The thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased (that is, as long as the voltage across the device has not reversed).

Some sources define silicon controlled rectifiers and thyristors as synonymous.

The Thyristor’s Function

The thyristor is a four-layer semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:

Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode
2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
3. Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the “holding current”

Function of the gate terminal

The thyristor has three p-n junctions (serially named J₁, J₂, J₃ from the anode).

When the anode is at a positive potential V_AK with respect to the cathode with no voltage applied at the gate, junctions J₁ and J₃ are forward biased, while junction J₂ is reverse biased. As J₂ is reverse biased, no conduction takes place (Off state). Now if V_AK is increased beyond the breakdown voltage V_BO of the thyristor, avalanche breakdown of J₂ takes place and the thyristor starts conducting (On state).

If a positive potential V_G is applied at the gate terminal with respect to the cathode, the breakdown of the junction J₂ occurs at a lower value of V_AK. By selecting an appropriate value of V_G, the thyristor can be switched into the on state suddenly.

It should be noted that once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the gate voltage, until both: (a) the potential V_G is removed and (b) the current through the device (anode−cathode) is less than the holding current specified by the manufacturer. Hence V_G can be a voltage pulse, such as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger voltage (V_GT) and gate trigger current (I_GT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.

Switching characteristics

In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. does not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (I_L). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (I_H).

A thyristor can be switched off if the external circuit causes the anode to become negatively biased. In some applications this is done by switching a second thyristor to discharge a capacitor into the cathode of the first thyristor. This method is called forced commutation.

After a thyristor has been switched off by forced commutation, a finite time delay must have elapsed before the anode can be positively biased in the off-state. This minimum delay is called the circuit commutated turn off time (t_Q). Attempting to positively bias the anode within this time causes the thyristor to be self-triggered by the remaining charge carriers (holes and electrons) that have not yet recombined.

For applications with frequencies higher than the domestic AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower values of t_Q are required. Such fast thyristors are made by diffusing into the silicon heavy metals ions such as gold or platinum which act as charge combination centres. Alternatively, fast thyristors may be made by neutron irradiation of the silicon.

In electronics and electrical engineering a fuse (from the Latin “fusus” meaning to melt) is a type of sacrificial overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overload or device failure is often the reason for excessive current.

A fuse interrupts excessive current (blows) so that further damage by overheating or fire is prevented. Wiring regulations often define a maximum fuse current rating for particular circuits. Overcurrent protection devices are essential in electrical systems to limit threats to human life and property damage. Fuses are selected to allow passage of normal current and of excessive current only for short periods.

A fuse was patented by Thomas Edison in 1890 as part of his successful electric distribution system.

Fuse Operation

A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-conducting and non-combustible housing. The fuse is arranged in series to carry all the current passing through the protected circuit. The resistance of the element generates heat due to the current flow. The size and construction of the element is (empirically) determined so that the heat produced for a normal current does not cause the element to attain a high temperature. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a soldered joint within the fuse, opening the circuit.

When the metal conductor parts, an electric arc forms between the un-melted ends of the element. The arc grows in length until the voltage required to sustain the arc is higher than the available voltage in the circuit, terminating current flow. In alternating current circuits the current naturally reverses direction on each cycle, greatly enhancing the speed of fuse interruption. In the case of a current-limiting fuse, the arc voltage builds up quickly enough to essentially stop the fault current before the first peak of the ac waveform. This effect significantly limits damage to downstream protected devices.

The fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and predictable characteristics. The fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess. The element must not be damaged by minor harmless surges of current, and must not oxidize or change its behavior after possibly years of service.

The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal. A dual-element fuse may contain a metal strip that melts instantly on a short-circuit, and also contain a low-melting solder joint that responds to long-term overload of low values compared to a short-circuit. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments.

The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc. Silica sand or non-conducting liquids may be used.

A trimmer or preset is a miniatur adjustable electrical component. It is meant to be ordered aright when installed in whatever device, and never seen or keyed by the device’s user. Trimmers can be potentiometers or varco (variable capacitors – trimmable inductors subsist but are rattling uncommon). They are ordinary in exactitude circuitry similar to  A/V components, and may need to be adjusted when the equipment is serviced. Unlike some another variable controls, trimmers are mounted direct on circuit boards, overturned with a small screwdriver and rated for some less adjustments over their lifetime.

Trimmers become in a difference of sizes and levels of precision; for example, multi-turn cut potentiometers exist, in which it takes individual turns of the fitting propellor to accomplish the modify value, allowing for rattling broad degrees of accuracy.

In 1952, Marlan Bourns patented the world’s prototypal cut potentiometer, trademarked “Trimpot”, a study today commonly consumed to intend to some cut potentiometer.

The electronic component, Light Dependent Resistor (LDR) or often called photoresistor or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referenced as a photoconductor.

A Light Dependent Resistor (LDR) is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

A LDR device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, e.g. silicon. In intrinsic devices the only available electrons are in the valence band, and hence the photon must have enough energy to excite the electron across the entire bandgap. Extrinsic devices have impurities, also called dopants, added whose ground state energy is closer to the conduction band; since the electrons do not have as far to jump, lower energy photons (i.e., longer wavelengths and lower frequencies) are sufficient to trigger the device. If a sample of silicon has some of its atoms replaced by phosphorus atoms (impurities), there will be extra electrons available for conduction. This is an example of an extrinsic semiconductor.

Applications

LDR come in many different types. Inexpensive cadmium sulfide cells can be found in many consumer items such as camera light meters, street lights, clock radios, alarms, and outdoor clocks.

They are also used in some dynamic compressors together with a small incandescent lamp or light emitting diode to control gain reduction.

Lead sulfide and indium antimonide LDRs are used for the mid infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.
Transducers are used for changing energy types.

This image explanation is very easy to understand, just click the image to enlarge and then save the image for future reference. Or download the picture here. You should be understand about how to read capacitors after looking the picture.

Ceramic capacitor have the highest voltage capability. This type is very popular nonpolarized capacitor that is small and inexpensive but has poor temeperature stability and poor accuracy. It contains a ceramic dielectric and a phenolic coating. It is often used for bypass and coupling applications. Tolerances range from +/-5 to +/-100 percent, while capacitances range from 1 pf to 2.2 uF, with maximum voltage rating from 3 V to 6 kV.

For better choice, use mylar capacitor. This type is a very popular nonpolarized capacitor that is reliable, inexpensive, and has low leakage current but poor temperature stability. Capacitances range from 0.001 to 10 uF, with voltage ratings from 50 to 600 V.

Electrolytic capacitor or electrolytics condensator or we often call “ELCO” is a type of capacitor that uses an ionic conducting liquid as one of its plates. Typically with a larger capacitance per unit volume than other types, they are valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations, in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not.

Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low corner frequencies.

Electrolytic Capacitor Construction

Aluminum electrolytic capacitors are constructed from two conducting aluminum foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil act as cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminium casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces. Both of these are shown in the picture.

Polarity

In aluminum electrolytic capacitors, the layer of insulating aluminum oxide on the surface of the aluminum plate acts as the dielectric, and it is the thinness of this layer that allows for a relatively high capacitance in a small volume. The aluminum oxide layer can withstand an electric field strength of the order of 10⁹ volts per meter. The combination of high capacitance and high voltage result in high energy density.

Unlike most capacitors, electrolytic capacitors have a voltage polarity requirement. The correct polarity is indicated on the packaging by a stripe with minus signs and possibly arrowheads, denoting the adjacent terminal that should have lower electrical potential (i.e. negative terminal). Also the negative terminal lead of radial electrolytic capacitors are shorter. This is necessary because a reverse-bias voltage above 1 to 1.5 V will destroy the center layer of dielectric material via electrochemical reduction (see redox reactions). Without the dielectric material the capacitor will short circuit, and if the short circuit current is excessive, then the electrolyte will heat up and either leak or cause the capacitor to explode.

Special capacitors designed for AC operation are available, usually referred to as “non-polar” or “NP” types. In these, full-thickness oxide layers are formed on both the aluminium foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentially, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series.

Modern capacitors have a safety valve, typically either a scored section of the can, or a specially designed end seal to vent the hot gas/liquid, but ruptures can still be dramatic. Electrolytics can withstand a reverse bias for a short period of time, but they will conduct significant current and not act as a very good capacitor. Most will survive with no reverse DC bias or with only AC voltage, but circuits should be designed so that there is not a constant reverse bias for any significant amount of time. A constant forward bias is preferable, and will increase the life of the capacitor.

Capacitor		Polarized Capacitor		Variable Capacitor

These are the different schematic symbols for electrolytic capacitors. Some schematic diagrams do not print the “+” adjacent to the symbol. Electrolytic capacitors are marked to show the polarity of the leads.

Capacitance

The capacitance value of any capacitor is a measure of the amount of electric charge stored per unit of potential difference between the plates. The basic unit of capacitance is a farad, however this unit has been too large for general use until the invention of the Double-layer capacitor, so microfarad, nanofarad and picofarad are more commonly used. These are usually abbreviated to μF or uF, nF and pF.

Many conditions determine a capacitor’s value, such as the thickness of the dielectric and the plate area. In the manufacturing process, electrolytic capacitors are made to conform to a set of preferred numbers. By multiplying these base numbers by a power of ten, any practical capacitor value can be achieved, which is suitable for most applications.

A standardized set of capacitor base numbers was devised so that the value of any modern electrolytic capacitor could be derived from multiplying one of the modern conventional base numbers 1.0, 1.5, 2.2, 3.3, 4.7 or 6.8 by a power of ten. Therefore, it is common to find capacitors with values of 10, 15, 22, 33, 47, 68, 100, 220, and so on. Using this method, values ranging from 0.1 to 4700 are common in most applications. Values are generally in microfarads (µF).

Many electrolytic capacitors have a tolerance range of 20 %, meaning that the manufacturer is stating that the actual value of the capacitor lies within 20 % of its labeled value. Selection of the preferred series ensures that any capacitor can be sold as a standard value, within the tolerance. Also many electrolytic caps have asymmetric tolerances, typically -20% but with much larger positive tolerance.^{[citation needed]} This eliminates any need to test and grade individual caps.

Variants

Unlike capacitors that use a bulk dielectric made from an intrinsically insulating material, the dielectric in electrolytic capacitors depends on the formation and maintenance of a microscopic metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume, but maintaining the integrity of the dielectric usually requires the steady application of the correct polarity of direct current else the oxide layer will break down and rupture, causing the capacitor to fail. In addition, electrolytic capacitors generally use an internal wet chemistry and they will eventually fail if the water within the capacitor evaporates.

Electrolytic capacitance values are not as tightly-specified as with bulk dielectric capacitors. Especially with aluminum electrolytics, it is quite common to see an electrolytic capacitor specified as having a “guaranteed minimum value” and no upper bound on its value. For most purposes (such as power supply filtering and signal coupling), this type of specification is acceptable.

As with bulk dielectric capacitors, electrolytic capacitors come in several varieties:

• Aluminum electrolytic capacitor: compact but lossy, these are available in the range of <1 µF to 1 F with working voltages up to several hundred volts DC. The dielectric is a thin layer of aluminum oxide. They contain corrosive liquid and can burst if the device is connected backwards. The oxide insulating layer will tend to deteriorate in the absence of a sufficient rejuvenating voltage, and eventually the capacitor will fail if voltage is not applied. Bipolar electrolytics (also called Non-Polarised or NP capacitors) contain two capacitors connected in series opposition and are used when the DC bias voltage must occasionally reverse. Bad frequency and temperature characteristics make them unsuited for high-frequency applications. Typical ESL values are a few nH.^[4]

• Tantalum: compact, low-voltage devices up to several hundred µF, these have a lower energy density and are more accurate than aluminum electrolytics. Tantalum capacitors are also polarized because of their dissimilar electrodes. The cathode electrode is formed of sintered tantalum grains, with the dielectric electrochemically formed as a thin layer of oxide. The thin layer of oxide and high surface area of the porous sintered material gives this type a very high capacitance per unit volume. The cathode electrode is formed either of a liquid electrolyte connecting the outer can or of a chemically deposited semi-conductive layer of manganese dioxide, which is then connected to an external wire lead. A development of this type replaces the manganese dioxide with a conductive plastic polymer (polypyrrole) that reduces internal resistance and eliminates a self-ignition failure.^[5]

Compared to aluminum electrolytics, tantalum capacitors have very stable capacitance, little DC leakage, and very low impedance at high frequencies. However, unlike aluminum electrolytics, they are intolerant of voltage spikes and are destroyed (often exploding violently) if connected in the circuit backwards or exposed to spikes above their voltage rating.

Tantalum capacitors are more expensive than aluminum-based capacitors and generally only usable at low voltage, but because of their higher capacitance per unit volume and lower impedance at high frequencies, they are popular in miniature applications such as cellular telephones.

LEDs are widely used as indicator lights on electronic devices and increasingly in higher power applications such as flashlights and area lighting. An LED is usually a small area (less than 1 mm2) light source, often with optics added to the chip to shape its radiation pattern and assist in reflection. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be infrared, visible, or ultraviolet. Besides lighting, interesting applications include using UV-LEDs for sterilization of water and disinfection of devices, and as a grow light to enhance photosynthesis in plants.

Discovery and development
The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round of Marconi Labs. Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored, and no practical use was made of the discovery for several decades. Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955.[9] Braunstein observed infrared emission generated by simple diode structures using GaSb, GaAs, InP, and Ge-Si alloys at room temperature and at 77 K. In 1961, experimenters Bob Biard and Gary Pittman working at Texas Instruments, found that gallium arsenide gave off infrared radiation when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode.

The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company. He later moved to the University of Illinois at Urbana-Champaign. Holonyak is seen as the “father of the light-emitting diode”. M. George Craford, a former graduate student of Holonyak’s, invented the first yellow LED and 10x brighter red and red-orange LEDs in 1972.

Shuji Nakamura of Nichia Corporation of Japan demonstrated the first high-brightness blue LED based on InGaN borrowing on critical developments in GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN which were developed by I. Akasaki and H. Amano in Nagoya. In 1995, Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs demonstrating very high result by using a transparent contact made of indium tin oxide (ITO) on (AlGaInP/GaAs) LED. The existence of blue LEDs and high efficiency LEDs quickly led to the development of the first white LED, which employed a Y3Al5O12:Ce, or “YAG”, phosphor coating to mix yellow (down-converted) light with blue to produce light that appears white. Nakamura was awarded the 2006 Millennium Technology Prize for his invention.

The development of LED technology has caused their efficiency and light output to increase exponentially, with a doubling occurring about every 36 months since the 1960s, in a similar way to Moore’s law. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally called Haitz’s Law after Dr. Roland Haitz.

Practical use
The first commercial LEDs were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches (see list of signal applications). These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level, causing LEDs to become bright enough to be used for illumination, in various applications such as lamps and other lighting fixtures.

Most LEDs were made in the very common 5 mm T1³⁄₄ and 3 mm T1 packages, but with higher power, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages adapted for efficient heat dissipation are becoming common. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs.

Physical principles

The inner workings of an LED

I-V diagram for a diode an LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2-3 Volt

Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped, with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

Light extraction

The refractive index of most LED semiconductor materials is quite high, so in almost all cases the light from the LED is coupled into a much lower-index medium. The large index difference makes the reflection quite substantial (per the Fresnel coefficients). The produced light gets partially reflected back into the semiconductor, where it may be absorbed and turned into additional heat; this is usually one of the dominant causes of LED inefficiency. Often more than half of the emitted light is reflected back at the LED-package and package-air interfaces.

The reflection is most commonly reduced by using a dome-shaped (half-sphere) package with the diode in the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The refractive index of the package material should also match the index of the semiconductor, to minimize back-reflection. An anti-reflection coating may be added as well.

The package may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted.

Other strategies for reducing the impact of the interface reflections include designing the LED to reabsorb and reemit the reflected light (called photon recycling) and manipulating the microscopic structure of the surface to reduce the reflectance, by introducing random roughness, creating programmed moth eye surface patterns. Recently photonic crystal have also been used to minimize back-reflections. In December 2007, scientists at Glasgow University claimed to have found a way to make LEDs more energy efficient, imprinting billions of holes into LEDs using a process known as nanoimprint lithography.

Electrical polarity

Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.

Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage above this threshold. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel.

Advantages of using LEDs

• Efficiency: LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
• Color: LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• Size: LEDs can be very small (>2 mm²) and are easily populated onto printed circuit boards.
• On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds^[34]. LEDs used in communications devices can have even faster response times.
• Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.
• Dimming: LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.
• Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
• Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.
• Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

Disadvantages of using LEDs

• High price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps
• Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.
• Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.^[37]
• Light quality: Most white LEDs have spectra that differ significantly from a black body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the color of objects to be perceived differently under LED illumination than sunlight or incandescent sources, due to metamerism,^[38] red surfaces being rendered particularly badly by typical phosphor based LEDs white LEDs. However, the color rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.
• Area light source: LEDs do not approximate a “point source” of light, so cannot be used in applications needing a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.
• Blue Hazard: There is increasing concern that blue LEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.
• Blue pollution: Because white LEDs emit much more blue light than conventional outdoor light sources such as high-pressure sodium lamps, the strong wavelength dependence of Rayleigh scattering means that LEDs can cause more light pollution than other light sources. It is therefore very important that LEDs are fully shielded when used outdoors. Compared to low-pressure sodium lamps, which emit at 589.3nm, the 460 nm emission spike of white and blue LEDs is scattered about 2.7 times more by the Earth’s atmosphere. LEDs should not be used for outdoor lighting near astronomical observatories.

The transformer is effectively a magnetic circuit. The transformer has two or more coils of wire wrapped about a common core.

The ideal relationship is:

If a transformer has an iron core it will be shown with lines in the centre,

To deal with a transformer in a circuit analysis we need to pay attention to the polarity of the coils, and we may consider the inductance of each coil at times.

Consider the example below, from [Nilsson, pg. 450]. We want to find the power delivered to the 1ohm resistor. We will use the mesh current method,

Next we can solve the remaining three equations and three unknown currents using a matrix approach,

Finally we find the power in the resistor,

Some basic schematic symbols for capacitors:

Figure 2(b) shows a polarised (electrolytic) capacitor. This type of capacitor has its dielectric formed electrically, and must only be used in DC circuits. Figures 2(c), (d), and (e) are all variable capacitors. Figure 2(d) is often called a trimmer capacitor, as it is adjusted with a small screwdriver and is often used to “trim” the capacitance in a circuit. Sometimes two or more variable capacitors are used on the one shaft, so that when the shaft is turned, all of the capacitors are made to vary. This is shown in 2(e). With such a capacitor, all the capacitors on the common shaft do not have to be the same.

Some capacitor packages:

Figures 3(a) and (c) – polyester dielectric capacitors; 3(b) – Trimmer capacitor; 3(d) and (e) electrolytic (polarised) capacitors. Photos not to scale.

The interesting stuff starts to happen when we connect an EMF to the plates of a capacitor. Have a look at the test circuit of figure 4.

The capacitor can be switched, so that it will be connected to the 10 volt battery when the switch is thrown to the left, or when the switch is thrown to the right it will be connected to the resistor. The capacitor can be connected by the switch to the battery or the resistor, but not both at the same time. The symbols -Ve and +Ve on the battery are shorthand for negative and positive charge.

The negative terminal of the battery has an excess of electrons on it, created by the chemical action in the battery. The positive terminal has a deficiency of electrons. Now recall that unlike charges attract. If there were any way for the electrons to get from the negative terminal of the battery to the positive, they would. I just want you to imagine a battery by itself with two terminals, for a moment. There is an electrostatic field between the two terminals of any battery created by the unlike charges on each terminal. In other words there is a very slight tugging from the positive terminal, and a very slight pushing from the negative terminal in a vain attempt to move electrons from the negative to the positive terminal. No current flows between the unconnected terminals of a battery because the electrostatic fields are very small due to the spacing of the battery terminals and the very high air resistance between them.

If you find this hard to imagine look at figure 5 below of two charges separated by air.

Here we have two lumped charges Q1 and Q2. There is an electrostatic field between them. This field is trying to pull electrons from the negative charge over to the positive charge. There is an electrical strain here. No current flows because the two charges are too far apart. Let’s say they are 100mm to start with. Now move the two lumped charges so that they are only 50mm apart. The electrostatic field will now be stronger. The strain will be stronger. Still no current flows. Let’s push the issue. Move the two lumped charges so that they are only 5mm apart. Now (depending on the charges) the electrostatic field will be immense. The tugging of electrons from the negative charge will be normous. Still no current flows, as the air insulation between the charges is too great.

The electrons on the negative lumped charge want to traverse the gap to the positive charge. Do you think the electrons would be evenly distributed on the negative lump? On the side of the negative lump closest to the positive lump (the inside) the electrons will be crowding up trying to jump the gap.

Can you figure out what will happen if we continue to move them closer, say, to 1 mm?
Well I think you will agree that a point will be reached where the electrostatic field is so strong that electrons will jump off the negative lump and flow through the air to the positive lump. There will be an arcing of electric current. This is what happens in a lightning storm.

Back to our capacitor connected at the moment to the resistor. The switch is thrown to the right. The plates of the capacitor have a very large area. The dielectric between the plates is extremely thin but still a very good insulator. When we throw the switch to the left as in figure 6, we are in fact extending the charges on the battery terminals to the plates of the capacitor, and there will be a strong electrostatic field across the plates of the capacitor and through the dielectric.

Electrons are going to move from the negative terminal of the battery and bunch up on the top plate of the capacitor. Similarly, electrons are going to move from the bottom plate and travel to the positive terminal of the battery. No electrons will be able to flow though the dielectric. Its insulating properties are too good. So, if you like, there is going to be a redistribution of charge. This movement of charge will continue until the electrons flowing into and out of the capacitor create a potential difference on the plates of the capacitor equal to the battery voltage, namely 10 volts. The capacitor is now said to be charged.

This charging of the capacitor does not happen instantaneously – it takes a little time.
Suppose now we move the switch so the capacitor is disconnected from the circuit, as
shown in figure 7.

The capacitor is now left charged, even though it has been disconnected from the battery.
The capacitor has 10 volts across it. Older capacitors would not hold this charge for very long as a current would slowly leak through the dielectric and the capacitor would eventually self-discharge. Some modern capacitors can hold their charge for days or longer.

The capacitor has stored energy in it in the form of a charge on the plates. If we connect a circuit to the capacitor it will discharge, as shown in figure 8.

Electrons will now flow from the top plate of the capacitor through the resistance, until the capacitor becomes discharged. If the resistor was a small light bulb it would flash brightly at first and then slowly dim as the capacitor discharges.

At no time did current flow through the dielectric of the capacitor

Suppose the resistor was a lamp. Also, suppose we continue to rapidly move the switch back and forth between the left and right position. The lamp would perhaps flicker a bit, but be continuously lit.
So you should now see that a capacitor can be used to store charge and we can use that charge to do something.

Where is the energy stored?
Though we have said that energy has been stored by the charge on the plates, it is more correct to say that the energy is stored in the electric field. It is the charge on the plates that forms the electric field between the plates. When current flows into a capacitor, charging it, the electric field becomes stronger (stores more energy). When the current flows out of the capacitor, the voltage across the plates decreases and hence the strength of the electric field decreases (energy moves out of the electric field).

UNIT OF CAPACITANCE
The unit of capacitance is called the Farad. The farad is the measure of a capacitors ability to store a charge. If one volt is applied to the plates of a capacitor and this causes a charge of 1 Coulomb to be stored on the plates, the capacitance is 1 Farad.

In practice 1 farad is an enormous capacitance. More practical sub-units of the farad are used. Microfarad and picofarad are the most common sub-units.

PERMITTIVITY OR DIELECTRIC CONSTANT
The insulating material between the plates (dielectric) determines the concentration of electric line of force. Just like different materials will concentrate magnetic lines of force to a greater of lesser extent, materials also vary in their ability to concentrate electric lines of force.

If the dielectric was air, then a certain number of lines of force will be set up. Some papers have a dielectric constant twice that of air, which would cause twice as many lines of force to be set up and the capacitance would be double. The higher the dielectric constant the greater the capacitance for a given plate area.

Suppose an air dielectric capacitor (dielectric constant close enough to 1) of 8 microfarads had its air dielectric replaced with mica, without changing the distance between the plates.
The capacitance would increase in direct proportion to the dielectric constant. In other words, the capacitance would increase from 8 microfarads to 5-7 times that value, or 40 to 56 microfarads.

DIELECTRIC CONSTANTS

FACTORS DETERMINING CAPACITANCE

For examination purposes you do not have to use this equation. However you most definitely do need to know what the equation says about the factors determining capacitance.

Capacitance is directly proportional to the dielectric constant (Ke).
Capacitance is directly proportional to the area of one of the plates (A).
Capacitance is inversely proportional to the distance between the plates (d).

CAPACITORS IN SERIES AND PARALLEL

In figure 9 above all the capacitors are the same. Let’s say they are 10 microfarads. Think about what happens when you connect two identical capacitors in series. Think about it in terms of the factors that we just discussed, that affect capacitance. Can you visualise by looking at the series capacitors (on the left of figure 9) that we have actually doubled the thickness of the dielectric? Doubling the thickness of the dielectric is exactly the same thing as doubling the distance between the plates.
If the distance between the plates is doubled, and capacitance is inversely proportional to the distance between the plates, then the capacitance must be half of a single capacitor on its own The total capacitance of two 10 microfarad capacitors in series must then be 5uF. The equation for any number of capacitors of any value in series is:

The easiest way to use this equation on a calculator is:
1. Find the reciprocal of each capacitance.
2. Add all of the reciprocals together (sum them).
3. Find the reciprocal of this sum.
In other words, the reciprocal of the sum of the reciprocals.
Worked example
Take three capacitors of 7, 8 and 12 microfarads in series.
Find the reciprocal of 7: 1/7 = 0.143 (rounded to 3 decimal places)
The reciprocal of 8 : 1/8 = 0.125
The reciprocal of 12: 1/12 = 0.083

The sum of the above is: 0.351 Find the reciprocal of this sum: 1/0.351 = 2.85 microfarads.
Three capacitors of 7, 8 and 12 microfarads in series has a total capacitance of 2.85 μF.
Notice how the total capacitance is always less then the lowest value capacitor.
Note: If just two capacitors are in series then you can use the simplified product over sum
formula.
For two capacitors (only) in series:

When the same two capacitors are connected in parallel, the distance between the plates and all other factors remain the same, except we have doubled the effective area of the plates. So the capacitance has doubled.
The equation for any number of capacitors of any value in parallel is:

These equations are opposite to that of resistances in parallel and series, so be careful not to confuse the two. The shortcuts we took with resistors in parallel work the same for capacitors in series. For example, if we have just two capacitors of any value in series we can use the product over sum method to find the total capacitance.

VOLTAGE ACROSS SERIES CAPACITORS
If two equal capacitors were connected in series across a 100 volt DC supply and we were to measure the voltage across each capacitor we would get 50 volts across each. Since the capacitors are equal we get an equal voltage drop.

For unequal capacitances in series the voltage across each C is inversely proportional to its capacitance. In other words the smallest capacitance would have the largest voltage drop. The largest capacitance would have the smallest voltage drop.
The amount of charge on a capacitor is given by:

Q = CE

If a 10 uF capacitor was charged to 10 volts, then the charge in coulombs on the capacitors would be:

The equation can be transposed for voltage across a capacitor and we get:

E = Q/C

In a series circuit, each capacitor regardless of its capacitance will have the same charge. Q is the same for all capacitances in series. For a smaller capacitance to have the same charge in a series circuit it must have a higher E.

We could do mathematical examples, however, you do not need this in practice or for examination purposes. You do need to understand and be able to visualise the voltage drops across capacitances in a series circuit.

Example. Two capacitances of 1uF and 2uF are connected in series across a 900 volt DC supply. What is the voltage drop across each capacitor?

Now, the voltage drops have to be unequal because each capacitor will have the same charge (Q).

E = Q/C

E is inversely proportional to C. Therefore the smallest C (1uF) must have the greatest voltage drop. But how much greater? Since the 1uF capacitor is half the value of the 2uF capacitor, it must have twice the voltage to achieve the same charge. The 1uF capacitor must then have 600 volts across it, leaving 300 volts on the 2 uF capacitor.

VOLTAGE RATING OF CAPACITORS
All capacitors are given a maximum voltage rating. This is necessary as the dielectric of capacitors can breakdown and conduct, causing the capacitor to fail and in most cases be destroyed. Some capacitors, if placed across a voltage which is too high, will create gas within them and explode fairly violently.