Fixed resistors are discrete units typically made in cylindrical or planar form. The most common cylindrical style is the axial-leaded resistor, as shown in Fig. 1. The resistive element is wound or deposited on a cylindrical core, and a cap with a lead wire is positioned on each end. The resistive elements include resistive wire (wirewound), metal film, carbon film, cermet, and metal oxide. Resistor networks and chip resistors are examples of planar resistors. All fixed resistors are rated for a nominal resistance value in ohms over the range of fractions of an ohm to thousands of ohms (kilohms), or millions of ohms (megohms). Other electrical ratings include:
Figure 1
Some resistors also have additional ratings for electrical noise, parasitic inductance, and parasitic capacitance. Resistors exhibit unwanted parasitics of inductance and capacitance because of their construction. These effects must be considered by the designer when selecting resistors for unusual or specialized applications such as their use in instrumentation. A resistor’s ability to dissipate power is directly related to its size. With the exception of those specified for power supplies, most resistors for electronic circuits are rated under 5 W, usually less than 1 W. A 5-W cylindrical resistor is about 1 in (25.4 mm) long with a diameter of 1⁄4 in (6.4 mm). The 1⁄2-, 1⁄4-, and 1⁄8-W resistors are correspondingly smaller.
- Resistive tolerance as a percentage of nominal value in ohms
- Power dissipation in watts (W)
- Temperature coefficient (tempco) in parts per million per degree Celsius of temperature
- change (ppm/°C)
- Maximum working voltage in volts (V)
CARBON-COMPOSITION RESISTORS
A carbon-composition resistor, as shown in Fig. 2, is made by mixing powdered carbon with a phenolic binder to form a viscous bulk resistive material, which is placed in a mold with embedded lead ends and fired in a furnace. Because their resistive elements are a bulk material, they can both withstand wider temperature excursions and absorb higher electrical transients than either carbon- or metal-film resistors. These qualities are offset by their typically wider resistive tolerances of +-10 to 20 percent and tendency to absorb moisture in humid environments, causing their values to change. However, the benefits of carboncomposition resistors are less important in low-voltage transistorized circuits, so demand for them has declined. These resistors have ratings of 1 ohm to 100 megohms, but values in the 10- to 100-ohm range were most popular. Power ratings are 1⁄8 to 2 W.
Figure 2
CARBON-FILM RESISTORS
A carbon-film resistor, as shown in Fig.3, is made by screening carbon-based resistive ink on long ceramic rods or mandrels and then firing them in a furnace. The rod is then sliced to form individual resistors. After leaded end caps are attached, the resistance values are set precisely in a laser trimming machine that trims away excess resistive film under closed-loop control. The trimmed resistors are then coated with an insulating plastic jacket. Resistive tolerances of carbon-film resistors are typically +-10 percent. Standard resistors have power ratings of 1⁄2, 1⁄4, and 1⁄8 W.
Figure 3
WIREWOUND RESISTORS
A wirewound resistor, as shown in Fig.4, is made by winding fine resistive wire on a plastic or ceramic mandrel. The most commonly used resistance wire is nickel-chromium (nichrome). The axial leads and end caps are attached to the ends of the wire winding and welded to complete the electrical circuit. There are both general-purpose and power wirewound resistors. General-purpose units have resistive values of 10 ohms to 1 megohm, resistance tolerances of +-2 percent, and temperature coefficients of +-100 ppm/°C. Power
units rated for more than 5 W have tolerances that can exceed +-10 percent.
Figure 4
Wirewound resistors are generally limited to low-frequency applications because each is a solenoid that exhibits inductive reactance in an AC circuit, which adds to its DC resistive value. The inductive reactance can be reduced or eliminated at low or medium frequencies by bifilar winding. This is done by folding the entire length of resistive wire back on itself, hairpin fashion, before winding it on the mandrel. As a result, opposing inductive fields cancel each other, lowering or eliminating inductive reactance.
Wirewound resistors are made with both axial and radial leads. Epoxy or silicone insulation is applied to some low-power wirewound resistors, but high-power units are encased in ceramic or placed in heat-dissipating aluminum cases. This reduces the danger of the hot resistor igniting nearby flammable materials or burning fingertips if accidentally touched.
Figure 5
METAL-FILM RESISTORS
A metal-film resistor, as shown in Fig. 5, is made by the same general method as a carbon-film resistor. A thin metal film is sputtered or vacuum deposited on an alumina (aluminum-oxide) mandrel in a vacuum chamber, or a thick metal film is applied in air. Tin oxide or nickel-chromium are widely used thin films, and a thick film made from powdered precious metal and glass (frit) in a volatile binder is a common cermet resistive ink. These resistors are laser trimmed to precise values under closed-loop control after firing. Metal-film resistors are offered in two grades: (1) those with resistive tolerances of +-1 percent and temperature coefficients of 25 to 100 ppm/°C, and (2) those with resistive tolerances of +-5 percent and temperature coefficients of 200 ppm/°C. Demand is highest for 1⁄4- and 1⁄8-W units, but 1⁄20-W units are available. Resistive values up to 100 megohms are available as catalog items, but they are generally rated for less than 10 kilohms.
Figure 6
RESISTOR NETWORKS
A resistor network, as shown in Fig. 6, consists of two or more resistive elements on the same insulating substrate. These networks are specified where 6 to 15 low-value resistors are required in a restricted space. Most commercial networks contain thick-film resistors, and they are packaged in dual-in-line packages (DIPs) or single-in-line packages (SIPs). Standard DIPs have 14 or 16 pins, and standard SIPs have 6, 8, or 10 pins. Resistor networks are used for “pull-up” and “pull-down” transitions between logic circuits operating at different voltage values, for sense amplifier termination, and for light-emitting diode (LED) display current limiting.
Alumina ceramic is the most widely used network substrate. Conductive traces are formed by screening an ink made from a powdered silver-palladium mix in a volatile binder on the bare ceramic substrate. After firing, the ink bonds with the ceramic to form hard, lowresistance paths. Resistive inks made from a powdered ruthenium-cermet mix with a powdered glass frit and a volatile binder are then screened over the ends of the conductors to form the resistive elements. This ink is also fired, and when it bonds with the ceramic it forms a hard, resistive element. Network resistors are laser trimmed under closed-loop control to precise resistance values. Standard network resistance values are from 10 ohms to 10 megohms with tolerances of +-2 percent. Most networks can safely dissipate less than 1⁄2 W.
Where more precise resistance values are required, thin-film networks are specified. They are made formed from compositions that include nickel-chromium, chrome-cobalt, and tantalum nitride, deposited or sputtered on alumina ceramic substrates. Unpackaged thin-film resistor networks are also sold as hybrid-circuit substrates. Thin-film resistivecapacitive (RC) networks are also packaged in metal and ceramic flatpacks.
Figure 7
CERAMIC-CHIP RESISTORS
A ceramic-chip resistor, as shown in Fig. 7, is made by screening and firing cermet resistive inks or sputtering tantalum nitride or nickel-chromium on an alumina substrate. The deposited resistive surface is then coated with glass for protection. The substrate is then diced into individual chips, and a silver-based ink is applied to the end surfaces and fired as the first step in forming leadless terminals. A barrier layer of nickel plating is then applied to prevent the migration of silver from the inner electrode. Finally, the terminations are coated with lead-tin solder for improved adhesion during reflow soldering.
Chip resistors were originally made for hybrid circuits, but surface-mount technology (SMT) has increased demand for them. Surface-mount chip resistor dimensions have been standardized to 1.6 × 3.2 mm for handling by automatic pick-and-place machines. (This is the same size as the 1206 chip capacitor that measures 0.063 × 0.125 in.) Chip resistors are typically rated for 1⁄8 W or less. An alternative form of SMT resistor is the leadless cylinder with solder-coated bands around each end for reflow solder bonding.
