Heat
In thermodynamics, heat is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The mechanisms include conduction, through direct contact of immobile bodies, or through a wall or barrier, impermeable to matter; when there is a suitable path between two systems with different temperatures, heat transfer occurs immediately, spontaneously from the hotter to the colder system. Thermal conduction occurs by the stochastic motion of microscopic particles. In contrast, thermodynamic work is defined by mechanisms that act macroscopically and directly on the system's whole-body state variables; the definition of heat transfer does not require. For example, a bolt of lightning may transfer heat to a body. Convective circulation allows one body to heat another, through an intermediate circulating fluid that carries energy from a boundary of one to a boundary of the other. Though spontaneous, convective circulation does not and occur because of temperature difference.
Like thermodynamic work, heat transfer is a process involving two systems, not a property of any one system. In thermodynamics, energy transferred as heat contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy; this is to be distinguished from the ordinary language conception of heat as a property of the system. Although heat flows from a hotter body to a cooler one, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems. In contrast, a heat engine reduces an existing temperature difference to do work on another system; the amount of heat transferred in any process can be defined as the total amount of transferred energy excluding any macroscopic work, done and any energy contained in matter transferred. For the precise definition of heat, it is necessary that it occur by a path that does not include transfer of matter; as an amount of energy, the SI unit of heat is the joule.
The conventional symbol used to represent the amount of heat transferred in a thermodynamic process is Q. Heat is measured by its effect on the states of interacting bodies, for example, by the amount of ice melted or a change in temperature; the quantification of heat via the temperature change of a body is called calorimetry. As a form of energy, heat has the unit joule in the International System of Units. However, in many applied fields in engineering the British thermal unit and the calorie are used; the standard unit for the rate of heat transferred is the watt, defined as one joule per second. Use of the symbol Q for the total amount of energy transferred as heat is due to Rudolf Clausius in 1850: "Let the amount of heat which must be imparted during the transition of the gas in a definite manner from any given state to another, in which its volume is v and its temperature t, be called Q"Heat released by a system into its surroundings is by convention a negative quantity. Heat transfer rate, or heat flow per unit time, is denoted by Q ˙.
This should not be confused with a time derivative of a function of state since heat is not a function of state. Heat flux is defined as rate of heat transfer per unit cross-sectional area. In 1856, Rudolf Clausius, referring to closed systems, in which transfers of matter do not occur, defined the second fundamental theorem in the mechanical theory of heat: "if two transformations which, without necessitating any other permanent change, can mutually replace one another, be called equivalent the generations of the quantity of heat Q from work at the temperature T, has the equivalence-value:" Q T. In 1865, he came to define the entropy symbolized by S, such that, due to the supply of the amount of heat Q at temperature T the entropy of the system is increased by Δ S = Q T In a transfer of energy as heat without work being done, there are changes of entropy in both the surroundings which lose heat and the system which gains it; the increase, ΔS, of entropy in the system may be considered to consist of two parts, an increment, ΔS′ that matches, or'compensates', the change, −ΔS′, of entropy in the surroundings, a further increment, ΔS′′ that may be considered to be'generated' or'produced' in the system, is said therefore to be'uncompensated'.
Thus Δ S = Δ S ′ + Δ S ′ ′. {\displaystyl
Mimeograph
The stencil duplicator or mimeograph machine is a low-cost duplicating machine that works by forcing ink through a stencil onto paper. The mimeograph process should not be confused with the spirit duplicator process. Mimeographs, along with spirit duplicators and hectographs, were a common technology in printing small quantities, as in office work, classroom materials, church bulletins. Early fanzines were printed with this technology, because it was cheap. In the late 1960s, spirit duplicators, hectographs began to be displaced by photocopying. Use of stencils is an ancient art, but—through chemistry and presses—techniques advanced in the late nineteenth century: A description of the Papyrograph method of duplication was published by David Owen: A major beneficiary of the invention of synthetic dyes was a document reproduction technique known as stencil duplicating, its earliest form was invented in 1874 by Eugenio de Zuccato, a young Italian studying law in London, who called his device the Papyrograph.
Zuccato’s system involved writing on a sheet of varnished paper with caustic ink, which ate through the varnish and paper fibers, leaving holes where the writing had been. This sheet – which had now become a stencil – was placed on a blank sheet of paper, ink rolled over it so that the ink oozed through the holes, creating a duplicate on the second sheet; the process was commercialized and Zuccato applied for a patent in 1895 having stencils prepared by typewriting. Thomas Edison received US patent 180,857 for Autographic Printing on August 8, 1876; the patent covered the electric pen, used for making the stencil, the flatbed duplicating press. In 1880 Edison obtained a further patent, US 224,665: "Method of Preparing Autographic Stencils for Printing," which covered the making of stencils using a file plate, a grooved metal plate on which the stencil was placed which perforated the stencil when written on with a blunt metal stylus; the word mimeograph was first used by Albert Blake Dick when he licensed Edison's patents in 1887.
Dick received Trademark Registration no. 0356815 for the term "Mimeograph" in the US Patent Office. It is listed as a dead entry, but shows the A. B. Dick Company of Chicago as the owner of the name. Over time, the term is now an example of a genericized trademark. In 1891, David Gestetner patented his Automatic Cyclostyle; this was one of the first rotary machines that retained the flatbed, which passed back and forth under inked rollers. This invention provided for more automated, faster reproductions since the pages were produced and moved by rollers instead of pressing one single sheet at a time. By 1900, two primary types of mimeographs had come into use: a single-drum machine and a dual-drum machine; the single-drum machine used a single drum for ink transfer to the stencil, the dual-drum machine used two drums and silk-screens to transfer the ink to the stencils. The single drum machine could be used for multi-color work by changing the drum - each of which contained ink of a different color.
This was spot color for mastheads. Colors could not be mixed; the mimeograph became popular because it was much cheaper than traditional print - there was neither typesetting nor skilled labor involved. One individual with a typewriter and the necessary equipment became their own printing factory, allowing for greater circulation of printed material; the image transfer medium was a stencil made from waxed mulberry paper. This became an immersion-coated long-fibre paper, with the coating being a plasticized nitrocellulose; this flexible waxed or coated sheet is backed by a sheet of stiff card stock, with the two sheets bound at the top. Once prepared, the stencil is wrapped around the ink-filled drum of the rotary machine; when a blank sheet of paper is drawn between the rotating drum and a pressure roller, ink is forced through the holes on the stencil onto the paper. Early flatbed machines used a kind of squeegee; the ink had a lanolin base. And became an oil in water emulsion; this emulsion used Turkey-Red Oil which gives it a distinctive and heavy scent.
For printed copy, a stencil assemblage is placed in a typewriter. The part of the mechanism which lifts the ribbon must be disabled so that the bare, sharp type element strikes the stencil directly; the impact of the type element displaces the coating, making the tissue paper permeable to the oil-based ink. This is called "cutting a stencil."A variety of specialized styluses were used on the stencil to render lettering, illustrations, or other artistic features by hand against a textured plastic backing plate. Mistakes can be corrected by brushing them out with a specially formulated correction fluid, retyping once it has dried. Stencils were made with a thermal process; the common machine was a Thermofax. Another device, called an electrostencil machine, sometimes was used to make mimeo stencils from a typed or printed original, it worked by scanning the original on a rotating drum with a moving optical head and burning through the blank stencil with an electric spark in the places where the optical head detected ink.
It produced ozone. Text from electrostencils had lower resolution than that from typed stencils, although the process was good for reproducing illustrations. A skilled mimeo operator using an electrostencil and a coarse halftone screen could make acceptable printed
Etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio in the metal. In modern manufacturing, other chemicals may be used on other types of material; as a method of printmaking, it is, along with engraving, the most important technique for old master prints, remains in wide use today. In a number of modern variants such as microfabrication etching and photochemical milling it is a crucial technique in much modern technology, including circuit boards. In traditional pure etching, a metal plate is covered with a waxy ground, resistant to acid; the artist scratches off the ground with a pointed etching needle where he or she wants a line to appear in the finished piece, so exposing the bare metal. The échoppe, a tool with a slanted oval section, is used for "swelling" lines; the plate is dipped in a bath of acid, technically called the mordant or etchant, or has acid washed over it. The acid "bites" into the metal where it is exposed, leaving behind lines sunk into the plate.
The remaining ground is cleaned off the plate. The plate is inked all over, the ink wiped off the surface, leaving only the ink in the etched lines; the plate is put through a high-pressure printing press together with a sheet of paper. The paper picks up the ink from the etched lines; the process can be repeated many times. The work on the plate can be added to by repeating the whole process. Etching has been combined with other intaglio techniques such as engraving or aquatint. Etching by goldsmiths and other metal-workers in order to decorate metal items such as guns, armour and plates has been known in Europe since the Middle Ages at least, may go back to antiquity; the elaborate decoration of armour, in Germany at least, was an art imported from Italy around the end of the 15th century—little earlier than the birth of etching as a printmaking technique. Printmakers from the German-speaking lands and Central Europe perfected the art and transmitted their skills over the Alps and across Europe.
The process as applied to printmaking is believed to have been invented by Daniel Hopfer of Augsburg, Germany. Hopfer was a craftsman who decorated armour in this way, applied the method to printmaking, using iron plates. Apart from his prints, there are two proven examples of his work on armour: a shield from 1536 now in the Real Armeria of Madrid and a sword in the Germanisches Nationalmuseum of Nuremberg. An Augsburg horse armour in the German Historical Museum, dating to between 1512 and 1515, is decorated with motifs from Hopfer's etchings and woodcuts, but this is no evidence that Hopfer himself worked on it, as his decorative prints were produced as patterns for other craftsmen in various media; the oldest dated etching is by Albrecht Dürer in 1515, although he returned to engraving after six etchings instead of developing the craft. The switch to copper plates was made in Italy, thereafter etching soon came to challenge engraving as the most popular medium for artists in printmaking.
Its great advantage was that, unlike engraving where the difficult technique for using the burin requires special skill in metalworking, the basic technique for creating the image on the plate in etching is easy to learn for an artist trained in drawing. On the other hand, the handling of the ground and acid need skill and experience, are not without health and safety risks, as well as the risk of a ruined plate. Prior to 1100 AD, the New World Hohokam independently utilized the technique of acid etching in marine shell designs. Jacques Callot from Nancy in Lorraine made important technical advances in etching technique, he developed the échoppe, a type of etching-needle with a slanting oval section at the end, which enabled etchers to create a swelling line, as engravers were able to do. Callot appears to have been responsible for an improved, recipe for the etching ground, using lute-makers' varnish rather than a wax-based formula; this enabled lines to be more bitten, prolonging the life of the plate in printing, greatly reducing the risk of "foul-biting", where acid gets through the ground to the plate where it is not intended to, producing spots or blotches on the image.
The risk of foul-biting had always been at the back of an etcher's mind, preventing too much time on a single plate that risked being ruined in the biting process. Now etchers could do the detailed work, the monopoly of engravers, Callot made full use of the new possibilities. Callot made more extensive and sophisticated use of multiple "stoppings-out" than previous etchers had done; this is the technique of letting the acid bite over the whole plate stopping-out those parts of the work which the artist wishes to keep light in tone by covering them with ground before bathing the plate in acid again. He achieved unprecedented subtlety in effects of distance and light and shade by careful control of this process. Most of his prints were small—up to about six inches or 15 cm on their longest dimension, but packed with detail. One of his followers, the Parisian Abraham Bosse, spread Callot's innovations all over Europe with the first published manual of etching, translated into Italian, Dutch and English.
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Chromolithography
Chromolithography is a unique method for making multi-colour prints. This type of colour printing stemmed from the process of lithography, includes all types of lithography that are printed in colour; when chromolithography is used to reproduce photographs, the term photochrome is used. Lithographers sought to find a way to print on flat surfaces with the use of chemicals instead of raised relief or recessed intaglio techniques. Chromolithography became the most successful of several methods of colour printing developed by the 19th century. Hand-colouring remained important; the initial technique involved the use of multiple lithographic stones, one for each colour, was still expensive when done for the best quality results. Depending on the number of colours present, a chromolithograph could take very skilled workers months to produce; however much cheaper prints could be produced by simplifying both the number of colours used, the refinement of the detail in the image. Cheaper images, like advertisements, relied on an initial black print, on which colours were overprinted.
To make an expensive reproduction print as what was once referred to as a "chromo", a lithographer, with a finished painting in front of him created and corrected the many stones using proofs to look as much as possible like the painting, sometimes using dozens of layers. Chromolithography is a chemical process; the process is based on the rejection of grease by water. The image is applied to stone, grained zinc or aluminium surfaces, with a grease-based crayon or ink. Limestone and zinc are two used materials in the production of chromolithographs, as aluminium corrodes easily. After the image is drawn onto one of these surfaces, the image is gummed-up with a gum arabic solution and weak nitric acid to desensitize the surface. Before printing, the image is proved before inking up the image with oil based transfer or printing ink; the inked image under pressure is transposed onto a sheet of paper using a flat-bed press. This describes the direct form of printing; the offset indirect method uses a rubber-covered cylinder that transfers the image from printing surface to the paper.
Colours may be overprinted by using additional stones or plates to achieve a closer reproduction of the original. Accurate registration for multi-coloured work is achieved by the use of a key outline image and registration bars which are applied to each stone or plate before drawing the solid or tone image. Ben-Day medium uses a raised gelatin stipple image to give tone gradation. An air-brush sprays ink to give soft edges; these are just two methods used to achieve gradations of tone. The use of twelve overprinted colours would not be considered unusual; each sheet of paper will therefore pass through the printing press as many times as there are colours in the final print. In order that each colour is placed in the right position, each stone or plate must be precisely'registered,' or lined up, on the paper using a system of register marks. Chromolithographs are considered to be reproductions that are smaller than double demi, are of finer quality than lithographic drawings which are concerned with large posters.
Autolithographs are prints where the artist draws and prints his or her own limited number of reproductions. This is the true lithographic art form. Alois Senefelder, the inventor of lithography, introduced the subject of colored lithography in his 1818 Vollstaendiges Lehrbuch der Steindruckerey, where he told of his plans to print using colour and explained the colours he wished to be able to print someday. Although Senefelder recorded plans for chromolithography, printers in other countries, such as France and England, were trying to find a new way to print in colour. Godefroy Engelmann of Mulhouse in France was awarded a patent on chromolithography in July 1837, but there are disputes over whether chromolithography was in use before this date, as some sources say, pointing to areas of printing such as the production of playing cards; the first American chromolithograph—a portrait of Reverend F. W. P. Greenwood—was created by William Sharp in 1840. Many of the chromolithographs were purchased in urban areas.
The paintings were used as decoration in American parlours as well as for decoration within middle-class homes. They were prominent after the Civil War because of their low production costs and ability to be mass-produced, because the methods allowed pictures to look more like hand-painted oil paintings. Production costs were only low if the chromolithographs were cheaply produced, but top-quality chromos were costly to produce because of the necessary months of work and the thousands of dollars worth of equipment that had to be used. Although chromos could be mass-produced, it took about three months to draw colours onto the stones and another five months to print a thousand copies. Chromolithographs became so popular in American culture that the era has been labeled as "chromo civilization". Over time, during the Victorian era, chromolithographs populated children's and fine arts publications, as well as advertising art, in trade cards and posters, they were once used for advertisements, popular prints, medical or scientific books.
Though chromolithographs served many uses within society at the time, many were opposed to the idea of them because of their perceived lack of authenticity. The new forms of art were sometimes tagged as "
Color
Color, or colour, is the characteristic of human visual perception described through color categories, with names such as red, yellow, blue, or purple. This perception of color derives from the stimulation of cone cells in the human eye by electromagnetic radiation in the visible spectrum. Color categories and physical specifications of color are associated with objects through the wavelength of the light, reflected from them; this reflection is governed by the object's physical properties such as light absorption, emission spectra, etc. By defining a color space, colors can be identified numerically by coordinates, which in 1931 were named in global agreement with internationally agreed color names like mentioned above by the International Commission on Illumination; the RGB color space for instance is a color space corresponding to human trichromacy and to the three cone cell types that respond to three bands of light: long wavelengths, peaking near 564–580 nm. There may be more than three color dimensions in other color spaces, such as in the CMYK color model, wherein one of the dimensions relates to a color's colorfulness).
The photo-receptivity of the "eyes" of other species varies from that of humans and so results in correspondingly different color perceptions that cannot be compared to one another. Honeybees and bumblebees for instance have trichromatic color vision sensitive to ultraviolet but is insensitive to red. Papilio butterflies may have pentachromatic vision; the most complex color vision system in the animal kingdom has been found in stomatopods with up to 12 spectral receptor types thought to work as multiple dichromatic units. The science of color is sometimes called chromatics, colorimetry, or color science, it includes the study of the perception of color by the human eye and brain, the origin of color in materials, color theory in art, the physics of electromagnetic radiation in the visible range. Electromagnetic radiation is characterized by its intensity; when the wavelength is within the visible spectrum, it is known as "visible light". Most light sources emit light at many different wavelengths.
Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary among different species, to a lesser extent among individuals within the same species. In each such class the members are called metamers of the color in question; the familiar colors of the rainbow in the spectrum—named using the Latin word for appearance or apparition by Isaac Newton in 1671—include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate wavelengths for various pure spectral colors; the wavelengths listed are as measured in vacuum. The color table should not be interpreted as a definitive list—the pure spectral colors form a continuous spectrum, how it is divided into distinct colors linguistically is a matter of culture and historical contingency.
A common list identifies six main bands: red, yellow, green and violet. Newton's conception included a seventh color, between blue and violet, it is possible that what Newton referred to as blue is nearer to what today is known as cyan, that indigo was the dark blue of the indigo dye, being imported at the time. The intensity of a spectral color, relative to the context in which it is viewed, may alter its perception considerably; the color of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the color of the light leaving their surfaces, which depends on the spectrum of the incident illumination and the reflectance properties of the surface, as well as on the angles of illumination and viewing; some objects not only reflect light, but transmit light or emit light themselves, which contributes to the color. A viewer's perception of the object's color depends not only on the spectrum of the light leaving its surface, but on a host of contextual cues, so that color differences between objects can be discerned independent of the lighting spectrum, viewing angle, etc.
This effect is known as color constancy. Some generalizations of the physics can be drawn, neglecting perceptual effects for now: Light arriving at an opaque surface is either reflected "specularly", scattered, or absorbed – or some combination of these. Opaque objects that do not reflect specularly have their color determined by which wavelengths of light they scatter strongly. If objects scatter all wavelengths with r
Atari 8-bit family
The Atari 8-bit family is a series of 8-bit home computers introduced by Atari, Inc. in 1979 and manufactured until 1992. All of the machines in the family are technically similar and differ in packaging, they are based on the MOS Technology 6502 CPU running at 1.79 MHz, were the first home computers designed with custom co-processor chips. This architecture enabled graphics and sound capabilities that were more advanced than contemporary machines at the time of release, gaming on the platform was a major draw. Star Raiders is considered the platform's killer app; the original Atari 400 and 800 models launched with a series of plug-n-play peripherals that used the Atari SIO serial bus system, an early analog of the modern USB. To meet stringent FCC requirements, the early machines were enclosed in a cast aluminum block, which made them physically robust but expensive to produce. Over the following decade, the 400 and 800 were replaced by the XL series the XE; the XL and XE are much lighter in construction and less expensive to build, while having Atari BASIC built-in and reducing the number of joystick ports from 4 to 2.
The 130XE, released in 1985, increased the memory to 128K of bank-switched RAM. The Atari 8-bit computer line sold two million units during its major production run between late 1979 and mid-1985, they were not only sold through dedicated computer retailers, but department stores such as Sears, using an in-store demo to attract customers. The primary competition in the worldwide market came several years when the Commodore 64 was introduced in 1982; this was the first computer to offer similar graphics performance, went on to be the best selling computer of the 8-bit era. Atari found a strong market in Eastern Europe and had something of a renaissance in the early 1990s as these countries joined a uniting Europe. In 1992, Atari Corp. dropped all remaining support of the 8-bit line. Some time in 1975, Steve Jobs called his former boss at Atari, Al Alcorn, Vice President of Engineering. Jobs was sourcing components for the soon-to-released Apple II, asked Alcorn if he knew of a good switched mode power supply.
Such devices were commercially available. Alcorn instead suggested. Holt worked in Atari's consumer division and had become a leading expert on power supplies, at that time was between projects. Instead, Jobs hired Holt away from Atari, offering him "a ton of stock". Jobs began hiring many Atari engineers and refused to stop this behaviour when asked. In response, Joe Keenan, one of Atari's co-founders, began a project to make an Apple II clone machine, which Atari could produce for much less than Apple, they began design work. It was not long after that news of this project reached Jobs, two weeks he agreed to stop poaching Atari staff; the project was cancelled, ending Atari's first attempt at a personal computer. Design of the 8-bit series of machines started at Atari as soon as the Atari 2600 games console was released in late 1977. While designing the 2600 in 1976, the engineering team from Atari Grass Valley Research Center felt that the 2600 would have about a three-year lifespan before becoming obsolete.
They started blue sky designs for a new console that would be ready to replace it around 1979. What they ended up with was a updated version of the 2600, fixing its more obvious limitations but sharing a similar overall design philosophy; the newer design would be faster than the 2600, have better graphics, would include much better sound hardware. Work on the chips for the new system continued throughout 1978 and focused on much-improved video hardware known as the CTIA. During the early development period, the home computer era began in earnest in the form of the TRS-80, Commodore PET, Apple II family—what Byte Magazine would dub the "1977 Trinity". Nolan Bushnell sold Atari to Warner Communications for $28 million in 1976 in order to raise funds for the launch of the 2600. Warner had sent Ray Kassar to act as the CEO of the company. Kassar felt. In order to adapt the machine to this role, it would need to support character graphics, include some form of expansion for peripherals, run the then-universal BASIC programming language.
The 2600 had no bitmap graphics support or a character generator, all on-screen graphics were created using Player-Missile graphics and a simple background using fixed patterns. The CTIA was designed on the same model, mainly used sprites for drawing. Instead of expanding the CTIA to handle these tasks, the designers introduced an new chip for this purpose, the Alphanumeric Television Interface Controller, or ANTIC; the CTIA and ANTIC worked together to produce a complete display, with the CTIA in charge of sprites and producing color video output, the ANTIC in charge of bitmap and character graphics. Management identified two sweet spots for the new computers: a low-end version known as "Candy", a higher-end machine known as "Colleen"; the primary difference between the two models was marketing. Colleen included user-accessible expansion slots for RAM and ROM, two 8 KB ROM cartridge slots, RF and monitor output and a full keyboard. Candy was designed as a games console, lacking a keyboard and input/output ports, although an external keyboard was planned tha
Thermal resistance
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. Thermal resistance R in K/W is a property of a particular component. For example, a characteristic of a heat sink. Specific thermal resistance or thermal resistivity Rλ in /W is a material constant. Thermal insulance has the units / W in / Btu in imperial units, it is the thermal resistance of unit area of a material. In terms of insulation, it is measured by the R-value. Absolute thermal resistance is the temperature difference across a structure when a unit of heat energy flows through it in unit time, it is the reciprocal of thermal conductance. The SI units of thermal resistance are the equivalent degrees Celsius per watt; the thermal resistance of materials is of great interest to electronic engineers because most electrical components generate heat and need to be cooled. Electronic components malfunction or fail if they overheat, some parts need measures taken in the design stage to prevent this.
Electrical engineers are familiar with Ohm's law and so use it as an analogy when doing calculations involving thermal resistance. Mechanical and Structural engineers are more familiar with Hooke's law and so use it as an analogy when doing calculations involving thermal resistance; the heat flow can be modelled by analogy to an electrical circuit where heat flow is represented by current, temperatures are represented by voltages, heat sources are represented by constant current sources, absolute thermal resistances are represented by resistors and thermal capacitances by capacitors. The diagram shows an equivalent thermal circuit for a semiconductor device with a heat sink. Consider a component such as a silicon transistor, bolted to the metal frame of a piece of equipment; the transistor's manufacturer will specify parameters in the datasheet called the absolute thermal resistance from junction to case, the maximum allowable temperature of the semiconductor junction. The specification for the design should include a maximum temperature at which the circuit should function correctly.
The designer should consider how the heat from the transistor will escape to the environment: this might be by convection into the air, with or without the aid of a heat sink, or by conduction through the printed circuit board. For simplicity, let us assume that the designer decides to bolt the transistor to a metal surface, guaranteed to be less than Δ T H S above the ambient temperature. Note: THS appears to be undefined. Given all this information, the designer can construct a model of the heat flow from the semiconductor junction, where the heat is generated, to the outside world. In our example, the heat has to flow from the junction to the case of the transistor from the case to the metalwork. We do not need to consider where the heat goes after that, because we are told that the metalwork will conduct heat fast enough to keep the temperature less than Δ T H S above ambient: this is all we need to know. Suppose the engineer wishes to know how much power he can put into the transistor before it overheats.
The calculations are as follows. Total absolute thermal resistance from junction to ambient = R θ J C + R θ B where R θ B is the absolute thermal resistance of the bond between the transistor's case and the metalwork; this figure depends on the nature of the bond - for example, a thermal bonding pad or thermal transfer grease might be used to reduce the absolute thermal resistance. Maximum temperature drop from junction to ambient = T J m a x −. We use the general principle that the temperature drop Δ T across a given absolute thermal resistance R θ with a given heat flow Q through it is: Δ T = Q × R θ. Substituting our own symbols into this formula gives: T J m a x − = Q m a x × {\displaystyle T_-=Q_\times (R_+R_{\theta {\
