November 2008 Electrical Apparatus

November 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus November 2008 featured technical article,  by Richard L. Nailen, P.E.     

Whether a-c or d-c, in the microchip or power grid, all electrical circuits contain resistance. Resistance has the peculiar characteristic of exhibiting either zero efficiency (wasting as heat all the energy generated within it) or 100% efficiency, as in the heating element with the sole purpose of warming the environment.

Resistance is derived from resistivity, that fundamental property of all materials that expresses opposition to the flow of electric current. Resistivity is defined as a number of ohms’ resistance of a material having a specific size and length (such as a wire 1 micrometer in diameter and 1 meter long). For common metals, the property is temperature-dependent, increasing as the material is heated. Insulating materials are simply those with extremely high (though not infinite) resistivity.

Joints between one conductor and another add additional resistance to a circuit. If permanent, they may be brazed, soldered, or crimped; otherwise, conductors may be fastened together by bolt, screw, or spring pressure. A common operating problem is unexpectedly high resistance within such an interface. Resulting from corrosion, thermal cycling, or chemical contamination, such resistance may cause local overheating because of restricted contact between conductor surfaces. This overheating in turn accelerates corrosion, and the condition worsens. Heat can lead to rapid deterioration of any insulation present.

Concern is often expressed about high resistance in soldered or brazed joints. Some technologists assert that the alloy used as filler metal in such joints must therefore have low resistivity. However, it can easily be shown that such alloys need to be chosen only for their strength, usability, and resistance to chemical attack, because the influence of their resistance alone is negligible. Furthermore, because of unavoidable imperfections in metals-joining methods, the areas of such joints are normally made several times as large as the areas of the conductors being joined. That more than compensates for any added resistance within the joint.

Brazing, sometimes called hard soldering, produces the strongest joint. The alloy used is typically a form of bronze, melting at 450 to 870 degrees C. The tin-based soft solders, melting at 180-230 degrees C, form a much weaker joint, that is quite satisfactory for a circuit board or small wiring joint.

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