Charging Currents in Transmission Lines

Any two conductors separated by an insulating medium constitutes a condenser or capacitor.In case of overhead transmission lines, two conductors form the two plates of the capacitor and the air between the conductors behaves as dielectric medium. Thus an overhead transmission line can be assumed to have capacitance between the conductors throughout the length of the line. The capacitance is uniformly distributed over the length of the line and may be considered as uniform series of condensers connected between the conductors.

When an alternating voltage is applied across the transmission line it draws the leading current even when supplying no load. This leading current will be in quadrature with the applied voltage and is termed as charging current. It must be noted that charging current is due to the capacitive effect between the conductors of the line and does not depend on the load. The strength of the charging currents depends on the voltage of transmission, the capacitance of the line and frequency of the ac supply.

If the capacitance of the overhead line is high, the line draws more charging currents which cancels out the lagging component of the load current (normally load is inductive in nature). Hence the resultant current flowing in the line is reduced. The reduction in the resultant current flowing through the transmission line for given load current results in:

• Reduction of the line losses and so increase of transmission efficiency.

• Reduction in the voltage drop in the system or improvement of the voltage regulation.

• Increased load capacity and improved power factor

Significance of Charging currents:

Capacitance effect (responsible for charging currents) of the short transmission lines are negligible. However they are significant in medium and long distance transmission lines.

In long distance transmission lines, during light loaded conditions receiving end voltage will be higher than sending end voltage. This is because of the charging currents and capacitive effect of the line.

Synchroscopes

Synchroscopes are electrodynamic instruments, which rely on the interaction of magnetic fields to rotate a pointer. In most types,there is no restoring spring torque for the magnetically produced torques to overcome therefore  pointer system is free to rotate continually. Synchroscopes have a damping vane to smooth out vibration of the moving system.

A polarized-vane synchroscope has a field winding with a phase-shifting network arranged to produce a rotating magnetic field. The field windings are connected to the incoming machine. A single phase polarizing winding is connected to the running system. It is mounted perpendicular to the field winding and produces a magnetic flux that passes through the moving vanes. The moving vanes turn a shaft that carries a pointer moving over a scale. If the frequency of the source connected to the polarizing winding is different from the source connected to the field winding, the pointer rotates continually at a speed proportional to the difference in system frequencies.

The scale is marked to show the direction of rotation corresponding to the incoming machine running faster than the running system. When the frequencies match, the moving vanes will rotate to a position corresponding to the phase difference between the two sources. The incoming machine can then be adjusted in speed  and than phase sequence is checked.

EM Wave Propogation

There are two main types of waves. Mechanical wave and Electromagnetic wave.
Mechanical waves propagate through a medium, and the substance of this medium is deformed. The deformation reverses itself owing to restoring forces resulting from its deformation. For example, sound waves propagate via air molecules colliding with their neighbors. When air molecules collide, they also bounce away from each other (a restoring force). This keeps the molecules from continuing to travel in the direction of the wave.

The second main type of wave, electromagnetic waves, do not require a medium. Instead, they consist of periodic oscillations of electrical and magnetic fields generated by charged particles, and can therefore travel through a vacuum. These types of waves vary in wavelength, and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. An electromagnetic wave (i.e., a light wave) is produced by accelerating electric charge. As the wave moves through the vacuum of empty space, it travels at a speed of c (3 x 108 m/s). This value is the speed of light in a vacuum. When the wave impinges upon a particle of matter, the energy is absorbed and sets electrons within the atoms into vibrational motion. If the frequency of the electromagnetic wave does not match the resonant frequency of vibration of the electron, then the energy is reemitted in the form of an electromagnetic wave. This new electromagnetic wave has the same frequency as the original wave and it too will travel at a speed of c through the empty space between atoms. The newly emitted light wave continues to move through the interatomic space until it impinges upon a neighboring particle. The energy is absorbed by this new particle and sets the electrons of its atoms into vibration motion. And once more, if there is no match between the frequency of the electromagnetic wave and the resonant frequency of the electron, the energy is reemitted in the form of a new electromagnetic wave. 

The cycle of absorption and reemission continues as the energy is transported from particle to particle through the bulk of a medium. Every photon (bundle of electromagnetic energy) travels between the interatomic void at a speed of c; yet time delay involved in the process of being absorbed and reemitted by the atoms of the material lowers the net speed of transport from one end of the medium to the other. Subsequently, the net speed of an electromagnetic wave in any medium is somewhat less than its speed in a vacuum - c (3 x 10^8 m/s).
How much the wave will delay will depend upon the optical density of material.
The optical density of a medium is not the same as its physical density. The physical density of a material refers to the mass/volume ratio. The optical density of a material relates to the tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before reemitting it as a new electromagnetic disturbance. The more optically dense that a material is, the slower that a wave will move through the material.

Speed Regulator (TRIAC)

TRIAC Basics

The TRIAC is a component that is effectively based on the thyristor. It provides AC switching for electrical systems. Like the thyristor, the TRIACs are used in many electrical switching applications. They find particular use for circuits in light dimmers,fan speed regulators, etc., where they enable both halves of the AC cycle to be used. This makes them more efficient in terms of the usage of the power available. While it is possible to use two thyristors back to back, this is not always cost effective for low cost and relatively low power applications.

It is possible to view the operation of a TRIAC in terms of two thyristors placed back to back.

TRIAC equivalent as two thyristors

One of the drawbacks of the TRIAC is that it does not switch symmetrically. It will often have an offset, switching at different gate voltages for each half of the cycle. This creates additional harmonics which is not good for EMC performance and also provides an imbalance in the system

In order to improve the switching of the current waveform and ensure it is more symmetrical is to use a device external to the TRIAC to time the triggering pulse. A DIAC placed in series with the gate is the normal method of achieving this.

DIAC and TRIAC connected together

Basic Circuit:

This is the circuit diagram of the simplest lamp dimmer or fan regulator.The circuit is based on the principle of power control using a Triac.The circuit works by varying the firing angle of the Triac . Resistors R1 ,R2 and capacitor C2 are associated with this. The firing angle can be varied by varying the value of any of these components. Here R1 is selected as the variable element . By varying the value of R1 the firing angle of Triac changes (i.e. how much time should Triac conduct) changes. This directly varies the load power, since load is driven by Triac. The firing pulses are given to the gate of Triac T1 using Diac D1. The most basic wavefor(i.e ignoring all losses and harmonics) is shown below.

                   

The waveform shown below demonstrates the output voltage of TRIAC before and after rectification.

Alpha is firinf angel of thyristers.

From the two figures shown below  we can see the output waveform by changing firing angel. In the first figure  the output will be half power of the input power.

In the second figure as firing angel is zero ,therefore output power will be same as input.

              

The Thing

There is a diaphram in the eye of eagle which get vibrated when sound wave fall on it . It consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna; it had no power supply or active electronic components. The device became active only when a radio signal of the correct frequency was sent to the device from an external transmitter. 

Sound waves caused the membrane to vibrate, which varied the capacitance of the circuit.When capacitance get changed , operating frequency get changed.When this changed frequency supply reached Antenna circuit, different EM wave is produced which get modulated with incoming EM wave and is get re-transmitted by the Thing. A receiver demodulated the signal so that sound picked up by the microphone could be heard, just as an ordinary radio receiver demodulates radio signals and outputs sound.

Q Factor

The Q, quality factor, of a resonant circuit is a measure of the goodness or quality of a resonant circuit. A higher value for this figure of merit correspondes to a more narrow bandwith, which is desirable in many applications. More formally, Q is the ration of power stored to power dissipated in the circuit reactance and resistance.  

           

                      

                                          

Series Resonance

The resonance of a series RLC circuit occurs when the inductive and capacitive reactances are equal in magnitude but cancel each other because they are 180 degrees apart in phase. The sharp minimum in impedance which occurs is useful in tuning applications. The sharpness of the minimum depends on the value of R and is characterized by the "Q" of the circuit.

The frequency response of the circuits current magnitude above, relates to the “sharpness” of the resonance in a series resonance circuit. The sharpness of the peak is measured quantitatively and is called the Quality factor, Q of the circuit. The quality factor relates the maximum or peak energy stored in the circuit (the reactance) to the energy dissipated (the resistance) during each cycle of oscillation meaning that it is a ratio of resonant frequency to bandwidth and the higher the circuit Q, the smaller the bandwidth. 

                    

Parallel Resonance

The Q-factor of a parallel resonance circuit is the inverse of the expression for the Q-factor of the series circuit. Also in series resonance circuits the Q-factor gives the voltage magnification of the circuit, whereas in a parallel circuit it gives the current magnification.

The selectivity or Q-factor for a parallel resonance circuit is generally defined as the ratio of the circulating branch currents to the supply current and is given as: 

               

  

The Q-factor of a parallel resonance circuit is the inverse of the expression for the Q-factor of the series circuit. Also in series resonance circuits the Q-factor gives the voltage magnification of the circuit, whereas in a parallel circuit it gives the current magnification

Resonant circuits are used to respond selectively to signals of a given frequency while discriminating against signals of different frequencies. If the response of the circuit is more narrowly peaked around the chosen frequency, we say that the circuit has higher selectivity. A quality factor Q, is a measure of that selectivity, and we speak of a circuit having a high Q if it is more narrowly selective. 

An example of the application of resonant circuits is the selection of AM radio stations by the radio receiver. The selectivity of the tuning must be high enough to discriminate strongly against stations above and below in carrier frequency, but not so high as to discriminate against the "sidebands" created by the imposition of the signal by amplitude modulation. 

Consider a circuit where R, L and C are all in parallel. The lower the parallel resistance, the more effect it will have in damping the circuit and thus the lower the Q.

Radar

A radar system has a transmitter that emits radio waves called radar signals in predetermined directions. When these come into contact with an object they are usually reflected or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity—especially by most metals, by sea water and by wet lands. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either toward or away from the transmitter, there is a slight equivalent change in the frequency of the radio waves, caused by the Doppler effect.

If electromagnetic waves traveling through one material meet another, having a very different dielectric constant or diamagnetic constant from the first, the waves will reflect or scatter or refract  from the boundary between the materials. This means that a solid object in air or in a vacuum, or a significant change in atomic density between the object and what is surrounding it, will usually scatter radio waves from its surface.

Radar receivers are usually, but not always, in the same location as the transmitter. Although the reflected radar signals captured by the receiving antenna are usually very weak, they can be strengthened by electronic amplifiers.

There are basically two types of radar:

1) Primary radar

2) Secondary radar

Primary radar has lots of limitations. It works best with large all-metal aircraft, not so well on small, composite aircraft, and not at all with some of the new "stealth" technology. Its range is limited by terrain and precipitation. It's rather indiscriminite about what it detects: airplanes, trucks, hills, trees. And it only reports a target's  and range, not its altitude (only 2D).

Secondary radar was invented to overcome these limitations. It depends on a transponder in the aircraft to respond to interrogations (type of signal) from the ground station. Depending on the type of interrogation, the transponder sends back an identification code  or altitude information.

Radar ground stations:

 It consist of three separate antennas. The biggest  one is the primary radar antenna, which looks like a parabolic dish that goes round and round . This antenna transmits powerful pulses and then listens for echoes. It is used to detect aircraft skin paint and also can detect weather to some degree. 

The second ground station antenna, called the directional antenna, is used to send interrogations to airborne transponders and to receive replies from those transponders, providing secondary radar capability. It is a bar-shaped  that is usually perched atop the primary radar antenna and rotates along with it. It's called directional because, like the primary radar antenna, it is designed to beam the interrogations and to receive the replies only from the direction it is pointed. 

However, the directional antenna is less than perfectly directional. To design a perfectly directional antenna, we have to make it infinitely large and thus not practical . Real-world directional antennas have weaker side lobes in addition to the main lobe. The side lobes are too weak to be a problem for distant aircraft, but for aircraft close to the antenna site they are a big problem. Unless something was done about them, the side lobes would cause a close-in aircraft to show up as three or four different targets on the controller's screen, causing  confusion.

Side lobe supression:

That's where the third antenna comes in. It's called the omnidirectional antenna because it radiates equally in all directions. Every time the directional antenna sends out an interrogation (which consists of a pair of pulses), the omnidirectional antenna sends out its own pulse . The signal from the omnidirectional antenna is designed to be much weaker than the main lobe of the directional antenna, but stronger than its side lobes. 

When the transponder receives an interrogation, it compares the strength of the three pulses it receives and according to the strength of the signals it provides information. 

Magnetron

All cavity magnetrons consist of a  cathode with a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path, a consequence of the Lorentz force. Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. This principle of cavity resonator is very similar to blowing a stream of air across the open top of a glass pop bottle. A portion of the field is extracted with a short antenna that is connected to a waveguide .The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar.

The magnetron is called a "crossed-field" device in the industry because both magnetic and electric fields are employed in its operation, and they are produced in perpendicular directions so that they cross. The applied magnetic field is constant and applied along the axis of the circular device illustrated. The power to the device is applied to the center cathode which is heated to supply energetic electrons which would, in the absence of the magnetic field, tend to move radially outward to the ring anode which surrounds it.

  

Electrons are released at the center  cathode by the process of thermionic emission and have an accelerating field which moves them outward toward the anode. The axial magnetic field exerts a magnetic force on these charges which is perpendicular to their initially radial motion, and they tend to be swept around the circle. In this way, work is done on the charges and therefore energy from the power supply is given to them. As these electrons sweep toward a point where there is excess negative charge, that charge tends to be pushed back around the cavity, imparting energy to the oscillation at the natural frequency of the cavity. This driven oscillation of the charges around the cavities leads to radiation of electromagnetic waves, the output of the magnetron.

The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load impedance, with changes in the supply current, and with the temperature of the tube. This is not a problem in uses such as heating, or in some forms of radar where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices such as the klystron are used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be synchronized with the build-up of oscillator output.

  

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1 kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.).

Mag Flowmeter

Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls. When the fluid moves faster, more voltage is generated. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.

A magnetic flow meter (mag flowmeter) is a volumetric flow meter which does not have any moving parts and is ideal for wastewater applications or any dirty liquid which is conductive or water based. Magnetic flowmeters will generally not work with hydrocarbons, distilled water and many non-aqueous solutions). Magnetic flowmeters are also ideal for applications where low pressure drop and low maintenance are required.

Principle of Operation

The operation of a magnetic flowmeter or mag meter is based upon Faraday's Law, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field is proportional to the velocity of that conductor.

Faraday's Formula:

E is proportional to V x B x D where:

E = The voltage generated in a conductor

V = The velocity of the conductor

B = The magnetic field strength

D = The length of the conductor

 To apply this principle to flow measurement with a magnetic flowmeter, it is necessary first to state that the fluid being measured must be electrically conductive for the Faraday principle to apply. As applied to the

design of magnetic flowmeters, Faraday's Law indicates that signal voltage (E) is dependent on the average liquid velocity (V) the magnetic field strength (B) and the length of the conductor (D) (which in this instance is the distance between the electrodes). 

Magnetic flowmeters measure the velocity of conductive liquids in pipes, such as water, acids, caustic, and slurries.However fluids with low conductivity, such as de-ionized water, boiler feed water, or hydrocarbons, can cause the flowmeter to turn off and measure zero flow.

This flowmeter does not obstruct flow, so it can be applied to clean, sanitary, dirty, corrosive and abrasive liquids. Magnetic flowmeters can be applied to the flow of liquids that are conductive, so hydrocarbons and gases cannot be measured with this technology due to their non-conductive nature and gaseous state respectively.

Ion Thruster

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field. Electromagnetic ion thrusters use the Lorentz force to accelerate the ions. Note that the term "ion thruster" frequently denotes the electrostatic or gridded ion thrusters only.

The thrust created in ion thrusters is very small compared to conventional chemical rockets, but a very high specific impulse, or propellant efficiency, is obtained. This high propellant efficiency is achieved through the very frugal propellant consumption of the ion thruster propulsion system. They do, however, use a large amount of power. Given the practical weight of suitable power sources, the accelerations given by these types of thrusters is of the order of one thousandth of standard gravity.

Due to their relatively high power needs, given the specific power of power supplies, and the requirement of an environment void of other ionized particles, ion thrust propulsion is currently only practical beyond planetary atmosphere (in space).

Gridded electrostatic ion thrusters :

Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from a hot cathode filament and when accelerated in the electrical field of the cathode, fall to the anode (Kaufman type ion thruster). Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radio frequency ion thruster).

The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust.

Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging-up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam. This also prevents the beam of ions from returning to the spacecraft and thereby cancelling the thrust.

Electromagnetic Wave

The electromagnetic field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwell's equations and the Lorentz force law.It is further classified as near field and far field.

The near field and far field and the transition zone are regions of time-varying electromagnetic field around any object that serves as a source for the field. The different terms for these regions describe the way characteristics of an electromagnetic (EM) field change with distance from the charges and currents in the object that are the sources of the changing EM field. The more distant parts of the far-field are identified with classical electromagnetic radiation.

The basic reason an EM field changes in character with distance from its source is that Maxwell's equations prescribe different behaviors for each of the two source-terms of electric fields and also the two source-terms for magnetic fields. Electric fields produced by charge distributions have a different character than those produced by changing magnetic fields. Similarly, Maxwell's equations show a differing behavior for the magnetic fields produced by electric currents, versus magnetic fields produced by changing electric fields. For these reasons, in the region very close to currents and charge-separations, the EM field is dominated by electric and magnetic components produced directly by currents and charge-separations, and these effects together produce the EM "near field." However, at distances far from charge-separations and currents, the EM field becomes dominated by the electric and magnetic fields indirectly produced by the change in the other type of field, and thus the EM field is no longer affected (or much affected) by the charges and currents at the EM source. This more distant part of the EM field is the "radiative" field or "far-field," and it is the familiar type of electromagnetic radiation seen in "free space," far from any EM field sources (origins).

The far-field thus includes radio waves and microwaves several wavelengths from most types of antennas etc.

Schumann resonance

Lightning discharges are considered to be the primary natural source of Schumann resonance excitation; lightning channels (e.g clouds) behave like huge antennas that radiate electromagnetic energy at frequencies below 100 hz.These signals are very weak at large distances from the lightning source, but the Earth–ionosphere waveguide behaves like a resonator at ELF frequencies and amplifies the spectral signals from lightning at the resonance frequency.

Waveguide:

Waves in open space propagate in all directions, as spherical waves. In this way they lose their power proportionally to the square of the distance; that is, at a distance R from the source, the power is the source power divided by R2. The waveguide confines the wave to propagation in one dimension, so that (under ideal conditions) the wave loses no power while propagating.Conductors used in waveguides have small skin depth and hence large surface resistance.

Waves are confined inside the waveguide due to total reflection from the waveguide wall, so that the propagation inside the waveguide can be described approximately as a "zigzag" between the walls. This description is exact for electromagnetic waves in a hollow metal tube with a rectangular or circular cross-section. 

The real Earth–ionosphere waveguide is not a perfect electromagnetic resonant cavity.

Cavity resonators:

A cavity resonator is a hollow conductor blocked at both ends and along which an electromagnetic wave can be supported. It can be viewed as a waveguide short-circuited at both ends .

The cavity's interior surfaces reflect a wave of a specific frequency. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss (due to standing waves which are formed due to interaction of waves with different velocities). As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity.

Losses due to finite ionosphere electrical conductivity lower the propagation speed of electromagnetic signals in the cavity, resulting in a resonance frequency that is lower than would be expected in an ideal case, and the observed peaks are wide (rather sharp).In addition, there are a number of horizontal asymmetries – day-night difference in the height of the ionosphere, latitudinal changes in the Earth magnetic field, sudden ionospheric disturbances, polar cap absorption, variation in the Earth radius of +/- 11 km from equator to geographic poles, etc. that produce other effects in the Schumann resonance power specification.

Schumann resonances are recorded at many separate research stations around the world. The sensors used to measure Schumann resonances typically consist of two horizontal magnetic inductive coils for measuring the north-south and east-west components of the magnetic field, and a vertical electric dipole antenna for measuring the vertical component of the electric field. A typical passband of the instruments is 3–100 Hz. The Schumann resonance electric field amplitude (~300 microvolts per meter) is much smaller than the static fair-weather electric field (~150 V/m) in the atmosphere. Similarly, the amplitude of the Schumann resonance magnetic field (~1 picotesla) is many orders of magnitude smaller than the Earth magnetic field (~30–50 microteslas. Specialized receivers and antennas are needed to detect and record Schumann resonances. 

CCD Vs CMOS sensors

Digital cameras have become extremely common as the prices have come down. One of the drivers behind the falling prices has been the introduction of CMOS image sensors. CMOS sensors are much less expensive to manufacture than CCD sensors. 

Both CCD (charge-coupled device) and CMOS (complementary metal-oxide semiconductor) image sensors start at the same point -- they have to convert light into electrons. One simplified way to think about the sensor used in a digital camera (or camcorder) is to think of it as having a 2-D array of thousands or millions of tiny solar cells, each of which transforms the light from one small portion of the image into electrons. Both CCD and CMOS devices perform this task using a variety of technologies. 

In a CCD device, the charge is actually transported across the chip and read at one corner of the array. An analog-to-digital converter turns each pixel's value into a digital value. In most CMOS devices, there are several transistors at each pixel that amplify and move the charge using more traditional wires. The CMOS approach is more flexible because each pixel can be read individually. 

CCDs use a special manufacturing process to create the ability to transport charge across the chip without distortion. This process leads to very high-quality sensors in terms of fidelity and light sensitivity. CMOS chips, on the other hand, use traditional manufacturing processes to create the chip -- the same processes used to make most microprocessors. Because of the manufacturing differences, there have been some noticeable differences between CCD and CMOS sensors.

CCD sensors, as mentioned above, create high-quality, low-noise images. CMOS sensors, traditionally, are more susceptible to noise.

Because each pixel on a CMOS sensor has several transistors located next to it, the light sensitivity of a CMOS chip tends to be lower. Many of the photons hitting the chip hit the transistors instead of the photodiode.

CMOS traditionally consumes little power. Implementing a sensor in CMOS yields a low-power sensor.

CCDs use a process that consumes lots of power. CCDs consume as much as 100 times more power than an equivalent CMOS sensor.

CMOS chips can be fabricated on just about any standard silicon production line, so they tend to be extremely inexpensive compared to CCD sensors.

CCD sensors have been mass produced for a longer period of time, so they are more mature. They tend to have higher quality and more pixels.

CMOS sensors are just now improving to the point where they reach near parity with CCD devices in some applications. CMOS cameras are usually less expensive and have great battery life.

Pyroelectricity

Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled.The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to a voltage across the crystal. If the temperature stays constant at its new value, the pyroelectric voltage gradually disappears due to leakage current (the leakage can be due to electrons moving through the crystal, ions moving through the air, current leaking through a voltmeter attached across the crystal, etc.).

Pyroelectricity should not be confused with thermoelectricity: In a typical demonstration of pyroelectricity, the whole crystal is changed from one temperature to another, and the result is a temporary voltage across the crystal. In a typical demonstration of thermoelectricity, one side of the material is kept at one temperature and the other side at a different temperature, and the result is a permanent voltage across the crystal.

Pyroelectricity can be visualized as one side of a triangle, where each corner represents energy states in the crystal: kinetic, electrical and thermal energy. The side between electrical and thermal corners represents the pyroelectric effect and produces no kinetic energy. The side between kinetic and electrical corners represents the piezoelectric effect and produces no heat.

Although artificial pyroelectric materials have been engineered,effect was first discovered in minerals such as tourmaline. The pyroelectric effect is also present in both bone and tendon.

Pyroelectric charge in minerals develops on the opposite faces of asymmetric crystals. The direction in which the propagation of the charge tends toward is usually constant throughout a pyroelectric material, but in some materials this direction can be changed by a nearby electric field. These materials are said to exhibit ferroelectricity. All pyroelectric materials are also piezoelectric, the two properties being closely related. However, note that some piezoelectric materials have a crystal symmetry that does not allow pyroelectricity.

Very small changes in temperature can produce an electric potential due to a materials' pyroelectricity. Passive infrared sensors are often designed around pyroelectric materials, as the heat of a human or animal from several feet away is enough to generate a difference in charge.

The pyroelectric coefficient may be described as the change in the spontaneous polarization vector with temperature.The total pyroelectric coefficient measured at constant stress is the sum of the pyroelectric coefficients at constant strain (primary pyroelectric effect) and the piezoelectric contribution from thermal expansion (secondary pyroelectric effect). Thermal expansion is due to heating and cooling.

Artificial pyroelectric materials, usually in the form of a thin film, are of gallium nitride (GaN), caesium nitrate (CsNO3), polyvinyl fluorides, derivatives of phenylpyridine, and cobalt phthalocyanine.