diode

A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other.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. This is a crystalline piece of semiconductor material connected to two electrical terminals. A vacuum tube diode (now little used except in some high-power technologies) 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 to pass in one direction (called the diode's 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 to extract modulation from radio signals in radio receivers.
However, diodes can have more complicated behavior than this simple on-off action. This is due to their complex non-linear electrical characteristics, which can be tailored by varying the construction of their P-N junction. These are exploited in special purpose diodes that perform many different functions. For example, specialized diodes are used to regulate voltage (Zener diodes), to electronically tune radio and TV receivers (varactor diodes), to generate radio frequency oscillations (tunnel diodes), and to produce light (light emitting diodes). Tunnel diodes exhibit negative resistance, which makes them useful in some types of circuits.
CURRENT VOLTAGE CHARACTERISTICS
A semiconductor diode’s behavior in a circuit is given by its current–voltage characteristic, or I–V graph (see graph above). 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. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (vacant places for electrons) with which the electrons “recombine”. When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor (dopant) on the N-side and negatively charged acceptor (dopant) on the P-side. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator.
However, the width of the depletion region (called the depletion width) cannot grow without limit. For each electron-hole pair that recombines, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a “built-in” potential across the depletion zone.
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).
At very large reverse bias , beyond the peak inverse voltage or PIV, a process called reverse breakdown occurs which causes a large increase in current (i.e. a large number of electrons and holes are created at, and move away from the pn junction) that usually damages the device permanently. The avalanche diode is deliberately designed for use in the avalanche region. In the zener diode, the concept of PIV is not applicable. 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, such that the reverse voltage is “clamped” to a known value (called the zener voltage), and avalanche does not occur. Both devices, however, do have a limit to the maximum current and power in the clamped reverse voltage region. Also, following the end of forward conduction in any diode, there is reverse current for a short time. The device does not attain its full blocking capability until the reverse current ceases.
The second region, at reverse biases more positive than the PIV, has only a very small reverse saturation current. In the reverse bias region for a normal P-N rectifier diode, the current through the device is very low (in the µA range). However, this is temperature dependent, and at sufficiently high temperatures, a substantial amount of reverse current can be observed (mA or more).
The third region is forward but small bias, where only a small forward current is conducted.
As the potential difference is increased above an arbitrarily defined “cut-in voltage” or “on-voltage” or “diode forward voltage drop (Vd)”, the diode current becomes appreciable (the level of current considered “appreciable” and the value of cut-in voltage depends on the application), and the diode presents a very low resistance. The current–voltage curve is exponential.
REVERSE-RECOVERY EFFECT
Following the end of forward conduction in a PN type diode, a reverse current flows for a short time. The device does not attain its full blocking capability until the reverse current ceases.
The effect can be significant when switching large currents very quickly (di/dt on the order of 100 A/µs or more). A certain amount of "reverse recovery time" tr (on the order of tens of nanoseconds) may be required to remove the "reverse recovery charge" Qr (on the order of tens of nanocoulombs) from the diode. During this recovery time, the diode can actually conduct in the reverse direction. In certain real-world cases it can be important to consider the losses incurred by this non-ideal diode effect. However, when the slew rate of the current is not so severe (di/dt on the order of 10 A/µs or less), the effect can be safely ignored.[12] For most applications, the effect is also negligible for Schottky diodes.
The reverse current ceases abruptly when the stored charge is depleted, which is exploited in step recovery diodes for generation of extremely short pulses.
APPLICATIOS
-Radio demodulation
The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the radio article. In summary, an AM signal consists of alternating positive and negative peaks of voltage, whose amplitude or “envelope” is proportional to the original audio signal. The diode (originally a crystal diode) rectifies the AM radio frequency signal, leaving an audio signal which is the original audio signal, minus atmospheric noise. The audio is extracted using a simple filter and fed into an audio amplifier or transducer, which generates sound waves.
-Power conversion
Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode, which rectifies the AC into DC, provides better performance than the commutator of earlier dynamo. Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages.
-Over-voltage protection
Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting). For example, diodes are used in (stepper motor and H-bridge) motor controller and relay circuits to de-energize coils rapidly without the damaging voltage spikes that would otherwise occur. (Any diode used in such an application is called a flyback diode). Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors. Specialized diodes are used to protect from over-voltages at higher power (see Diode types above).
-Logic gates
Diodes can be combined with other components to construct AND and OR logic gates. This is referred to as diode logic.
-Ionizing radiation detectors
In addition to light, mentioned above, semiconductor diodes are sensitive to more energetic radiation. In electronics, cosmic rays and other sources of ionizing radiation cause noise pulses and single and multiple bit errors. This effect is sometimes exploited by particle detectors to detect radiation. A single particle of radiation, with thousands or millions of electron volts of energy, generates many charge carrier pairs, as its energy is deposited in the semiconductor material. If the depletion layer is large enough to catch the whole shower or to stop a heavy particle, a fairly accurate measurement of the particle’s energy can be made, simply by measuring the charge conducted and without the complexity of a magnetic spectrometer or etc. These semiconductor radiation detectors need efficient and uniform charge collection and low leakage current. They are often cooled by liquid nitrogen. For longer range (about a centimetre) particles they need a very large depletion depth and large area. For short range particles, they need any contact or un-depleted semiconductor on at least one surface to be very thin. The back-bias voltages are near breakdown (around a thousand volts per centimetre). Germanium and silicon are common materials. Some of these detectors sense position as well as energy. They have a finite life, especially when detecting heavy particles, because of radiation damage. Silicon and germanium are quite different in their ability to convert gamma rays to electron showers.
Semiconductor detectors for high energy particles are used in large numbers. Because of energy loss fluctuations, accurate measurement of the energy deposited is of less use
-Temperature measurements
A diode can be used as a temperature measuring device, since the forward voltage drop across the diode depends on temperature, as in a Silicon bandgap temperature sensor. From the Shockley ideal diode equation given above, it appears the voltage has a positive temperature coefficient (at a constant current) but depends on doping concentration and operating temperature (Sze 2007). The temperature coefficient can be negative as in typical thermistors or positive for temperature sense diodes down to about 20 kelvins. Typically, silicon diodes have approximately −2 mV/˚C temperature coefficient at room temperature.
-Current steering
Diodes will prevent currents in unintended directions. To supply power to an electrical circuit during a power failure, the circuit can draw current from a battery. An Uninterruptible power supply may use diodes in this way to ensure that current is only drawn from the battery when necessary. Similarly, small boats typically have two circuits each with their own battery/batteries: one used for engine starting; one used for domestics. Normally both are charged from a single alternator, and a heavy duty split charge diode is used to prevent the higher charge battery (typically the engine battery) from discharging through the lower charged battery when the alternator is not running.

Diodes are also used in electronic musical keyboards. To reduce the amount of wiring needed in electronic musical keyboards, these instruments often use keyboard matrix circuits. The keyboard controller scans the rows and columns to determine which note the player has pressed. The problem with matrix circuits is that when several notes are pressed at once, the current can flow backwards through the circuit and trigger "phantom keys" that cause “ghost” notes to play. To avoid triggering unwanted notes, most keyboard matrix circuits have diodes soldered with the switch under each key of the musical keyboard. The same principle is also used for the switch matrix in solid state pinball machines

zener diode.

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 conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by circuitry, the diode will be permanently damaged due to overheating. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.
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. By contrast with the conventional device, a reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of very nearly 3.2 V across a wide range of reverse currents. The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications.
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.[1] In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature-critical applications. Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.
All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode
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.
Zener diodes are also used in surge protectors to limit transient voltage spikes.
Another notable application of the zener diode is the use of noise caused by its avalanche breakdown in a random number generator that never repeats
Avalanche breakdown - is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. It is a type of electron avalanche. The Avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bond electrons.
Explanation
Materials conduct electricity if they contain mobile charge carriers. There are two types of charge carriers in a semiconductor: free electrons and electron holes. A fixed electron in a reverse-biased diode may break free due to its thermal energy, creating an electron-hole pair. If there is a voltage gradient in the semiconductor, the electron will move towards the positive voltage while the hole will "move" towards the negative voltage. Most of the time, the electron and hole will just move to opposite ends of the crystal and stop. Under the right circumstances, however, (ie. when the voltage is high enough) the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (ie. more charge carriers), increasing the current. Fast-"moving" holes may also result in more electron-hole pairs being formed. In a fraction of a nanosecond, the whole crystal begins to conduct.
Avalanche breakdown usually destroys regular diodes, but avalanche diodes are designed to break down this way at low voltages and can survive the reverse current.
The voltage at which the breakdown occurs is called the breakdown voltage. There is a hysteresis effect; once avalanche breakdown has occurred, the material will continue to conduct if the voltage across it drops below the breakdown voltage. This is different from a Zener diode, which will stop conducting once the reverse voltage drops below the breakdown voltage.

ac circuit part 2

RL-CIRCUIT
The impedance of an RL circuit is the total opposition to AC current flow caused by the resistor (R) and the reactance of the inductor (XL).
The equation for the impedance of an RL circuit is:

where:
Z = the total impedance in ohms
XL = the inductive reactance in ohms
R = the resistance in ohms
Although you can use an ordinary ohmmeter to measure resistance, there are no common lab instruments for directly measuring reactance and impedance. For all practical purposes, then, you must calculate reactance and impedance from other circuit values that are more readily available.

In practical RL circuits, you can readily determine or directly measure the values of VT, f, R, and L. There is more than enough information among these items to calculate the values of XL and Z.
RC-CIRCUIT
A resistor–capacitor circuit (RC circuit), or RC filter or RC network, is an electric circuit composed of resistors and capacitors driven by a voltage or current source. A first order RC circuit is composed of one resistor and one capacitor and is the simplest type of RC circuit.
RC circuits can be used to filter a signal by blocking certain frequencies and passing others. The four most common RC filters are the high-pass filter, loSeries RLC Circuit
If an AC emf given by is used to drive current through a resistor, a capacitor, and an inductor connected in series, then the current through each element must be the same. The voltages across the various elements obey the rules given above, and the sum of these voltages must, by the loop theorem, be equal to the applied emf. This sum must be taken at a particular instant of time, which is complicated because each voltage difference will be at a different part of its cycle.

Note that the amplitude of I is given by , which looks about like the DC relation I = V / R. The quantity Z is called the impedance, it has units of ohms, and it plays the same role in AC circuits as resistance does in DC circuits. Unlike the resistance, however, it depends on the driving frequency, so the current that flows in the circuit depends sensitively on the driving frequency. Be careful, however; this formula for the impedance only applies to the series RLC circuit. Each different circuit has its own impedance formula.
Resonance: Consider the series RLC circuit discussed above. The formula for the current makes it easy to see how things should be adjusted to get as much current as possible from a given driving emf : simply make Z as small as possible. If the circuit value of R is fixed (as is usually the case) then the only way to get more current is by fiddling with and . And it is clear that the smallest value of Z will be obtained when , which a little algebra shows is equivalent to . But this simply says that things should be adjusted so that the driving frequency is equal to the natural frequency of the circuit (without the correction due to resistance). So, if the driving frequency is near the natural frequency, very large currents can result. When a circuit is driven near its natural frequency, we say that it is being driven at resonance. And the formula for Z shows that the smaller the resistance of the circuit, the larger the response at resonance will be. This is what makes the radio tuner work. The antenna of the radio picks up radio signals from every station in the area, but only the station whose frequency matches the natural frequency of the tuning circuit will cause large currents to flow in the circuit. These currents, when amplified, are the ones that produce the sound you hear. If the circuit is not properly tuned, then it may pick up two stations equally well, an effect you have probably heard many times.
w-pass filter, band-pass filter, and band-stop filter.

simple a.c circuit, part 1

When an alternator produces AC voltage, the voltage switches polarity over time, but does so in a very particular manner. When graphed over time, the “wave” traced by this voltage of alternating polarity from an alternator takes on a distinct shape, known as a sine wave.
In the voltage plot from an electromechanical alternator, the change from one polarity to the other is a smooth one, the voltage level changing most rapidly at the zero (“crossover”) point and most slowly at its peak. If we were to graph the trigonometric function of “sine” over a horizontal range of 0 to 360 degrees, we would find the exact same pattern.
The reason why an electromechanical alternator outputs sine-wave AC is due to the physics of its operation. The voltage produced by the stationary coils by the motion of the rotating magnet is proportional to the rate at which the magnetic flux is changing perpendicular to the coils (Faraday's Law of Electromagnetic Induction). That rate is greatest when the magnet poles are closest to the coils, and least when the magnet poles are furthest away from the coils. Mathematically, the rate of magnetic flux change due to a rotating magnet follows that of a sine function, so the voltage produced by the coils follows that same function.
A more popular measure for describing the alternating rate of an AC voltage or current wave than period is the rate of that back-and-forth oscillation. This is called frequency. The modern unit for frequency is the Hertz (abbreviated Hz), which represents the number of wave cycles completed during one second of time. In the United States of America, the standard power-line frequency is 60 Hz, meaning that the AC voltage oscillates at a rate of 60 complete back-and-forth cycles every second. In Europe, where the power system frequency is 50 Hz, the AC voltage only completes 50 cycles every second. A radio station transmitter broadcasting at a frequency of 100 MHz generates an AC voltage oscillating at a rate of 100 million cycles every second.
An instrument called an oscilloscope, Figure below, is used to display a changing voltage over time on a graphical screen.
watch out for the next issue.

THE HISTORY OF CHEMICAL ENGINEERING

THE HISTORY OF CHEMICAL ENGINEERING
Chemical engineering as a profession was established over 100 years when it arose from the combination of mechanical engineering and chemistry1. In the developing chemical industry in the late 19th century there was a critical need for engineers involved in the design, construction, and operation of chemical factories to have a solid understanding of both engineering and chemical principles, particularly chemical reaction stoichiometry and kinetics. The first degrees in chemical engineering in the USA were awarded at that time and the American Institute of Chemical Engineers was established in 1908.

Throughout the 20th century, the profession of chemical engineering developed as the chemical and petrochemical industries2, 3 expanded and as more universities developed formalized chemical engineering curricula that incorporated solid fundamental education in the physical and chemical sciences. As the educational component of chemical engineering developed to include deep fundamental analysis of physical and physical chemical processes, particularly after the 2nd World War with the study of mass, momentum, and energy transport4, 5, the role of chemical engineers in industry and society broaden considerably6.

Because of the emphasis on fundamental principles in the education as opposed to the equipment-specific emphasis of the earlier years, chemical engineering employment diversified into new fields such as microelectronics processing and environmental engineering and pollution control7. The solid fundamental training of chemical engineers in the analysis of complex multiphase systems with chemical reactions and molecular transport was recognized in many industries and government laboratories. Today, the education of modern chemical engineers includes the full range of fundamental subjects in chemistry, physics, and engineering sciences such as thermodynamics, mechanics, fluid mechanics, heat and mass transport, and chemical reactor design. Understanding of these subjects, coupled with the capability to utilize advanced computational methods to solve systems of partial differential equations that describe how materials behave under a very large range of conditions, provide the modern chemical engineer the ability and skills to tackle many problems to improve our modern way of life.
In Nigeria, it started at obafemi awolowo university as department of chemical technology in1969. It became a constituent of chemical technology on the first of October 1971. In 1972 the name was changed to department of chemical engineering, the first set of students graduated 1973.
The post graduate program started in1980 and was developed to provide advance training in industrial development of the country.

Chemical engineers

The Role of Chemical Engineers
Chemical engineers are considered to be "universal engineers." They use chemistry, physics, biology, microbiology, biochemistry and mathematics to design programs, machines and processes that turn raw materials into valuable products for human use and for use in the environment. Chemical engineers play a very important role in making modern society as we know it from creating simple products such as paper, plastic, rubber, pharmaceuticals, gasoline and cement.
Responsibilities and Duties
Chemical engineers have multiple job responsibilities and duties to perform on a daily basis. Meticulous research is an important first step in order to cover all issues before the production of products. Chemical engineers
are also responsible for designing programs, machinery and processes that are used to manufacture their products. Performing a multitude of tests and analyzing data, problems and engineering design during the creation process is also key work of the chemical engineer. In addition, chemical engineers must prepare multiple reports and consult with other individuals on their findings.
The role that a chemical engineer plays in today’s world is an important one to put it quite simply. There are many responsibilities which a chemical engineer must undertake on a daily basis and various specific duties which must be carried out as well. It is important to look at these items to determine why the role of chemical engineer is so important and whether one is suited for a job of this type.
Specific Duties of a Chemical Engineer
Beneath all of the general responsibilities listed above, a chemical engineer must engage in numerous specific duties on a daily basis. The first duty which the chemical engineer is responsible for completing is research. The chemical engineer must take careful steps to ensure that what they are looking to manufacture and how they are looking to manufacture a product is the right avenue to pursue. The way to resolve this issue is by doing a lot of research on a variety of topics relating to chemical engineering.

The chemical engineer is also responsible for designing a variety of items and this is a very important duty which they must complete. A chemical engineer must design various items such as measurement and control systems, chemical manufacturing equipment and chemical manufacturing processes. This is a major duty on the part of the chemical engineer and one which must be carried out with preciseness at all levels and stages.

A chemical engineer must also engage in a wide array of analyses. The things which the chemical engineer must analyze include test data, engineering design, design problems and research findings. The chemical engineer must take painstaking measures to adequately analyze these items as the outcome of the project could very well depend on the analysis which is undertaken by the chemical engineer.

One who is an engineer must develop certain procedures and policies as well so that there will be smooth operations all the way around the board. Various procedures and policies such as safety procedures, data tables and employment policies may all be in the hands of the chemical engineer. A senior level chemical engineer will have more to do with regard to developing policies and procedures within the company or corporation.

The preparation of multiple reports is also in the hands of the chemical engineer. The chemical engineer must prepare data which specifically details the findings of certain tests and evaluations. These reports can be text or tables depending on the type of report which is needed.

A chemical engineer will also deal with other individuals a great deal. The reason for doing so is to relay the results and findings as well as oversee other chemical engineers and related workers in their field. From time to time, chemical engineers must lecture to their peers and the general public regarding their job and role in society.