Metal Foil Resistors
Metal foil resistors are made from relatively thick (a few microns or more) metal foil attached to an insulating substrate. At the time of writing, they are the technology of choice when accuracy and stability are required, with tolerances down to ±0.005% and temperature coefficients below 1 PPM/°C. Other resistors have power consumption ratings of a few or tens of watts while maintaining excellent stability over time and temperature; such resistors are well suited for trade-related metrology and other applications where maintaining accuracy under a range of conditions is very important.
Metal Oxide Resistors
A metal oxide resistor is a thin film type resistor in which the resistive component is formed from an oxide of some metal rather than the metal itself. Essentially, the metal oxide is the residual product of the burning of the parent metal in oxygen again, and as can be expected, these materials are quite resistant to high temperatures; since it is difficult to ignite something that has completely burned ...... the main advantage of metal oxide resistors comes from this property; they have a higher maximum operating temperature and better pulse load handling characteristics than the usual carbon and metal film resistors and better pulse load handling characteristics. Initial tolerances are comparable to those found in carbon film products, and oxide products offer lower temperature coefficients; metal film products offer improvements in both areas compared to oxide products. Oxide film resistors are expected to offer better noise characteristics than carbon based resistors, although not as good as those offered by metal film types. Most commonly found in through-hole packages rated at 1 - 5 watts, oxide film technology is typically used to produce very high resistance values in the 10 MΩ and higher range, although a full range of products from less than 1 ohm level to higher resistance values is available.
Thick Film Resistors
Thick film resistors in which the resistive components are deposited on a substrate by a method similar to screen printing. This process is less expensive than thin film deposition, and the components are typically tens to hundreds of microns thick. While the actual component thicknesses may overlap between "thin" and "thick" film products, the deposition method is the differentiating factor.
The paste material used for the composition in thick film resistors contains a mixture of a finely divided resistive material and a material with more electrically insulating properties, which is baked and cured into a glass-like substance with a conductive reticular microstructure similar to that of a carbide resistor. The material differences and the use of a hot fusion process instead of an organic binder yield resistive compositions that are significantly less resistive than carbide resistors and more stable in their parameters, but still relatively noisy and temperature sensitive compared to many other resistive compositions. However, the total mass of the resistive composition tends to be much smaller than that of a carbide resistor, so thick film resistors do have the potential to be sensitive to ESD and damage from similar transients, even though they can provide much higher maximum operating temperatures than carbide resistors. The trimming method used to achieve the final value and the specific formulation of the thick film composition used have a significant impact on the robustness of the terminating resistor, which is usually a little less robust than that of a thin film resistor. Thick film resistors, primarily in surface mount form, currently dominate as a general purpose resistor and are well suited for applications requiring lower cost (relative to thin film resistors) with less noise increase and stability requirements. Such applications might include, for example, ballast resistors for small indicator LEDs or pull-up resistors for logic signals.
Wirewound Resistors
Wirewound resistors are constructed in a wire-wound fashion and are primarily available in through-hole and chassis mount form. They are characterized by low current noise, good short-term overload tolerance, and often have very significant parasitic inductance, which in some products is mitigated by the use of a close-fitting wirewound configuration. Tolerances and temperature coefficients vary from product to product; design accuracies of less than 0.1% and temperature coefficients of a few tens of PPM/°C can be achieved, although at the time of this writing, multiple available products are specified with tolerances closer to 1% and 200 PPM/°C or less.
Summary
Resistors are a high number of components used in electronic circuits, and we choose the right resistors to optimize the regulation and stabilization of current and voltage in the circuit according to the circuit requirements, allowing the design to achieve more accurate and reliable results in the application.
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