NEED OF RESISTORS

Resistors can play any of numerous different roles in electrical and electronic equip- ment. Here are a few of the more common ways resistors are used.
Voltage division You’ve already learned a little about how voltage dividers can be designed using resis- tors. The resistors dissipate some power in doing this job, but the resulting voltages are needed for the proper biasing of electronic transistors or vacuum tubes. This ensures that an amplifier or oscillator will do its job in the most efficient, reliable possible way.
Biasing In order to work efficiently, transistors or tubes need the right bias. This means that the control electrode—the base, gate, or grid—must have a certain voltage or current. Net- works of resistors accomplish this. Different bias levels are needed for different types of circuits. A radio transmitting amplifier would usually be biased differently than an os- cillator or a low-level receiving amplifier. Sometimes voltage division is required for bias- ing. Other times it isn’t necessary. Figure 6-1 shows a transistor whose base is biased using a pair of resistors in a voltage-dividing configuration.

Current limiting

Resistors interfere with the flow of electrons in a circuit. Sometimes this is essential to prevent damage to a component or circuit. A good example is in a receiving amplifier. A resistor can keep the transistor from using up a lot of power just getting hot. Without re- sistors to limit or control the current, the transistor might be overstressed carrying di- rect current that doesn’t contribute to the signal. An improperly designed amplifier might need to have its transistor replaced often, because a resistor wasn’t included in the design where it was needed, or because the resistor isn’t the right size. Figure 6-2 shows a current-limiting resistor connected in series with a transistor. Usually it is in the emitter circuit as shown in this diagram, but it can also be in the collector circuit.

Power dissipation

 Dissipating power as heat is not always bad. Sometimes a resistor can be used as a “dummy” component, so that a circuit “sees” the resistor as if it were something more complicated. In radio, for example, a resistor can be used to take the place of an an- tenna. A transmitter can then be tested in such a way that it doesn’t interfere with sig- nals on the airwaves. The transmitter output heats the resistor, without radiating any signal. But as far as the transmitter “knows,” it’s hooked up to a real antenna (Fig. 6-3). Another case in which power dissipation is useful is at the input of a power amplifier. Sometimes the circuit driving the amplifier (supplying its input signal) has too much power for the amplifier input. A resistor, or network of resistors, can dissipate this ex- cess so that the power amplifier doesn’t get too much drive.

MAGNETIC UNITS

The size of a magnetic field is measured in units called webers, abbreviated Wb. One weber is mathematically equivalent to one volt-second. For weaker magnetic fields, a smaller unit, called the maxwell, is sometimes used. One maxwell is equal to 0.00000001 (one hundred-millionth) of a weber, or 0.01 microvolt-second. The flux density of a magnetic field is given in terms of webers or maxwells per square meter or per square centimeter. A flux density of one weber per square meter (1 Wb/m2) is called one tesla. One gauss is equal to 0.0001 weber, or one maxwell per square centimeter.
Magnetic units 39
2-12 Magnetic flux lines around a coil of wire. The fines converge at the magnetic poles.
In general, the greater the electric current through a wire, the greater the flux den- sity near the wire. A coiled wire will produce a greater flux density than a single, straight wire. And, the more turns in the coil, the stronger the magnetic field will be. Sometimes, magnetic field strength is specified in terms of ampere-turns (At). This is actually a unit of magnetomotive force. A one-turn wire loop, carrying 1 A of current, produces a field of 1 At. Doubling the number of turns, or the current, will dou- ble the number of ampere-turns. Therefore, if you have 10 A flowing in a 10-turn coil, the magnetomotive force is 10 10, or 100 At. Or, if you have 100 mA flowing in a 100-turn coil, the magnetomotive force is 0.1 100, or, again, 10 At. (Remember that 100 mA 0.1 A.) Another unit of magnetomotive force is the gilbert. This unit is equal to 0.796 At.

MAGNETISM

Electric currents and magnetic fields are closely related. Whenever an electric current flows—that is, when charge carriers move—a mag- netic field accompanies the current. In a straight wire, the magnetic lines of flux surround the wire in circles, with the wire at the center (Fig. 2-11). Actually, these aren’t really lines or circles; this is just a convenient way to represent the magnetic field. You might sometimes hear of a certain number of flux lines per unit cross-sectional area, such as 100 lines per square centimeter. This is a relative way of talking about the intensity of the magnetic field.

Magnetic fields can be produced when the atoms of certain materials align them- selves. Iron is the most common metal that has this property. The iron in the core of the earth has become aligned to some extent; this is a complex interaction caused by the rotation of our planet and its motion with respect to the magnetic field of the sun. The magnetic field surrounding the earth is responsible for various effects, such as the con- centration of charged particles that you see as the aurora borealis just after a solar eruption. When a wire is coiled up, the resulting magnetic flux takes a shape similar to the flux field surrounding the earth, or the flux field around a bar magnet. Two well-defined magnetic poles develop, as shown in Fig. 2-12. The intensity of a magnetic field can be greatly increased by placing a special core inside of a coil. The core should be of iron or some other material that can be readily magnetized. Such substances are called ferromagnetic. A core of this kind cannot actually increase the total quantity of magnetism in and around a coil, but it will cause the lines of flux to be much closer together inside the material. This is the principle by which an electromagnet works. It also makes possible the operation of electrical trans- formers for utility current. Magnetic lines of flux are said to emerge from the magnetic north pole, and to run inward toward the magnetic south pole. But this is just a semantical thing, about which theoretical physicists might speak. It doesn’t need to concern you for ordinary electri- cal and electronics applications.