Electrical relays, bell ringers, electric hammers, and other mechanical devices make use of solenoids.
Sophisticated electromagnets, sometimes in conjunction with permanent magnets, allow engineers to construct motors, meters, generators, and other electromechanical devices. Let’s look at a few
examples.
Figure 1-12 illustrates a bell ringer, also called a chime. The ferromagnetic core has a hollow region in the center along its axis, through which a steel rod, called the hammer, passes. The coil has many turns of wire, so the electromagnet produces a strong field if high current passes through the coil. When no current flows in the coil, gravity holds the rod down so that it rests on the base plate.
When a pulse of current passes through the coil, the rod jumps up and hits the ringer plate.
FIGURE 1-12 A bell ringer, also known as a chime.
Quick Question, Quick Answer
• In a chime, such as the one shown in Fig. 1-12, the magnetic field “wants” the ends of the rod, which has the same length as the core, to align with the ends of the core. Why doesn’t the rod stop at that point? Why does it continue on up to hit the ringer?
• The rod’s upward momentum makes it fly through the core and keep going for a while, even as the magnetic field from the solenoid “tries” to pull it back. The rod travels far enough to hit the ringer and then falls back, allowing the ringer to reverberate.
You can’t always locate switches near the devices they control. For example, imagine that you want to switch a communications system between two different antennas from a control point a few
hundred meters away. Wireless antenna systems carry high-frequency AC (the radio signals) that must remain within certain parts of the circuit. You can’t let those signals follow control wires that go to a simple switch; doing that would interfere with the workings of the antenna system. A relay makes use of a solenoid to allow remote-control switching in a situation of that sort.
Figure 1-13 illustrates a simple relay. A movable lever, called the armature, is held to one side (upward in this diagram) by a flexible, “springy strip” of metal or plastic when no current flows through the coil. Under these conditions, terminal X connects to terminal Y, but X does not contact
terminal Z. When a sufficient current flows in the coil, the armature moves to the other side (downward in this illustration), disconnecting X from Y, and connecting X to Z.
FIGURE 1-13 Simplified drawing of a relay.
A normally closed relay completes the circuit when no current flows in the coil, and breaks the circuit when coil current flows. (“Normal” in this sense means the absence of coil current.) A
normally open relay does the opposite, completing the circuit when coil current flows, and breaking the circuit when coil current does not flow. The relay shown in Fig. 1-13 can function as a normally open or normally closed switch, depending on which contacts you select. It can also switch a single line between two different circuits.
Did You Know?
These days, engineers install relays in circuits and systems that must handle massive current or high voltage (or both). In applications where the current and voltage remain low to moderate, electronic semiconductor switches, which have no moving parts, offer better performance and reliability than relays.
In a DC motor, you connect a source of electricity to a set of coils that produce small-scale, but nevertheless powerful, magnetic fields. The attraction of opposite poles, and the repulsion of like poles, is manipulated so that a constant torque (rotational force) results inside the device. As the coil current increases, so does the torque that the motor can produce, and so does the energy it takes to operate the motor at a constant speed.
Figure 1-14 illustrates the functional aspects of a DC motor. The armature coil rotates along with the motor shaft. A pair of field coils remains stationary. The field coils function as electromagnets.
(Some motors use a pair of permanent magnets instead of the field coils.) Every time the shaft
completes half a rotation, the commutator reverses the current direction in the armature coil so that the shaft’s torque keeps going in the same direction. The shaft’s angular (rotational) momentum carries it around so that it doesn’t stop at those instants in time when the current reverses.
FIGURE 1-14 Simplified drawing of a DC motor.
The construction of an electric generator resembles the construction of an electric motor, although the two devices function in the opposite sense. A motor constitutes an electromechanical transducer because it converts electrical energy to mechanical motion. You might call a generator a specialized mechanoelectrical transducer (although I’ve never heard anybody use that term).
A basic electric generator produces AC when a coil rotates in a strong magnetic field. You can drive the shaft with a gasoline-fueled engine, a turbine, or some other source of mechanical energy.
Some generators employ commutators to produce pulsating DC output, which you can filter to obtain pure DC for use with precision equipment, just as you would do with the pulsating DC from an AC power supply.
Did You Know?
Some generators can operate as motors, and some motors can operate as generators. The experts call such devices motor-generators.
Semiconductors
During the 1960s, semiconductor materials acquired a dominating role in consumer electronic devices of all kinds. The term “semiconductor” arises from the fact that the substance’s conductivity can be controlled to generate, amplify, modify, mix, rectify, and switch electrical currents or electronic signals. Various mixtures of elements and compounds can function as semiconductors. The two most common semiconductor media are based on the element silicon (Si) or a compound of gallium and arsenic known as gallium arsenide (GaAs).
In some semiconductor-based devices (also sometimes called solid-state devices), the power supply can be modest indeed, comprising a couple of 1.5-volt size AA or AAA “flashlight” cells or a 9-volt “transistor battery.” A single integrated circuit (IC or chip), smaller than your thumbnail, can do the work of thousands of discrete electronic components, such as diodes, transistors, capacitors, and resistors. You’ll find an excellent example of IC technology in any computer. In 1950, a personal computer (if such a thing had existed) would have occupied a large building, required thousands of watts to operate, and probably cost over a million dollars. Today you can buy one for a few hundred dollars and carry it in a small portfolio jacket.
Silicon is widely used in diodes, transistors, and ICs. Other substances, called impurities or
dopants, are added to the silicon to give it the desired properties. If, on the other hand, you hear about
“gasfets” and “gas ICs,” you’re hearing about GaAs technology. Gallium arsenide works better than silicon in several ways. A GaAs device needs less voltage than an equivalent Si device does. In
addition, GaAs will function at higher frequencies than Si will. GaAs devices are relatively immune to the effects of ionizing radiation, such as x rays and gamma rays. GaAs devices are used in
light-emitting diodes (LEDs), infrared-light-emitting diodes (IREDs), laser diodes, visible-light and infrared (IR) detectors, ultra-high-frequency (UHF) amplifying devices, and a variety of computer chips. The
primary disadvantage of GaAs is the fact that it costs more than silicon to fabricate into semiconductor components.
Elemental selenium (Se) exhibits resistance that varies depending on the intensity of visible-light, infrared (IR) radiation, or ultraviolet (UV) radiation that falls on it. All semiconductor materials exhibit this property to some degree, but Se is especially affected. For this reason, Se makes excellent solar photocells and solar cells. This material is also used in certain types of rectifiers. Perhaps the main advantage of Se over Si is the fact that Se-based devices withstand power-line transients better than Si-based devices do.
Pure germanium (Ge) constitutes a rather poor electrical conductor, but it becomes a semiconductor when impurities are added. This substance was used extensively in the early years of semiconductor technology. Some diodes and transistors still use Ge, but it’s pretty much been replaced by Si. One big problem with Ge-based technologies is their sensitivity to heat. Technicians must take extreme care when soldering the leads of a Ge component, so that the heat from the soldering instrument doesn’t conduct through the wire leads and destroy the semiconducting properties of the Ge inside.
Some oxides of metals have properties that make them useful for semiconductor devices. When someone tells you about MOS (pronounced “moss”) or CMOS (pronounced “sea-moss”) technology, you’re hearing about metal-oxide semiconductor and complementary metal-oxide semiconductor devices, respectively. Chips made from these materials demand so little power that the battery in a MOS-based or CMOS-based portable electronic device lasts almost as long as it would just sitting on the shelf without being put to any use at all. Devices with MOS and CMOS chips work fast, a property that makes them useful at high frequencies, allowing computers to perform many millions of
calculations per second. In ICs, MOS and CMOS technology also allows for high component density: a
large number (sometimes millions) of discrete diodes, transistors, capacitors, and resistors on a single chip.
Fact or Myth?
Some people might tell you that an MOS or CMOS device can be permanently ruined by an action as simple as picking it up and looking at it. Are they telling the truth? How could such a thing happen? Well, those people aren’t lying or exaggerating. The main problem with MOS and CMOS technology arises from the fact that such devices are easily damaged by electrical discharges that can occur as a result of the accumulation of charge carriers somewhere. Even the slightest
electrostatic buildup (“static electricity”) on your fingers can abruptly “zero itself out” through the internal circuits of a MOS or CMOS device and destroy some of the microscopic components.
You must always use care when handling components of this type. Technicians actually go so far as to place metal straps on their wrists, connected to wires that end in a substantial electrical ground, ensuring that their bodies don’t acquire a charge sufficient to “fry” sensitive MOS or CMOS components.
For Nerds Only
Many of the elements found in semiconductors can be mined from the earth. Others are
“grown” as crystals under laboratory conditions, a process called epitaxy.