Electronic oscillator

 Electronic oscillator :

An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games.

A low-frequency oscillator (LFO) is an electronic oscillator that generates an AC waveform at a frequency below ≈20 Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an audio frequency oscillator.

Oscillators designed to produce a high-power AC output from a DC supply are usually called inverters.

There are two main types of electronic oscillator: the harmonic oscillator and the relaxation oscillator.

Harmonic oscillator :

The harmonic, or linear, oscillator produces a sinusoidal output. There are a few types of harmonic oscillators.

The basic form of a harmonic oscillator is an electronic amplifier with an electronic filter connected in the feedback loop. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop, being filtered and re-amplified until it increasingly resembles the desired signal.

Capacitive-inductive oscillators also known as LC oscillators are built by a tank circuit, which oscillates by charging and discharging a capacitor through an inductor and an active negative resistance circuit that compensates the internal LC losses. These oscillators are typically used when a tunable precision frequency source is necessary, such as with radio transmitters and receivers. Most LC oscillators use off-chip inductors. On-chip inductors suffer large resistive losses, so that the Q-factor of the resulting tank circuit is generally less than 10. As processes have made larger numbers of metal layers available (allowing designers to distance the inductor metal layer from the resistive substrate), on-chip inductors have become more useful.

A piezoelectric crystal (commonly quartz) may take the place of the filter to stabilise the frequency of oscillation, this is called a crystal oscillator. These kinds of oscillators contain quartz crystals that mechanically vibrate between two slightly different shapes. Crystals have very high Q-factor, and can only be tuned within a very small range of frequencies. Because the crystal is an off-chip component, it adds some cost and complexity to the system design, but the crystal itself is generally quite inexpensive.

Surface acoustic wave (SAW) devices are a kind of crystal oscillator, but achieve much higher frequencies by establishing standing waves on the surface of the quartz crystal.^{[citation needed]} These are more expensive than crystal oscillators, and are used in specialized applications which require a direct and very accurate high frequency reference, for example, in cellular telephones.

There are many ways to implement harmonic oscillators, because there are different ways to amplify and filter. Some of the different circuits are:

• Armstrong oscillator
• Hartley oscillator
• Colpitts oscillator
• Clapp oscillator
• Delay line oscillator
• Pierce oscillator (crystal)
• Phase-shift oscillator
• RC oscillator (Wien Bridge and "Twin-T")
• Cross-coupled LC oscillator
• Vackar oscillator
• Opto-Electronic Oscillator.

Relaxation oscillator :

Main article: relaxation oscillator

A relaxation oscillator produces a non-sinusoidal output, such as a square, sawtooth or triangle wave. It contains an energy-storing element (a capacitor or, more rarely, an inductor) and a trigger circuit (a latch, Schmitt trigger, negative resistor, etc.) that periodically charges/discharges the energy stored in the storage element thus causing abrupt changes in the output waveform.

Square-wave relaxation oscillators are used to provide the clock signal for sequential logic circuits such as timers and counters, although crystal oscillators are often preferred for their greater stability. Triangle wave or sawtooth oscillators are used in the timebase circuits that generate the horizontal deflection signals for cathode ray tubes in analogue oscilloscopes and television sets. In function generators, this triangle wave may then be further shaped into a close approximation of a sine wave.

Ring oscillators are built of a ring of active delay stages. Generally the ring has an odd number of inverting stages, so that there is no single stable state for the internal ring voltages. Instead, a single transition propagates endlessly around the ring.

Types of relaxation oscillator circuits include:

• multivibrator
• ring oscillator
• delay line oscillator
• rotary traveling wave oscillator.

Ohm's law

Ohm's law :

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them.

The mathematical equation that describes this relationship is:

where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. He presented a slightly more complex equation than the one above to explain his experimental results. The above equation is the modern form of Ohm's law.

In physics, the term Ohm's law is also used to refer to various generalizations of the law originally formulated by Ohm. The simplest example of this is:

where J is the current density at a given location in a resistive material, E is the electric field at that location, and σ is a material dependent parameter called the conductivity. This reformulation of Ohm's law is due to Gustav Kirchhoff.

see : http://en.wikipedia.org/wiki/Ohm%27s_law

Water Turbines

Water Turbine

Water turbine is a device that convert the energy in a stream of fluid into mechanical energy by passing the stream through a system of fixed and moving fan like blades and causing the latter to rotate. A turbine looks like a large wheel with many small radiating blades around its rim.

Classification of Water turbines
According to the type of flow of water : The water turbines used as prime movers in hydro electric power stations are of four types.They are

• axial flow : having flow along shaft axis
• inward radial flow : having flow along the radius
• tangential or peripheral : having flow along tangential direction
• mixed flow : having radial inlet axial outlet

If the runner blades of axial flow turbines are fixed,those are called propeller turbines.

According to the action of water on moving blades water turbines are of 2 types namely impulse ad reaction type turbines.
Impulse Turbines :These turbines change the direction of flow of a high velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid in the turbine rotor blades. Before reaching the turbine the fluid's Pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelton wheels and de Laval turbines use this process exclusively. Impulse turbines do not require a pressure casement around the runner since the fluid jet is prepared by a nozzle prior to reaching turbine. Newton's second law describes the transfer of energy for impulse turbines.

Reaction Turbines : These turbines develop torque by reacting to the fluid's pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages may be used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy for reaction turbines.

According to the Head and quantity of water available the water turbines are of 2 types.Those are high head - low flow and low to medium head and high to medium discharge turbines.

According to the name of the originator water turbines are of 3 types namely Pelton Wheel,Francis tubine and Kaplan turbine.

Nuclear Power

Nuclear Power

Nuclear power is the controlled use of nuclear reactions to release energy for work including propulsion, heat, and the generation of electricity. Use of nuclear power to do significant useful work is currently limited to nuclear fission and radioactive decay. Nuclear energy is produced when a fissile material, such as uranium-235 (235U), is concentrated such that nuclear fission takes place in a controlled chain reaction and creates heat — which is used to boil water, produce steam, and drive a steam turbine. The turbine can be used for mechanical work and also to generate electricity. Nuclear power provides 7% of the world's energy and 15.7% of the world's electricity and is used to power most military submarines and aircraft carriers.

The United States produces the most nuclear energy, with nuclear power providing 20% of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors—80% as of 2006. In the European Union as a whole, nuclear energy provides 30% of the electricity.Nuclear energy policy differs between countries, and some countries such as Austria, Australia and Ireland have no nuclear power stations.

Concerns about nuclear power

The use of nuclear power is controversial because of the problem of storing radioactive waste for indefinite periods, the potential for possibly severe radioactive contamination by accident or sabotage, and the possibility that its use in some countries could lead to the proliferation of nuclear weapons. Proponents believe that these risks are small and can be further reduced by the technology in the new reactors. They further claim that the safety record is already good when compared to other fossil-fuel plants, that it releases much less radioactive waste than coal power, and that nuclear power is a sustainable energy source. Critics, including most major environmental groups, claim nuclear power is an uneconomic and potentially dangerous energy source with a limited fuel supply, especially compared to renewable energy, and dispute whether the costs and risks can be reduced through new technology.

There is concern in some countries over North Korea and Iran operating research reactors and fuel enrichment plants, since those countries refuse adequate IAEA oversight and are believed to be trying to develop nuclear weapons. North Korea admits that it is developing nuclear weapons, while the Iranian government vehemently denies the claims against Iran.

Several concerns about nuclear power have been expressed, and these include:

• Concerns about nuclear reactor accidents, such as the Chernobyl disaster
• Vulnerability of plants to attack or sabotage
• Use of nuclear waste as a weapon
• Health effects of nuclear power plants
• Nuclear proliferation