Resistors share the spotlight with inductors and capacitors as the most basic units in electrical engineering. These passive components vary greatly in packaging, costs, size and composition material. Resistance tolerances based on heat, voltage and on frequency are widely divergent, too. In addition, different types of resistors are beset with different levels of stray inductance and capacitance, and it’s up to the engineer to know when money can be saved on a less expensive choice and when it can’t.
Carbon Composition Resistors
Carbon-composition resistors and carbon-film resistors are usually packaged in that once-ubiquitous design consisting of a cylindrical shape, axial leads and a banded color-band code to convey their specifications. Carbon-composition resistors use a mixture of powdered ceramic and carbon to determine resistance values. Today, though still very common, they are less often used because of their relatively poor tolerances and are now most often seen in higher-power applications.
Carbon Film Resistors
Carbon-film resistors are constructed on a ceramic base onto which a film of carbon is deposited during the manufacturing process. The resistance of the finished component is determined by the thickness of the deposited carbon layer, and also by cutting out spiral sections of the carbon layer as appropriate. These types of resistors often present a capacitance value of up to 1.0 pF. If their resistances have been modified by spiraling as it has been described, there may be also be an inherent inductance of several μH. One great advantage carbon-film resistors enjoy over the older carbon composition variety is that, because the carbon is pure, much less “noise” is produced, which can be a critical factor in many applications.
Thick-Film and Thin-Film Resistors
Thick-film resistors consist of a resistive film, generally on the order of 10’s of microns thick, that’s been deposited on a ceramic base. They are cheap to produce, and are today’s most commonly used component resistors. The composition of the film is an active area of development for manufacturers.
Thin-film resistors are also built on a ceramic base, but that’s where the similarity ends. The resistive element here is a metallic film of nichrome that’s vacuum deposited onto the base. The resulting film is about 1,000 times thinner than that of the thick-film resistor. Thin-film resistors are more expensive to produce than their thick-film counterparts, but they have low stray induction and capacitance, and they have excellent temperature coefficients. Thick-film resistors are more physically rugged, and can be fabricated to handle more current.
Thick-film and thin-film resistors are available in similar packaging. Choices include surface-mounted packages, SIP arrays, DIP arrays and radial-lead packages. High-amperage thick-film resistors are available in sturdy TO220 packages or even with built-in chassis-mountable heat sinks.
The Vishay VTF285BX, available from Arrow Electronics, is an example of a series of thin-film resistor arrays available in a SIP Package. Designers can utilize a datasheet, where they’ll notice they can order a 3-pin device that includes two resistors in one package designed to serve as a ratio divider. There are a variety of other options available, including an 8-pin device housing four independent resistors.
Metal-Oxide Resistors
Metal-oxide resistors are a good choice for applications in which there may be exposure to high temperatures. They can handle more power than other resistors of a similar size, and are also highly resistant to surges and overloads. However, like some of the carbon resistors, they are relatively noisy.
Metal-oxide resistors are built by the deposition of metal oxide film on a ceramic base. They often come in packages similar to carbon-composition and carbon-film resistors. Their final resistance is determined by the thickness of their coating, and by their trimming, which is accomplished by the affecting of a spiral cut through their length. As expected, for this reason, metal-oxide resistors exhibit relatively high inductance. These resistors can be specified with a very-tight-resistance tolerance.
Metal Film Resistors
Metal-film resistors are easily confused with thin-film resistors. One difference is that the final resistance value of a metal-film resistor is determined by spiral cut rather than by etching. By this method, tighter resistance tolerances can be achieved. Temperature Coefficients of Resistance (TCR) are excellent, and these resistors are very appropriate for use where noise could be a problem. However, they are easily damaged by surges.
Wire Wound Resistors
Wire-wound resistors are for high power, and often come packaged with heat sinks, which enhance their inherent ability to tolerate high levels of heat. The resistances of these devices vary little with temperature. Their big disadvantage, aside from their relatively high cost, is high inductance, due to the fact that they are constructed of wire coils. Coiling the resistive wiring back and forth over the same pathway can ameliorate some of the effect, in a manner exactly opposite to the way in which a transformer coil or an electromagnet is constructed.
The resistance value of wire wounds can be specified to an unusually high degree of precision. These devices can be specified to dissipate up to a few hundred watts, and are available with resistances of up to several hundred thousands of ohms.
OHMITE's D225K100K is an example of a wire-wound resistor. The datasheet reveals that this particular model can be used as a voltage divider or as an adjustable resistor.
Foil Resistors
Foil resistors are considered to have the best TCRs and precision of any resistor commonly available today. These low-noise components also have very low capacitance and no inductance at all. The resistance element of a foil resistor is a nickel and chromium foil, several micrometers thick, attached to a non-conductive base. They are then laser-trimmed to achieve exactly the resistance needed. These components have had the best stability of any resistor available. Unlike some of the carbon types, the resistance value specifically is remarkably stable over voltage.
The Most Common Resistors in the World
However, most of the resistors in use today don’t even exist as separate components, because they reside within ICs. In an IC, resistors aren’t added as separate components; instead, they are often fabricated, just as transistors and capacitors are. If the number of ohms desired is relatively low, it is usually formed in an N-region, because of the N-region’s lower resistivity. Conversely, higher-value resistors are fabricated in P-regions, because of that region’s higher resistivity. In both cases, the actual value of the resistor created is controlled by how deeply diffusion is affected, the width and length of the diffusion, and by the amount of impurity injected.
Of course, there is probably no place else on earth where space is at as great a premium as within an IC. As such, there are many different ways to build resistors within ICs, with each method optimized for specific situations.