The Circuit Designer's Companion - diagramas.diagram...

70 The Circuit Designer’s CompanionChapter 3Passive components3.1 ResistorsResistors are ubiquitous. Because of this their performance is taken for granted;provided they are operated within their power, voltage and environmental ratings thisis reasonable, since after millions of accumulated resistor-years experience there islittle left for their manufacturers to discover. But there are still applications wherespecifying and applying resistors needs to be handled with some care.Let us start with an appreciation of the different varieties of resistor that areavailable. Table 3.1 (opposite) is a guide to the common types that will be encounteredin general circuit design. There are more esoteric types which are not covered.3.1.1 Resistor typesSurface mount chipThe most common general purpose resistor is the thick film surface mount chip type.Available in huge quantities and very low prices, it is the workhorse of the resistorworld. The construction is very simple (Figure 3.1) and hardly varies frommanufacturer to manufacturer. An alumina (aluminium oxide ceramic) substrate withnickel plated terminations has a resistive ink film printed or otherwise deposited on itstop surface. The terminations are coated with a solder dip to ensure ease of wettingwhen the part is soldered into place, and the top of the part is coated with an epoxy orglass layer to protect the resistive element.Size Dimensions mm L x W x H0201 0.6 x 0.3 x 0.250402 1.0 x 0.5 x 0.250603 1.6 x 0.8 x 0.450805 2.0 x 1.25 x 0.51206 3.2 x 1.6 x 0.61210 3.2 x 2.6 x 0.62010 5.1 x 2.5 x 0.62512 6.5 x 3.2 x 0.6over-coatingresistive elementalumina substrateterminals – solder over nickelFigure 3.1 Chip resistor construction and sizesDifferent manufacturers make various claims for the ruggedness and performanceof their parts but the basic features are similar. Power dissipation is largely controlledby the thermal properties of the PCB pads to which the chip is soldered, and if you are