Although it might seem that resistance has a disadvantage in reducing the current in a circuit, actually resistors are probably the most common components in electronic equipment. A resistor is manufactured with a specific value of ohms for R. The most common resistors are carbon-composition and wire-wound.
The purpose of using a resistor in a circuit is either to reduce I to a specific value or to provide a desired IR voltage drop. For an example, a series resistor in the output circuit of a transistor amplifier accomplishes both of these functions. First, the value of R affects the amount of I in the transistor. Furthermore, the IR voltage drop provides a sample of the I external to the transistor, so that the amplified voltage can be connected to the next circuit. Another feature of resistance is that the effect is the same for dc and ac circuits.
The two main characteristics of a resistor are its resistance R in ohms and its power rating in watts, W. Resistors are available in a very wide range of R values, from a fraction of an ohm to many megohms. The power rating may be as high as several hundred watts or as low as 1/10 W.
The R is the resistance value required to provide the desired I or IR voltage drop. Also important is the wattage rating because it specifies the maximum power the resistor can dissipate without excessive heat. Dissipation means that the power is wasted as I2R loss, since the resultant heat is not used. Too much heat can make the resistor burn. The wattage rating of the resistor is generally more than the actual power dissipation, as a safety factor.
Wire-wound resistors are used where the power dissipation is about 5 W or more. Typical wire-wound resistors are shown in Fig. 7.1. For 2 W or less, carbon resistors are preferable because they are small and cost less. Between 2 and 5 W, combinations of carbon resistors can be used. Also, small wire-wound resistors are available in a 3- or 4-W rating.
Figure 2.1: Wire-wound resistors with 50-W power rating, (a) Fixed R, 5 in. long. (b) Variable R, 2 in. diameter. (P. R. Mallory)
Most common in electronic equipment are carbon resistors with a power rating of 1 W or less. Typical carbon-composition resistors are shown in Fig. 2.2. Fig. 2.3 shows a group of resistors to be mounted on a printed-circuit (PC) board. The resistors can be inserted automatically by machine.
Figure 2.2: Carbon-composition resistors. (a) Internal construction. Length in 3/4 in., without leads, for 1-W power rating. Color strips give R value in ohms. Tinned leads have coating of solder. (b) Group of resistors mounted on printed-circuit (PC) board.
Figure 7.3: Typical carbon-type fixed resistors. Leads are cut and formed for insertion into holes with 0.5-in. spacing on PC board.
Resistors with higher R values usually have smaller wattage ratings because they have less current. As an example, a common value is 1 Mῼ at 1/4 W, for a resistor only 1/2 in. long. The less the power rating, the smaller the actual physical size of the resistor. However, the resistance value is not related to physical size. Wire-Wound Resistors In this construction, a special type of wire called resistance wire is wrapped around an insulating core, as shown in Fig. 2.2. The length of wire used and its specific resistivity determine the R of the unit. Types of resistance wire include tungsten and manganin, “Conductors and Insulators.” The insulated core is commonly porcelain, a phenolic material like Bakelite, cement, or just plain pressed paper. Bare wire is used, but the entire unit is generally encased in an insulating material.
Since they are generally for high-current applications with low resistance and appreciable power, wire-wound resistors are available in wattage ratings from 5 W up to 100 W or more. The resistance can be less than 1 Ω up to several thousand ohms.
In addition, wire-wound resistors are used where accurate, stable resistance values are necessary. Examples are precision resistors for the function of an ammeter shunt or a precision potentiometer to adjust for an exact amount of R.
- Carbon-Composition Resistors
This type of resistor is made of finely divided carbon or graphite mixed with a powdered insulating material as a binder, in the proportions needed for the desired R value. As shown in Fig. 2.1 a, the resistor element is enclosed in a plastic case for insulation and mechanical strength. Joined to the two ends of the carbon resistance element are metal caps with leads of tinned copper wire for soldering the connections into a circuit. These are called axial leads because they come straight out from the ends.
Carbon resistors are commonly available in R values of 1 Ω to 20 MΩ.
There are two kinds. The carbon-film type has a thin coating around an insulator. Metal-film resistors have a spiral around a ceramic substrate (Fig. 2.4). Their advantage is more precise R values. The film-type resistors use metal end caps for the terminal leads, which makes the ends a little higher than the body.
Figure 2.4: Construction of metal-film resistor. (Stackpole Components Co.)
These have a carbon coating fired onto a solid ceramic substrate. The purpose is to have more precise R values and greater stability with heat. They are often made in a small square with leads to fit a printed-circuit (PC) board.
This type is a wire-wound resistor made to burn open easily when the power rating is exceeded. It then serves the dual functions of a fuse and a resistor to limit the current.
All the resistor types considered here are linear, meaning that they follow the Ohm’s-law relation 1 = V/R. In some applications, special characteristics are useful. One example is a thermistor, which increases its R with higher temperature. Another type is the varistor, which is a device whose R depends on the applied voltage.
Answers at End of Chapter Answer True or False.
An R of 10 Ω with a 10-W rating would be a wire wound resistor.
An R of 12 kΩ with a 1-W rating would be a carbon resistor.
Axial leads are not used for carbon resistors.
Because carbon resistors are small physically, they are color-coded to mark their R value in ohms. The basis of this system is the use of colors for numerical values, as listed in Table 2.1. In memorizing the colors, note that the darkest colors, black and brown, are for the lowest numbers, zero and one, through lighter colors to white for nine. The color coding is standardized by the Electronic Industries Association (EIA). These colors are also used for small capacitors, as summarized in App. G on all the color codes.
Table 2.1: Color Code
The use of bands or stripes is the most common system for color-coding carbon resistors, as shown in Fig. 2.5. Color stripes are printed at one end of the insulating body, which is usually tan. Reading from left to right, the first band close to the edge gives the first digit in the numerical value of R. The next band marks the second digit. The third band is the decimal multiplier, which gives the number of zeroes after the two digits.
Figure 2.5: How to read color stripes on carbon resistors.
In Fig. 2.6a, the first stripe is red for 2 and the next stripe is green for 5. The red multiplier in the third stripe means add two zeroes to 25, or “this multiplier is 102.” The result can be illustrated as follows: