carbon, a weak conductor, result in lower resistance. Carbon composition resistors were commonly used in the 1960s and earlier, but are not so popular for general use now as other types have better specifications, such as tolerance, voltage dependence, and stress (carbon composition resistors will change value when stressed with over-voltages). Moreover, if internal moisture content (from exposure for some length of time to a humid environment) is significant, soldering heat will create a non-reversible change in resistance value. These resistors, however, if never subjected to overvoltage nor overheating were remarkably reliable.
They are still available, but comparatively quite costly. Values ranged from fractions of an ohm to 22 mega ohms.
B. Carbon film:
A carbon film is deposited on an insulating substrate, and a helix cut in it to create a long, narrow resistive path. Varying shapes, coupled with the resistivity of carbon, (ranging from 90 to 400 nOm) can provide a variety of resistances.[1] Carbon film resistors feature a power rating range of 0.125 W to 5 W at 70 °C. Resistances available range from 1 ohm to 10 mega ohm. The carbon film resistor can operate between temperatures of -55 °C to 155 °C. It has 200 to 600 volts maximum working voltage range.[2]
C. Metal film:
A common type of axial resistor today is referred to as a metal-film resistor. Metal electrode leadless face (MELF) resistors often use the same technology, but are a cylindrically shaped resistor designed for surface mounting. Note that other types of resistors (e.g., carbon composition) are also available in MELF packages.
Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermets materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though that is one such technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix through the coating rather than by etching. (This is similar to the way carbon resistors are made.) The result is a reasonable tolerance (0.5, 1, or 2%) and a temperature coefficient of (usually) 25 or 50 ppm/K.
D. Wire wound:
Wire wound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. Wire leads in low power wire wound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wire wound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wire wound resistors may be rated for 1,000 watts or more.
Because wire wound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance.
E. Foil resistor:
The primary resistance element of a foil resistor is a special alloy foil several micrometers thick. Since their introduction in the 1960s, foil resistors have had the best precision and stability of any resistor available. One of the important parameters influencing stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further improved over the years. One range of ultra-precision foil resistors offers a TCR of 0.14 ppm/°C, tolerance ±0.005%, long-term stability (1 year) 25 ppm, (3 year) 50 ppm (further improved 5-fold by hermetic sealing), stability under load (2000 hours) 0.03%, thermal EMF 0.1 µV/°C, noise -42 dB, voltage coefficient 0.1 ppm/V, inductance 0.08 µH, capacitance 0.5 pF.[3]
2.2.4 Resistor marking:
Most axial resistors use a pattern of colored stripes to indicate resistance. Surface-mount resistors are marked numerically, if they are big enough to permit marking; more-recent small sizes are impractical to mark. Cases are usually tan, brown, blue, or green, though other colors are occasionally found such as dark red or dark gray.
Early 20th century resistors, essentially uninsulated, were dipped in paint to cover their entire body for color coding. A second color of paint was applied to one end of the element, and a color dot (or band) in the middle provided the third digit. The rule was "body, tip, dot", providing two significant digits for value and the decimal multiplier, in that sequence. Default tolerance was ±20%. Closer-tolerance resistors had silver (±10%) or gold-colored (±5%) paint on the other end.
Four-band resistors:
Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits.
For example, green-blue-yellow-red is 56×104 O = 560 kO ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000O at ±2% tolerance accuracy. 560,000O changes to 560 kO ±2% (as a kilo- is 103).
Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below.
Color 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient
Black 0 0 ×100
Brown 11 1 ×101 ±1% (F) 100 ppm
Red 2 2 ×102 ±2% (G) 50 ppm
Orange 3 3 ×103 15 ppm
Yellow 4 4 ×104 25 ppm
White 9 9 ×109
Gold ×10-1 ±5% (J)
Silver ×10-2 ±10% (K)
None ±20% (M)
Table No. 2.1
The operational temperature range distinguishes commercial grade, industrial grade and military grade components.
¢ Commercial grade: 0 °C to 70 °C
¢ Industrial grade: -40 °C to 85 °C (sometimes -25 °C to 85 °C)
¢ Military grade: -55 °C to 125 °C (sometimes -65 °C to 275 °C)
¢ Standard Grade -5 °C to 60 °C
Fig 2.3.1 A typical single-turn potentiometer
Type: passive
Electronic symbol (Europe)
(US)A potentiometer (colloquially known as a "pot") is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider.[1] If only two terminals are used (one side and the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on audio equipment. Potentiometers operated by a mechanism can be used as position transducers, for example, in a joystick.
Potentiometers are rarely used to directly control significant power (more than a watt). Instead they are used to adjust the level of analog signals (e.g. volume controls on audio equipment), and as control inputs for electronic circuits. For example, a light dimmer uses a potentiometer to control the switching of a TRIAC and so indirectly control the brightness of lamps.
Potentiometers are sometimes provided with one or more switches mounted on the same shaft. For instance, when attached to a volume control, the knob can also function as an on/off switch at the lowest volume.
2.3.1 Potentiometer construction:
Fig 2.3.2 Constructional diagram of potentiometer
Construction of a wire-wound circular potentiometer. The resistive element (1) of the shown device is trapezoidal, giving a non-linear relationship between resistance and turn angle. The wiper (3) rotates with the axis (4), providing the changeable resistance between the wiper contact (6) and the fixed contacts (5) and (9). The vertical position of the axis is fixed in the body (2) with the ring (7) (below) and the bolt (8) (above).
A potentiometer is constructed using a semi-circular resistive element with a sliding contact (wiper). The resistive element, with a terminal at one or both ends, is flat or angled, and is commonly made of graphite, although other materials may be used. The wiper is connected through another sliding contact to another terminal. On panel pots, the wiper is usually the center terminal of three. For single-turn pots, this wiper typically travels just under one revolution around the contact. "Multi turn" potentiometers also exist, where the resistor element may be helical and the wiper may move 10, 20, or more complete revolutions, though multi turn pots are usually constructed of a conventional resistive element wiped via a worm gear. Besides graphite, materials used to make the resistive element include resistance wire, carbon particles in plastic, and a ceramic/metal mixture called cermet.
One form of rotary potentiometer is called a String potentiometer. It is a multi-turn potentiometer operated by an attached reel of wire turning against a spring. It is used as a position transducer.
In a linear slider pot, a sliding control is provided instead of a dial control. The resistive element is a rectangular strip, not semi-circular as in a rotary potentiometer. Due to the large opening slot or the wiper, this type of pot has a greater potential for getting contaminated.
Potentiometers can be obtained with either linear or logarithmic relations between the slider position and the resistance (potentiometer laws or "tapers").
Manufacturers of conductive track potentiometers use conductive polymer resistor pastes that contain hard wearing resins and polymers, solvents, lubricant and carbon “ the constituent that provides the conductive/resistive properties. The tracks are made by screen printing the paste onto a paper based phenol substrate and then curing it in an oven. The curing process removes all solvents and allows the conductive polymer to polymerize and cross link. This produces a durable track with stable electrical resistance throughout its working life.[citation needed]
Fig 2.3.3 PCB mount trimmer potentiometers, or "trimpots", intended for infrequent adjustment.
2.3.2 Linear taper potentiometer:
A linear taper potentiometer (uses the letter 'B' in the designation eg 100kB) has a resistive element of constant cross-section, resulting in a device where the resistance between the contact (wiper) and one end terminal is proportional to the distance between them. Linear taper describes the electrical characteristic of the device, not the geometry of the resistive element. Linear taper potentiometers are used when an approximately proportional relation is desired between shaft rotation and the division ratio of the potentiometer; for example, controls used for adjusting the centering of (an analog) cathode-ray oscilloscope.
One of the advantages of the potential divider compared to a variable resistor in series with the source is that, while variable resistors have a maximum resistance where some current will always flow, dividers are able to vary the output voltage from maximum (VS) to ground (zero volts) as the wiper moves from one end of the pot to the other. There is, however, always a small amount of contact resistance.In addition, the load resistance is often not known and therefore simply placing a variable resistor in series with the load could have a negligible effect or an excessive effect, depending on the load.

Fig 2.4.1 Zener diode schematic symbol
Fig 2.4.2 Current-voltage characteristic of a Zener diode with a breakdown voltage of 17 volt. Notice the change of voltage scale between the forward biased (positive) direction and the reverse biased (negative) direction.
A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property.
A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of doping on both sides.[1] A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage.
However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.
The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available zener diodes can vary widely from 1.2 volts to 200 volts.
2.4.1 Uses:
2.4.3 Zener diode shown with typical packages. Reverse current - iZ is shown.
Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. From that point on, the relatively low impedance of the diode keeps the voltage across the diode at that value.
Fig 2.4.4 Zener diode voltage regulator
In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated down to a stable output voltage UOUT. The intrinsic voltage drop of diode D is stable over a wide current range and holds UOUT relatively constant even though the input voltage may fluctuate over a fairly wide range. Because of the low impedance of the diode when operated like this, Resistor R is used to limit current through the circuit.
In the case of this simple reference, the current flowing in the diode is determined using Ohms law and the known voltage drop across the resistor R. IDiode = (UIN - UOUT) / RO
The value of R must satisfy two conditions:
1. R must be small enough that the current through D keeps D in reverse breakdown. The value of this current is given in the data sheet for D. For example, the common BZX79C5V6 device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUT will be unregulated, and less than the nominal breakdown voltage (this differs to voltage regulator tubes where the output voltage will be higher than nominal and could rise as high as UIN). When calculating R, allowance must be made for any current through the external load, not shown in this diagram, connected across UOUT.
2. R must be large enough that the current through D does not destroy the device. If the current through D is ID, its breakdown voltage VB and its maximum power dissipation PMAX, then IDVB < PMAX.
A load may be placed across the diode in this reference circuit, and as long as the zener stays in reverse breakdown, the diode will provide a stable voltage source to the load.
2.5 Diode
Fig 2.5.1 Various semiconductor diodes. Bottom: A bridge rectifier
In electronics a diode is a two-terminal electronic component that conducts electric current in only one direction. The term usually refers to a semiconductor diode, the most common type today, which is a crystal of semiconductor connected to two electrical terminals, a P-N junction. A vacuum tube diode, now little used, is a vacuum tube with two electrodes; a plate and a cathode.
The most common function of a diode is to allow an electric current in one direction (called the forward direction) while blocking current in the opposite direction (the reverse direction). Thus, the diode can be thought of as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current to direct current, and remove modulation from radio signals in radio receivers.
2.5.1 Current“voltage characteristic:
A semiconductor diode™s behavior in a circuit is given by its current“voltage characteristic, or I“V curve (see graph at right). The shape of the curve is determined by the transport of charge carriers through the so-called depletion layer or depletion region that exists at the p-n junction between differing semiconductors.
If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron/hole pairs are actively being created in the junction by, for instance, light. see photodiode). This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed, resulting in substantial electric current through the p-n junction (i.e. substantial numbers of electrons and holes recombine at the junction).. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode such that the P-doped region is positive with respect to the N-doped region and the diode is said to be turned on as it has a forward bias.
Fig 2.5.2 I“V characteristics of a P-N junction diode (not to scale).
A diode™s I“V characteristic can be approximated by four regions of operation (see the figure at right).
2.5.2 Types of semiconductor diode:
Diode Zener
Silicon controlled rectifier
Fig 2.5.3 Some diode symbols.
2.5.3 Numbering and Coding schemes:
There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes; the most common European Pro Electron standard:
Pro Electron:
The European Pro Electron coding system for active components was introduced in 1966 and comprises two letters followed by the part code. The first letter represents the semiconductor material used for the component (A = Germanium and B = Silicon) and the second letter represents the general function of the part (for diodes: A = low-power/signal, B = Variable capacitance, X = Multiplier, Y = Rectifier and Z = Voltage reference), for example:
¢ AA-series germanium low-power/signal diodes (eg: AA119)
¢ BA-series silicon low-power/signal diodes (eg: BAT18 Silicon RF Switching Diode)
¢ BY-series silicon rectifier diodes (eg: BY127 1250V, 1A rectifier diode)
¢ BZ-series silicon zener diodes (eg: BZY88C4V7 4.7V zener diode)
Other common numbering / coding systems (generally manufacturer-driven) include:
¢ GD-series germanium diodes (ed: GD9) ” this is a very old coding system
¢ OA-series germanium diodes (eg: 0A47) ” a coding sequence developed by Mullard, a UK company
As well as these common codes, many manufacturers or organizations have their own systems too ” for example:
¢ HP diode 1901-0044 = JEDEC 1N4148
¢ UK military diode CV448 = Mullard type OA81 = GEC type GEX23

Electronic symbol Fig 2.6.1Pin configuration:
A light-emitting diode (LED) (pronounced /l.i'di/[1], or just /ld/) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962,[2] early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.
The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2), and integrated optical components are used to shape its radiation pattern and assist in reflection.
LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliance. However, they are relatively expensive and require more precise current and heat management than traditional light sources. Current LED products for general lighting are more expensive to buy than fluorescent lamp sources of comparable output.
They also enjoy use in applications as diverse as replacements for traditional light sources in automotive lighting (particularly indicators) and in traffic signals. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in advanced communications technology.
2.7 Construction of LED:
Fig 2.7.1 Parts of an LED
2.7.2 The inner workings of an LED
2.7.1 Colors and materials:
Conventional LEDs are made from a variety of inorganic semiconductor materials, the following table shows the available colors with wavelength range, voltage drop and material:
Color Wavelength (nm) Voltage (V) Semiconductor Material
> 760
V < 1.9
Gallium arsenide (GaAs)
Aluminum gallium arsenide (AlGaAs)Red
610 < < 760 1.63 < V < 2.03 Aluminum gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)Orange
590 < < 610 2.03 < V < 2.10 Gallium arsenide phosphide (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)Yellow
570 < < 590 2.10 < V < 2.18 Gallium arsenide phosphide (GaAsP)
Aluminum gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)Green
500 < < 570 1.9[32] < V < 4.0 Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Gallium(III) phosphide (GaP)
Aluminum gallium indium phosphide (AlGaInP)
Aluminum gallium phosphide (AlGaP)Blue
450 < < 500 2.48 < V < 3.7 Zinc selenide (ZnSe)
Indium gallium nitride (InGaN)
Silicon carbide (SiC) as substrate
Silicon (Si) as substrate ” (under development)Violet
400 < < 450 2.76 < V < 4.0 Indium gallium nitride (InGaN)Purple
multiple types 2.48 < V < 3.7 Dual blue/red LEDs,
blue with red phosphor,
or white with purple plastic
< 400 3.1 < V < 4.4 diamond (235 nm)[33]
Boron nitride (215 nm)[34][35]
Aluminum nitride (AlN) (210 nm)[36]
Aluminum gallium nitride (AlGaN)
Aluminum gallium indium nitride (AlGaInN) ” (down to 210 nm)[37]
Table No 2.7.1 Material used in LEDs

A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device triggered by light to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protection relays".
Basic design and operation:
Fig 2.8.1 Simple electromechanical relay
Fig 2.8.2 Small relay as used in electronics
A simple electromagnetic relay, such as the one taken from a car in the first picture, is an adaptation of an electromagnet. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.
When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.
If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.[1]
By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor

Electronic symbol
Fig 2.9.1 A typical electrolytic capacitor
A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric. When a potential difference exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called plates.
An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference
between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage.
Capacitors are widely used in electronic circuits to block the flow of direct current while allowing alternating current to pass, to filter out interference, to smooth the output of power supplies, and for many other purposes. They are used in resonant circuits in radio frequency equipment to select particular frequencies from a signal with many frequencies.
2.9.1 Theory of operation:
Fig 2.9.2 Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.
A capacitor consists of two conductors separated by a non-conductive region.[7] The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces,[8] and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits.
An ideal capacitor is wholly characterized by a constant capacitance 'C', defined as the ratio of charge ±'Q' on each conductor to the voltage 'V' between them:[7]
Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes:
In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.[9]
2.9.2 Current-voltage relation:
The current i (t ) through a component in an electric circuit is defined as the rate of change of the charge q (t ) that has passed through it. Physical charges cannot pass through the dielectric layer of a capacitor, but rather build up in equal and opposite quantities on the electrodes: as each electron accumulates on the negative plate, one leaves the positive plate. Thus the accumulated charge on the electrodes is equal to the integral of the current, as well as being proportional to the voltage (as discussed above). As with any anti derivative, a constant of integration is added to represent the initial voltage v (t0). This is the integral form of the capacitor equation,[11]
Taking the derivative of this, and multiplying by C, yields the derivative form,[12]
The dual of the capacitor is the inductor, which stores energy in the magnetic field rather than the electric field. Its current-voltage relation is obtained by exchanging current and voltage in the capacitor equations and replacing C with the inductance L.
2.9.3 Applications
Capacitors have many uses in electronic and electrical systems. They are so common that it is a rare electrical product that does not include at least one for some purpose.

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors”the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (VS) is in proportion to the primary voltage (VP), and is given by the ratio of the number of turns in the secondary (NS) to the number of turns in the primary (NP) as follows:
By appropriate selection of the ratio of turns, a transformer thus allows an alternating current (AC) voltage to be "stepped up" by making NS greater than NP, or "stepped down" by making NS less than NP.
In the vast majority of transformers, the coils are wound around a ferromagnetic core, air-core transformers being a notable exception.
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used to interconnect portions of national power grids. All operate with the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household ("mains") voltage. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical.
2.10.1 Basic principles:
The transformer is based on two principles: firstly, that an electric current can produce a magnetic field (electromagnetism) and secondly that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). Changing the current in the primary coil changes the magnetic flux that is developed. The changing magnetic flux induces a voltage in the secondary coil.
Fig 2.10.1 Transformer
An ideal transformer
An ideal transformer is shown in the adjacent figure. Current passing through the primary coil creates a magnetic field. The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron, so that most of the magnetic flux passes through both the primary and secondary coils.
2.10.2 Center tap:
In electronics, a center tap is a connection made to a point half way along a winding of a transformer or inductor, or along the element of a resistor or a potentiometer. Taps are sometimes used on inductors for the coupling of signals, and may not necessarily be at the half-way point, but rather, closer to one end. A common application of this is in the Hartley oscillator. Inductors with taps also permit the transformation of the amplitude of alternating current (AC) voltages for the purpose of power conversion, in which case, they are referred to as autotransformers, since there is only one winding. An example of an autotransformer is an automobile ignition coil. Potentiometer tapping provides one or more connections along the device's element, along with the usual connections at each of the two ends of the element, and the slider connection. Potentiometer taps allow for circuit functions that would otherwise not be available with the usual construction of just the two end connections and one slider connection.
2.10.3 Volts center tapped:
Volts center tapped (VCT) describes the voltage output of a center tapped transformer. For example: A 24 VCT transformer will measure 24 VAC across the outer two taps (winding as a whole), and 12VAC from each outer tap to the center-tap (half winding). These two 12 VAC supplies are 180 degrees out of phase with each other, thus making it easy to derive positive and negative 12 volt DC power supplies from them.
2.10.4 Common applications of center-tapped transformers:
¢ In a rectifier, a center-tapped transformer and two diodes can form a full-wave rectifier that allows both half-cycles of the AC waveform to contribute to the direct current, making it smoother than a half-wave rectifier. This form of circuit saves on rectifier diodes compared to a diode bridge, but has poorer utilization of the transformer windings. Center-tapped two-diode rectifiers were a common feature of power supplies in vacuum tube equipment. Modern semiconductor diodes are low-cost and compact so usually a 4-diode bridge is used (up to a few hundred watts total output) which produces the same quality of DC as the center-tapped configuration with a more compact and cheaper power transformer. Center-tapped configurations may still be used in high-current applications, such as large automotive battery chargers, where the extra transformer cost is offset by less costly rectifiers.
¢ In an audio power amplifier center-tapped transformers are used to drive push-pull output stages. This allows two devices operating in Class B to combine their output to produce higher audio power with relatively low distortion. Design of such audio output transformers must tolerate a small amount of direct current that may pass through the winding.
Hundreds of millions of pocket-size transistor radios used this form of amplifier since the required transformers were very small and the design saved the extra cost and bulk of an output coupling capacitor that would be required for an output-transformer less design. However, since low-distortion high-power transformers are costly and heavy, most consumer audio products now use a transformer less output stage.
The technique is nearly as old as electronic amplification and is well-documented, for example, in "The Radiotron Designer's Handbook, Third Edition" of 1940.
¢ In analog telecommunications systems center-tapped transformers can be used to provide a DC path around an AC coupled amplifier for signaling purposes.
¢ In electronic amplifiers, a center-tapped transformer is used as a phase splitter in coupling different stages of an amplifier.
¢ Power distribution, see 3 wire single phase.
¢ A center-tapped rectifier is preferred to the full bridge rectifier when the output DC current is high and the output voltage is low.
¢ The over/under voltage cut-off with ON-Time delay is used to protect loads such as refrigerator, T.V., and VCR from undesirable over and under line voltages, as well as surges caused due to sudden failure/resumption of main power supply
¢ With fine tuning of this circuit can be used in place of regulator since it provides various types of protection
1) Over-voltage protection.
2) Under-voltage protection.
3) Protection against transients.
4) Protection to load from frequent turning ON & OFF by providing time delay.
¢ The over/under voltage cut-off with ON-Time delay can be modified by increasing the rating of transformer and various components and can be made to use for industrial purpose.
¢ With fine tuning of this circuit can be used in place of regulator since it provides various types of protection
1) Over-voltage protection.
2) Under-voltage protection.
3) Protection against transients.
4) Protection to load from frequent turning ON & OFF by providing time delay.
¢ The over/under voltage cut-off with ON-Time delay provides various types of protection
1) Over-voltage protection.
2) Under-voltage protection.
3) Protection against transients.
4) Protection to load from frequent turning ON & OFF by providing time delay.
¢ The over/under voltage cut-off with ON-Time delay is much cheaper than a regulator and provides much more protection against various faults.
¢ Making the minor project lead us to learn the practical and actual aspects of all the components used and also make us to manage the various circuits according to the requirement.
¢ The minor project helped us in exploring the market and gathering information about the current market trends .
1. Elements of electronic engineering by N N Rao.
2. www.wikipedia.com
3. http://www.efy.com

hi i have the project

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