| MadSci Network: Engineering |
Hello Nora: The cooling fluid will affect your resistor design parameters. Pure water (distilled) is a very good medium for transferring heat but it is easily contaminated by impurities that makes it conduct electricity, therefore, it isn’t a good candidate for electrical circuit cooling. Special oil formulations are used for insulation and heat dissipation in power distribution transformers (for example, those mounted on power poles stepping down 12 to 15 thousand volts to 115/230 volts for household usage). The density of this type oil varies little with heat and therefore can be safely sealed inside transformer housings. As far as I know oil cooling has limited applications because it requires additional space and adds weight. The most convenient coolant is air. Heat is dissipated either by convective or forced circulation. Adding black fins to the resistor body increases the surface area for greater heat dissipation. There are a number of such “heat sinks” available in clamp-on styles to fit the majority of design packages. The color black is used to maximize thermal conductivity. The typical power resistor consists of nickel-chromium wires wound around a hollow ceramic core. A ceramic layer that acts as insulator coats each layer of windings until the desired value of resistance is reached. Normally the final ceramic layer is glazed for durability in handling. This fabrication technique introduces “reactance” in alternating current applications because it becomes an open core inductor. Brittanica on line explains the reasons: www.britannica.com/bcom/eb/article/4/0,5716,43304+1+42351,00.html? query=inductance In recent years semiconductors have displaced power resistors. These active devices dissipate only a fraction of the heat generated by power resistors and are electronically controllable over a wide range of resistance. Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices are a good example of this technology. http://www.ednmag.com/reg/1994/060994/12DI3.htm Most electronic circuits include fractional wattage, carbon composition or carbon film resistors. These resistors are more sensitive to heat than the power counterparts, usually becoming more conductive as the heat increases. Many circuit designs include miniature resistors packaged together on a carrier, the advantage being that the overall rise of temperature is uniform reducing the miss-matches. The following web site show a number of different resistor package designs. Unlike power resistors that have the ratings of Ohms and watts printed on their cases, smaller resistors use color-coded banding to identify the value and the tolerance in percent of the nominal value. http://www.techweb.com/encyclopedia/defineterm?term=resistor&Define=Define http://www.testeq.com/charts/resclr.lasso Your MAD.SCI Micro
Try the links in the MadSci Library for more information on Engineering.