Electronics - Electricity Basics

8/8/2019 Electronics - Electricity Basics

1/18

Page 1

University of Wisconsin-MadisonCollege of Letters and Science

Department of Communicative Disorders

A Poor Man's Tour of Basic Electricity and Electronics

Michael R. Chial, Ph.D. 1998

INTRODUCTION

Many of the devices we use in clinical and laboratory work are extremely simple: switches, lamps,connecting cables, and counters. Creating such systems requires very little knowledge, certainly not an engineeringdegree. What is required is an understanding a few basic terms and concepts and skill in the use of simple hand tools.These concepts and tools make it possible to create systems that work by design, rather than by luck or by trial anderror. (Throwing pigment at a canvas may produce a pleasing pattern, but not a landscape.) Creation by designrequires an understanding of basic laws--in the case of landscapes, the rules of perspective and light. And in the caseof simple electronic devices, Ohms law and basic electronic circuits. This Tour deals with these concepts.

ENTRY LEVEL SKILLS

Prior to using this Tour, you should be able to do each of the following:

(1) Correctly distinguish between quantities and units of measurement.(2) Correctly use SI systemprefixes to convert quantities expressed by one scale to another

(e.g., from milliseconds to seconds).

INSTRUCTIONAL OBJECTIVES

When you finish this material, you should be able to do each of the following correctly and without help.

(1) Distinguish between DCandAC signals.

(2) Distinguish between conductors and insulators.(3) Distinguish electron flow from conventional current flow.(4) State the units and symbols for electric charge, currrent, voltage, resistance, andpower.(5) State and explain Ohm's law for DC signal systems.(6) Describe simple series circuits and parallel circuits in terms of current and the effects of shorts and

opens.(7) State the differences betweenprimary and secondary batteries.(8) Identify male and female connectors of the following types: GR, phone, BNC, Canon, alligator and

RCA (phono).(9) Draw schematic diagrams illustrating the following switch functions: SPST, SPDT, DPST, DPDT.

(10) Describe the type of solder used in electronics.(11) List and explain six steps in soldering wires to wires or wires to terminals.(12) Describe the difference between a shortand an open in the context of connecting cables.

ELEMENTARY ELECTRICITY

Electricity (more precisely, electromagnetism) is a fundamental force of nature, like gravity and the forcethat binds atomic nuclei. The study of electricity is the study of the forces that cause electrons to move, whether innon-living or living matter. Electronics is the application of knowledge about such forces for practical tasks. Bothhave vast implications for society, culture and economics, as well as for the scientific, engineering and clinicaldisciplines that use them.


2/18

Page 2

CHARGE, CURRENT, AND VOLTAGE

Understanding electricity requires knowledge of a few physical laws and a simplified model of atomicstructure. Atoms and molecules contain electrons and protons. Electrons carry charges labeled negative and protonscarry an opposite charge (labeledpositive). In normal (non-ionized) matter, these charges balance each other:positive and negative charges are equal. If forces are applied to matter which cause an excess number of electrons orprotons, that matter has a static (unmoving) electrical charge. Electrical charge is a basic physical quantity, alongwith time, mass and length. The symbol for the physical quantity of electrical charge is Q. The amount of the

charge is determined by the number of electrons, and is indexed by the unit coulomb (C), named in honor of Charles-Augustin de Coulomb (1736-1806). One coulomb equals the charge associated with 6.25 x 10 +18electrons (a lot--

the superscript means move the decimal 18 places to the right); the charge of one electron equals 0.16 x 10 -18

coulombs.

Coulomb (the guy) was the first to show by measurement that electrical charges of the same sign (e.g.,negative and negative) repel each other and that charges of unlike signs (e.g., negative and positive) attract eachother. This pattern of attraction and repulsion creates the potential for movement of electrical charges, i.e., chargescan move from one place to another. A moving (dynamic) charge is called current. The symbol for the physicalquantity of electric current is I or i. The unit for electric current is the ampere (A), named in honor of Andre-MarieAmpere (1775-1836), who discovered that flowing current creates lines of force. One ampere equals 1 coulomb ofcharge moving past a point in 1 second.

Separating electrons and protons to create a charge requires physical work, that is, a force acting through adistance (force is mass times acceleration). A battery creates a charge (does work) chemically by depositing electronson one pole of the battery and depleting them from the other pole. Thus, a difference in electric charge existsbetween the two poles of a battery. We also can create electric charge mechanically. Hans Christian Oersted (1777-1851) discovered that when current flows in a wire, a magnetic field is generated in the region of the wire. MichaelFaraday (1791-1867) discovered that the reverse is also true: when a wire passes through the field created by amagnet, current flows through the wire. Such a current is induced by motion through a magnetic field. Because theamount and direction of induced current depend upon motion, this kind of charge is dynamic (moving). These ideasunderpin electric motors and the generation of electricity for use in homes and industry.

Because unlike charges attract and like charges repel, differing charges have different potentials for doingwork, or more precisely for undoing the work that created the difference in charge. This potential energy (orpotential work) is called electromotive force (emf), electricpotential, potential difference, orvoltage. Symbols forthis quantity include emf, E, e, and V. The unit for potential difference is thevolt (V), named after Alessandro Volta(1745-1827). One volt equals the work or enegy required to move 1 coulomb of charge between two points. Onevolt also equals 1 joule of energy (1 joule equals 1 newton-meter). Electric potential is required to produce current.

The idea of energy is somewhat elusive. The word energy comes from the Greek, meaning the workwithin. It was first used in 1807 by the British physicist Thomas Young (1773-1829) in an effort to identify acharacteristic of nature that made work possible (work is a force acting through a distance). Another Britton,James Prescott Joule (1818-1889) studied energy in the forms of heat and motion. Joule (a brewer by trade) observedin 1847 that if the heat generated by motion is taken into account, energy remains constant. In other words, energyis conserved: although energy can be changed from one form to another (e.g., from motion to heat or vice versa), itcannot be created or destroyed, at least not by ordinary means. This concept later became known as the first law ofthermodynamics: the total amount of energy in a closed system is constant. The second law of thermodynamics(generalized by Rudolf J. E. Clausius in 1850) states that enegy in a closed system always migrates toward heat--inother words, entropy (disorder) always increases. In 1905 Albert Einstein (1879-1955) stated his special theory ofrelativity which established that energy and matter are intimately connected.

When a moving electric charge does not change direction, it is direct current(DC). An example of a DCsystem is a battery-operated flashlight. When current changes direction from positive to negative and back again, itis alternating current (AC). An example of a small amplitude AC signal is that which causes the loudspeaker of aportable radio to produce sound. Such signals may cover most of the audio range (from about 60-20,000 cycles persecond (Hertz, abbreviated Hz) and typically have amplitudes ranging from millivolts to a few volts. An example ofa large amplitude AC signal is the current available from wall outlets in the home. This signal has a fixed frequencyof 60 Hz and a nominal amplitude of 117 volts. Figure 1 illustrates DC, AC, and combined DC-AC signals.


3/18

Page 3

Time Time

Amp

Amp

A B

C D

Figure 1. Waveforms illustrating direct current and alternating current signals. Panel A is an invariant DCsignal. Panel B changes in magnitude, but because it does not change polarity, it is a DC signal. Panel C showsan AC signal that periodically changes polarity from positive to negative. Panel D illustrates a signal containingboth DC and AC components.

RESISTANCE: CONDUCTORS AND INSULATORS

Neither direct nor alternating current can flow unless a pathway exists to conduct electrons from one place

to another. Some elements and compounds have more free electrons than others. The less motile the electrons, thefewer will flow for the same applied EMF. Opposition to the flow of free electrons is resistance. The quantity ofresistance is symbolized by R. The unit is the ohm ( , the Greek letter omega), named after George Simon Ohm(1789-1854) who discovered basic relations among current, voltage and resistance. When 1 ampere of current flowsthrough 1 ohm of resistance, it produces 0.27 calorie of heat (a more formal definition of the ohm is the resistance of

a column of mercury 106.3 centimeters long with a cross-section of 1/100th of a square centimeter at 00 Celsius).Forms of matter with low resistance are called conductors (e.g., silver, copper, gold, tin, lead and water). Conductorsdiffer in how well they conduct electricity. Resistance to current flow can be expressed relative to copper, the mostcommon conductor (Mims, 1988).

Silver 0.94Copper 1.00Gold 1.40Aluminum 1.55

Brass 4.82Iron 5.80Stainless Steel 52.94

Forms of matter with very high resistance are called insulators (e.g., glass, air, dry wood, mica and mostplastics). Semi-conductors fall between the two (e.g., silicon treated with impurities for use in transistors andintegrated circuits--ICs). Super-conductors are semi-conductors managed by special methods (e.g., extreme cooling)to make them highly efficient as conductors.

Fuses are specialized conductors intentionally designed to serve as weak links in electronic and electricalsystems. They consist of conductors whose material and form (diameter) are rated by the amount of current inamperes (A) they will pass. Larger currents destroy the conductor by over-heating. Thus, fuses limit the amount of


4/18

Page 4

electric current that can pass through other conductors, wires, or systems. If a fuse blows, that is a signal thatsomething may be wrong; if this happens repeatedly, potential causes should be investigated. Home electricalsystems typically consist of several branch circuits, each fused at 10, 15, or 20 A. Electronic instruments are fusedat .5, 1.5, or 2.0 A, or some other current determined by the purpose and design of the instrument. Circuit breakersare fuses made with sandwiches of metal that mechanically deform when heated (i.e., by current flow that exceeds aspecified amount). After current is interrupted, the circuit breaker cools and can be reset mechanically.

CURRENT AND ELECTRON FLOW

A closed electrical circuit consists of (1) a source of potential difference (such as a battery), (2) a conductor(such as copper wire) to convey current from one side of the voltage source to the other, and usually (3) some sort ofresistance (such as a lamp) that generates heat or otherwise limits current in order to do something practical. Aflashlight is a simple example, represented schematically in Figure 2, where S1 designates a switch and L1designates a lamp.

Figure 2. Schematic diagram of a simple flashlight. Electrons flow from the negative pole of the battery,through a closed switch (S1) and an incandescent lamp (L1), back to the positive pole of the battery.Conventional current is in a direction opposite to that of electron flow (i.e., from positive to negative).

The labels used to describe the direction of current and electron flow differ. The original assignment ofpositive and negative polarities to protons and electrons (respectively) was arbitrary. Electron flow is described bymovement of electrons from a place where there are more to a place where there are less. Conventional current isdescribed by proton movement. Both electron flow and current always seek a path of least resistance. In theillustration above, the lamp offers some electrical resistance, causing heat and light. All the current (moving charge)passes through the lamp. If there were an alternate path between the poles of the battery that offered zero resistance,the current would take that path and the lamp would not light. If the lamp is burned out (i.e., the conductor isdestroyed because of metal fatigue due to heat), no current flows because the circuit is incomplete.

ELECTRICAL POWER

In the simple flashlight illustration above, flowing electrons (current) accomplish work by providingillumination. The rate at which work is done ispower(work divided by time). The unit of power is the watt,named after James Watt (1738-1819) and symbolized by the letter W. One watt equals the work done in 1 second by1 volt of potential difference in moving 1 coulomb of charge. Because 1 coulomb per second equals 1 ampere,power in watts is equal to voltage (in volts) times current flow (in amperes).

3.0 v

Battery

L1

S1

Electron Flow

Current Flow

Negative

Positive


5/18

Page 5

OHM'S LAW

Relations among current, voltage, and resistance in circuits obey a physical law that is both simple andrich, fortunately so because without it (and the ability to apply it) our lives would be vastly more primitive thanthey are. Formulated by George Simon Ohm in 1827, the DC version of the law bearing his name can be stated as:voltage (V) equals current (I) times resistance (R).

V = I * R (1)

The units for voltage, current, and resistance are defined above. Given the defining relation of equation (1), we cancalculate any one value if we know the other two.

I = V / R (2)

R = V / I (3)

As noted above, power is equal to the product of voltage and current:

W = V * A (4)

The quantities and units of electricity discussed thus far are summarized in Table 1. Note that commonlyused symbols for quantities do not always correspond to the standardized symbols for units of measurement.

Table 1. Summary of fundamental electrical quantities and units.

Quantity Uni t

Name Symbol Definition Name Symbol Definition

Electric charge Q, q (basic quantity) couloumb Cphysical standard:

charge = 6.25 * 10 +18

electrons

Electric current I, i moving charge ampere A

1 C / sec;

1 V / 1

Electric potentialE, e,

emf, Vpotential energy orwork volt V

the work needed to move1 coulomb;1 joule;1 A * 1

Electric resistance R, oppositionto electric current(moving charge)

ohm 1 V / 1 A

Electric power P, W work per unit time watt W1 joule / sec;

1 V * 1 A


6/18


7/18

Page 7

S1

L1

L2

1.5 vBattery

S1

S2 L1

1.5 v

Battery

Figure 3. Simple series circuits. The left-hand schematic illustrates two lamps (L1 and L2) connected inseries with a switch (S1) and a battery. When the switch closes, the same current flows through both lamps.The total battery voltage is divided among the lamps in direct proportion to their resistance: if they have equalresistance, they will divide the total voltage equally. If either lamp burns out, no current flows and theremaining lamp will not shine. The right-hand schematic illustrates a similar circuit, but with two switches(S1 and S2) and only one lamp (L1). Both switches must be closed for current to flow through the lamp.Thus, these serial switches amount to a logic equation that says, if and only if Switch 1 AND Switch 2 areclosed, Lamp 1 will be lighted.

Parallel connection . In parallel circuits, the source current divides among system elements in inverse proportion

to the resistance (impedance in an AC circuit) of the individual elements. Voltage is the same across each element.By Ohm's law, total current IT = V / RT, where the reciprocal of RT equals the sum of the reciprocals of the

individual resistances (i.e., 1/RT = 1/R1 + 1/R2 + 1/R3 +. . . 1/Rn). If one element is removed, the remaining

circuit remains intact. Power outlets (wallplugs) in homes are wired in parallel: each has the same voltage, but eachcan draw different amounts of current (hence power), up to the limit of the fuse. A non-electrical example is thestate lottery. If many individuals (each with varying opposition to wasting resources) participate, the total numberof flowing dollars is inversely related to individual resistances. Figure 4 illustrates parallel circuits.

L1 L21.5 v

Battery

S1S2

S1

L11.5 v

Battery

Figure 4. Simple parallel circuits. The left-hand schematic illustrates two lamps (L1 and L2) connected inparallel with each other and in series with a switch (S1). When the switch closes, the same voltage is applied toeach lamp. The amount of current flowing through each lamp is inversely proportional to the resistance of thelamp: if they have equal resistance, half the total current will flow through one lamp, the other half through theother lamp. If either lamp is burned out, the other continues to shine. The right-hand schematic shows twoswitches (S1 and S2) connected in parallel. If either switch closes, current flows through the lamp, L1. Thus,these parallel switches amount to a logic equation that says, if and only if Switch 1 OR Switch 2 is closed,Lamp 1 will be lighted.


8/18

Page 8

Practical electronic devices typically contain combinations of series and parallel circuits engineered toaccomplish well-defined tasks. Circuits that use AC signals (e.g., radio, television, audio systems) arrange parts toperform equally complex operations, essentially calculations on signals.

SIMPLE ELECTRONIC COMPONENTS

POWER SOURCES

Batteries are the most common power source for small electronic devices. Batteries consist of one or morecells, each of which is a chemical engine containing two different metals separated by an electrolyte in the form of apaste or fluid. Electrolytes concentrate free electrons on one of the metal surfaces of the battery. Each metal surfaceis attached to a terminal or pole, one labeled positive, the other negative. The schematic symbols for batteriesconsist of two parallel lines (see Figure 5). The short line designates the negative pole; the longer line indicates thepositive pole. The number of pairs of lines denotes the number of cells in the battery.

One-Cell

1.5 v

Battery

Negative

Pole

Positive

Pole

Two-Cell

3.0 v

Battery

Four-Cell

6.0 v

Battery

Figure 5. Schematic symbols for batteries. Each pair of parallel lines is one cell. The negative pole orterminal of the battery is shown by the shorter line.

Supermarket displays make it obvious that batteries come in a range of sizes and voltages. So-called"transistor" batteries (rated at 9 volts) are designed for applications that require relatively small currents. Standard 1.5volt sizes are designated by letter, from relatively large D cells, to smaller C cells, to smaller AA cells, to even

smaller AAA cells, and the tiny N cell. Even smaller are the batteries designed for watches and hearing aids.Physically larger batteries supply more current for longer periods. Batteries differ in other ways, too.

Primary batteries are the most common, in part because they are designed to be used, then discarded (e.g., inflashlights, toys, tape recorders, cameras, watches, calculators, hearing aids, etc.). Primary batteries are notrechargeable. Most commercially available primary batteries consist of single cells and provide 1.5 volts per cell.These can be combined in series (to produce a total voltage equal the the sum of the individual batteries), or inparallel (to produce a total current capability equal to the sum of the individual batteries). Different materials producedifferent characteristics. For example, carbon-zinc units (1.5 volts per cell) are inexpensive; alkaline units (1.5 voltsper cell) provide greater currents (e.g., for devices containing motors and incandescent lamps); mercury batteries (1.4volts per cell) give uniform voltage during use; and lithium batteries (3.0 volts per cell) offer a very long storage lifeand relatively high voltage in a small package.

Secondary (storage) batteries can be recharged many times. Lead-acid batteries (2.0 volts per cell) provide

very high current and operate well at low temperatures. Thus, they are used in cars, boats, and other vehicles.Nickel-cadmium batteries (1.2 volts per cell) have long storage life when discharged, but present memory problemswhen recharged without being fully discharged. Nickel-hydride batteries are similar to nickel-cadmium, but lack thememory problems of older technology devices.

Some safety issues should be mentioned about batteries. Batteries should not be heated or burned. Thisincludes soldering to battery terminals (soldering requires high heat--see below). Nor should the terminals of a batterybe directly connected to each other (i.e., shorted). Primary batteries should not be charged, but they should beremoved from equipment that is not being used--they may leak and damage their containers. Because batteriescontain toxic materials, they should be stored (or discarded) so they cannot be eaten or inhaled by children.


9/18

Page 9

Battery eliminators are devices powered by household current. Internal components transform relativelylarge voltage (117 volts) alternating current into one or more smaller DC voltages that simulate particular batteryconfigurations. Examples include 12 volts (the same as eight carbon-zinc cells), 9 volts (the same as six carbon-zinccells), 6 volts (the same as four carbon-zinc cells), and so forth. Battery eliminators have the advantage of long use-life, but they are less portable than batteries. Indeed, they require a power outlet. Because these devices requirehousehold current, and because they usually lack protective fuses, they present potential risk if used withinappropriately or carelessly. In most cases, the risk is not great, but in others (e.g., a child who twists or chews apower cord) the results can be serious.

Most electronic devices designed for connection to power outlets contain circuits that convert AC linevoltage into the DC signals required to operate other circuits internal to the devices. These specialized circuits, calledpower supplies, typically consist of transformers (which convert 117 v line current into other voltages), rectifiers(which change 60 Hz AC voltage into a changing DC voltage), filters (which smooth most of the remainingvariations in voltage), and regulators (which stabilize voltage output). Power supplies are can supply more powerthan batteries and differ greatly in complexity, size, and cost.

CONNECTING DEVICES

Cables are conductors designed to carry electrical signals from one device to another. Such signals may beAC or DC; they may serve functions of control or communication, or they may carry the information of primaryinterest in measurement. Connectors are the mechanical means by which cables are attached to devices or other

cables. When cables and connectors interconnect points, those points are electrically identical, even though theymay be physically separated. Adapters are specialized connectors that allow changing from one type of connector to

another.

Cables, connectors, and adapters have one, two or more electrical conductors. Multiple conductors areelectrically isolated from each other by insulating material. Most commonly, connectors have two conductors ofdifferent polarity: a signal positive or "high" conductor, and a signal ground, negative or "low" conductor. In audiowork, it is common to use three conductors, where the third is a neutral shield, sometimes called a system orinstrument groundwith the purpose of isolating the cable from stray electromagnetic radiation produced byinstruments, transducers, motors, lighting fixtures, etc. Another common three-conductor cable system is used forstereo applications (two signal lines, right and left, share a sole signal ground). Co-axial cable is so called because itcontains two or more conductors that share a common "axis," i.e., the conductors are physically joined and parallelto each other.

CABLES

Cables are designed with wires that differ in diameter (measured via the American Wire Gauge-AWG-standard; larger AWG numbers indicate smaller diameter wire), depending on the amount of current to be carried bythe wire (thicker wires are used for larger currents), and by the amount of resistance acceptable for a particularapplication (thicker wires offer less resistance). A two-conductor, coaxial cable commonly used for audio and videowork, designated RG-59/U, is illustrated in Figure 6. Conductor A is a solid copper wire. Conductor B is made thincopper wire braided into a cylinder that surrounds the inner insulation. Alternatives for audio work include cablewith three conductors: two for the electrical signal and a third (called a shield) which is connected to ground. Suchcable is used for microphones and in other situations where there may be stray electromagnetic fields that mightinterfere with transmission.


10/18

Page 10

Outer

Insulation

Conductor A

(Signal Positive)

Conductor B

(Signal Negative or Ground)

Inner

Insulation

A. Coaxial Audio Cable

Conductors

B. Lamp Cord

Insulation

Figure 6. The left panel illustrates coaxial cable with two conductors. Typically, the inner conductor (A) issolid wire and the outer conductor (B) is braided wire. The two conductors are separated by insulating materialand the entire cable is encased in an insulating jacket. The right panel illustrates lamp cord or zip cord of thetype commonly used for appliances. Each conductor consists of many individual strands of copper.

Another common cable illustrated in Figure 6 is lamp cord (also called "zip-cord") consisting of twoconductors, each made up of a separate bundle of fine wires separated by rubber or plastic insulation. Lamp cord isavailable in various AWG diameters and is used with loudspeakers, electric lights, and other devices moved fromplace to place (stranded wire is mechanically more robust). Many power extension cords used to bring AC powerfrom electrical outlets to electronic devices employ three conductors, one of which is connected to earth-ground forsafety reasons. Most (not all) power cords code conductors with colored insulation: white for positive, black fornegative, and green for earth-ground. All three conductors are packaged in a black outer sheath. Another commoncable, hook-up wire (usually a single solid copper conductor in gauges ranging from 22 to 18), is used inside deviceswhere mechanical stress is unlikely to harm the conductor. Stranded hook-up wire is used whenever motion andstress might damage solid-core cable.

CONNECTORS

Connectors are attached to the ends of cables, or to the panels of instruments (the latter are called bulkheadconnectors). Connectors are of two forms: male, with protruding contacts; andfemale, with recessed contacts.Male connectors also are calledplugs; female connectors also are calledjacks. In most cases, the connectors oncables are male, while bulkhead connectors are female. Connectors are attached to cables by stripping off insulationand soldering, or (for some types of connectors) with crimping tools designed for specific types of connectors. Mostelectronic instruments are designed to be connected in parallel with each other, not in series. Some connectors aremechanically pre-polarized--designed so that correct mechanical connection automatically produces proper signalconnection (i.e., positive-to-positive and negative-to-negative).

Connectors exist in many designs, usually distinguished by abbreviations based upon the names of thecompanies that originally developed them (e.g., "GR" connectors developed by the General Radio Corporation).

Some designs are available in different sizes, usually distinguished by diameter. See Figure 7.


11/18

Page 11

BNC

Alligator

Phone

RCA

(Phono)GR

Canon

Figure 7. Illustration of selected connectors designed for attachment to cables. All are male plugs.

1. GR (for General Radio Corporation) connectors, also called banana plugs because of appearance. Mostoften these are built in pairs (thus, dual-banana), but they also are available as single-conductorconnectors. GR plugs may have labels to designate which contact is signal ground, but they are not

mechanically pre-polarized. Most GR connectors are both male and female. GR connector contacts are

silver in color; the insulated housing may be black or some other color.

2. Phone connectors were developed by the Bell System in the days of manual telephone switchboards.Today they are used to connect microphones to other equipment, and to connect earphones to amplifiers,stereo pre-amplifiers, and audiometers. Phone connectors may be two-conductor (one channel) or three-conductor (two channel or stereo) devices. In the one-channel form, the tip of the plug is one conductor(signal) and the shaft is the other (signal ground). In the two-channel form, a ring near the tipaccommodates an additional conductor. Two sizes are common (for both mono and stereo plugs): 1/4-inch diameter and 1/8-inch diameter.

3. BNC(for Berkeley Nucleonics Corporation) connectors are two-conductor devices. The male conductorhas a rotating outer shell containing slots that mechanically latch to pins on the neck of the femaleconnector. BNC connectors are mechanically pre-polarized and usually silver in color. A thin center pinis for signal; the outer portion of the connector is signal ground. Most electronic test instruments areequipped with female BNC connectors. Home television signal distribution systems (TV cable) useconnectors similar to BNC connectors, but with screw threads rather than latch pins.

4. Canon orXLR connectors are three-conductor devices used exclusively with microphones. Thisconnector uses conductors numbered 1 (signal), 2 (signal ground), and 3 (shield or instrument ground).XLR connectors are common in professional recording applications to eliminate spuriouselectromagnetic noise.

5. Alligator clips are spring-loaded connectors with gripping teeth used for temporary connection betweentest equipment (such as voltmeters) and electronic components. One-conductor, non-polarized connectors,they are available in several sizes.

6. RCA (for Radio Corporation of America) connectors are two-conductor devices, also calledphono (forphonograph) connectors. Because these are commonly used with home-entertainment stereo gear, RCAcable assemblies often contain two sets of conductors. They also are commonly used with home videocameras and recorders. RCA connectors are pre-polarized and available in various colors.

Many other connectors exist for specialized applications, most of which also require specialized cables. Forexample, contemporary telephone systems employ pre-polarized plastic connectors with either four or six contacts.These are designed for attachment to light-gauge (low voltage) cable used in residential and commercial telephoneinstallations, to interconnect computers linked together in local networks, and to connect MODEMs (mod ulator-

dem odulators, devices which allow widely separated computers to communicate with each other over telephone lines)

to computers and to telephone systems. Telephone connectors (designated RJ-4 and RJ-6 to distinguish numbers ofconductors) are attached to flat telephone cable with a crimping tool designed for that purpose. Still another class of


12/18

Page 12

connectors is used to connect computers to printers and other peripheral devices. These differ in numbers of contacts(typically between three and 36) dictated by the details of the circuits with which they are used. The major reasonwhy there are so many different specialized connectors is to prevent incompatible devices from being inadvertentlyconnected to each other.

ADAPTERS

Adapters convert the form or function (or both) of dissimilar connectors to allow complete signal paths.Like connectors, they are designated by sex, size, and configuration. Another purpose of adapters is to allowsplitting or branching of signals. Some common adapters are (1) female BNC-to-male 1/4-inch phone, (2) femaleBNC-to-male RCA, (3) male BNC-to-dual female BNC, or "T," (4) female BNC-to-female BNC, and (5) femaleBNC-to-GR.

SWITCHES

Switches route current from one place in a circuit to another. Although they come in a wide variety ofsizes and shapes, most switches perform one of the four basic functions noted in Figure 8. These functions may beimplemented in various ways. Among the most common are mechanical switches activated with toggles, buttons orrockers. These accomplish switching by physical motion of solid conductors. Other switches (e.g., those used inhome thermostats) use a liquid conductor (mercury). Others are activated by electromagnets (e.g., solenoids and

relays). Switches provide either momentary or non-momentary closure of contacts. Momentary switches employsprings or other devices so that electrical contact is made only as long as the switch is mechanically activated. Stillother switches function electronically by means of diodes, transistors or integrated circuits. In general, electronicswitches use an electrical signal (typically a voltage) to gate current flow. Electronic switches are available whichrespond to light, heat, motion, proximity to people or objects, or other physical events. Switches are rated for themaximum current and voltage allowed by their design and construction.

SPST

DPST DPDT

SPDT

S1

S3

S2

S4

Figure 8. Schematic illustrations of four common switch functions. Switch 1 (S1) illustrates a single-pole,

single-throw (SPST) switch such as those found in table lamps and most simple home appliances. Switch 2(S2) is a single-pole, double-throw (SPDT) switch capable of routing current to one of two paths. Switch 3(S3) is a double-pole, single-throw (DPST) device capable of simultaneously turning on each of two circuits(e.g., a tape recorder motor and a lamp that indicates the recorder is turned on). Switch 4 (S4) is a double-pole,double-throw (DPDT) switch for more complex applications.


13/18


14/18

Page 14

TOOLS

The metal alloy used for soldering electronic parts is usually 60% tin and 40% lead. This 60/40combination melts at a temperature of about 370 degrees Fahrenheit (different proportions of tin and lead havedifferent melting points), a low enough temperature to minimize potential damage to the parts being soldered. Atcooler temperatures, 60/40 solder becomes plastic (pliable), then solid. At temperatures above the melting point,solder is liquid. Solder designed for use in electronics looks like silver wire wound around a spool. The wire isactually a hollow tube, containing liquid flux, a substance that chemically removes any oxide from the metal parts to

be soldered. When the solder melts, the flux is released to clean the metal parts. Acid once was used as flux; morecommon today is non-corrosive rosin. Solder intended for other applications (e.g., plumbing and hobbies such asstained-glass) should never be used for soldering electronic components.

Some additional tools are necessary or useful for soldering: a small vise to hold components duringsoldering (and to avoid burned fingers); a combination wire-cutter and wire-stripper; a long-nosed pliers (also calledneedle-nosed) for bending wires; steel wool or sandpaper (for cleaning large oxidized surfaces); and a spring-loadedheat sink to protect delicate components (e.g., transistors) from over-heating.

TECHNIQUE

In manual soldering, heat is applied by means of a soldering iron, an electrical hand tool consisting of apower cord, a handle, a heating element, and a tip. Most common are those that look like fat pencils. Solderingirons come in different sizes and different electrical powers. Both physical size and power (in watts) are directly

related to the amount of heat produced by the iron. Most electronic work is done with irons rated between 20 wattsand 40 watts that generate temperatures between about 700-900 degrees Fahrenheit. The tips of soldering irons areusually in the shape of chisels, cones, or pyramids, depending upon the work to be done. More expensive units havevariable temperature controls or thermal regulators. The tips, made of copper and plated with other metals, must betinned(coated with solder) before the iron can be used. During soldering, the tip must be cleaned frequently bywiping it on a damp sponge or rag. This removes dirt and excess solder. Use stands as resting places for solderingirons to avoid burning work surfaces (and yourself). Soldering irons must be unplugged when not in use.

Most soldering involves bonding one wire to another wire or to a lug of some sort. This can be done in afew simple steps as illustrated in Figure 9. First, prepare the wire by cutting and removing about a half-inch ofinsulation from the end of the wire. Tin the exposed wire by heating it, then applying a thin film of solder. Next,bend the exposed length of wire into a hoop or hook and insert it in (or wrap it snugly around) the lug. Periodicallyclean the tip of the iron by wiping it on a damp sponge or rag. Touch the tip of the soldering iron to both the wire

and the lug. Do this for 3-5 seconds to heat both surfaces at the same time. Now apply the solder to the side of the

joint opposite the tip of the soldering iron. The heated parts will melt the solder and cause it to flow around thejunction of wire and lug. Hold the tip to the work only long enough for the solder to melt and flow around thejoint, using only enough solder to cover the joint. Finally, remove the tip from the work, keeping the workimmobile for the 5-10 seconds needed for the solder to solidify.

Good solder bonds are shiny and smooth, conforming to the shape of the wire-lug junction. Poor bonds arelumpy, crystalline or grainy. Too little heat causes a residue of rosin--a brownish stain, sometimes with a lump ofsolder on top of the junction. This may result in a cold-solder jointin which the electrical connection is very poor.Too much heat (and too much solder) can produce another flaw, a solder bridge, in which solder unintentionallyflows to connect other wires, leads or lugs. Too much heat also can melt the electronic components being soldered.Unsoldering can be done with a special desoldering bulb consisting of a hollow, heat-resistant tube inserted in asqueezable rubber bulb. The solder is melted and the bulb is used to suck solder away from the connection.Another method is to use flux-coated desoldering braid, made with very fine wires woven together in a pattern thatabsorbs liquid solder. A section of braid is placed on the connection, then heated. The melted solder flows into thebraid by the action of the flux. For larger connections, still another desoldering technique is to use a long-nosepliers to lift wire away from heated solder. Each method requires several attempts to complete desoldering.


15/18

Page 15

Solder Lug

Terminal Strip

Secure Cableto Lug

3

Strip and Tin

Cable1

1/2-inch

Bend Cable2

Hold in Place&

Allow to Cool(5-10 sec)

6

Apply Iron to

Work (3-5 Sec)4

Apply Solderto OppositeSide of Work

5

60/40Rosin-Core

Solder

Soldering Iron

Figure 9. Illustration of solder technique for bonding a solid-core wire to a lug terminal. See text forexplanation of the six steps noted above.


16/18

Page 16

Another very common application of soldering is to attach a connector to the end of a cable. Although theprocedures just described also apply to such work, a few additional pointers are useful. Most coaxial cables employtwo or more conductors, one or both of which may be multi-stranded or braided wire. Before multi-strandedconductors are soldered, they should be twisted to form a single wire which should be heated and tinned with asoldering iron. Any protective housing should be placed on the cable before soldering the cable to the connector.

Because most cables used with connectors contain two or more conductors, and because attachment usually must beaccomplished in a confined space (i.e., the solder joints will be covered by a protective housing), special care must

be taken to avoid solder bridges. One way to do this is to isolate the two finished solder connections with a smalltab of insulating electrical tape. Another is to use heat-shrink tubing. Heat-shrink tubing (available in differentdiameters) can be slipped over wire prior to soldering. After the work is soldered, the tube is slipped over the jointto be insulated. When the tubing is heated with a match or soldering iron, it shrinks to a tight fit around the joint.

PROBLEMS WITH CABLES, CONNECTORS AND SWITCHES

The two most common problems with these devices are shorts and opens. A shorthappens when currentflow is present, but should be absent. An open happens when current flow is absent, but should be present. In thecoaxial cable illustrated in Figure 6, current should flow through conductor A and through conductor B; if it does notflow through one or the other conductor, that conductor is open (interrupted). Similarly, current should not flowbetween conductors A and B; if it does, the two conductors are shorted. Such flaws may be intermittent, occurringonly with mechanical displacement. They often happen because of poor soldering technique (cold-solder joints or

solder bridges) or physical abuse (e.g., disconnecting a cable by pulling the cable instead of the connector).Appendix B gives hints for troubleshooting problems with cables and connectors used for electronic measurement.

Shorts and opens usually cannot be found by visual inspection. Instead, a Volt-Ohm-Milliameter (VOM) isused to check electrical continuity. A VOM is a device that measures potential difference, resistance, and current.Electrical continuity is checked by measuring the electrical resistance between the various conductors of cables orconnectors without any power applied to the circuit (i.e., with batteries disconnected). Resistance is indexed by theposition of a needle on a meter face, or by a numerical display. If the resistance is low where it should be low andhigh where it should be high, then all is well. If resistance is low where it should be high, a short exists; ifresistance is high where it should be low, an open exits. Some VOMs have circuits that produce an audible signalwhen resistance is less than some nominally small amount (e.g., 200 ohms).

SUGGESTED REFERENCES

Adams, H. (1974). SI Metric Units: An Introduction, Rev. Ed. Toronto, ON: McGraw-Hill Ryerson,Limited.

Cleary, A. (1977). Instrumentation for Psychology. New York, NY: John Wiley & Sons.Cudahy, E. (1988). Introduction to Instrumentation in Speech and Hearing. Baltimore, MD: Williams and

Wilkins Co.Curtis, J., and Schultz, M. (1986). Basic Laboratory Instrumentation for Speech and Hearing. Boston, MA:

Little, Brown and Co.Grob, B. (1977). Basic Electronics, 4th Ed. New York, NY: McGraw-Hill Book Co.Mims, F. M. (1986). Engineer's Mini-Notebook: Basic Semiconductor Circuits. Fort Worth, TX: Radio

Shack, Inc.Mims, F. M. (1985). Engineer's Mini-Notebook: Digital Logic Circuits. Fort Worth, TX: Radio Shack,

Inc.Mims, F. M. (1988). Engineer's Mini-Notebook: Formulas, Tables and Basic Circuits. Fort Worth, TX:

Radio Shack, Inc.Mims, F. M. (1985). Engineer's Mini-Notebook: Op Amp IC Circuits. Fort Worth, TX: Radio Shack, Inc.Mims, F. M. (1986). Engineer's Mini-Notebook: Opto electronic Circuits. Fort Worth, TX: Radio Shack,

Inc.Mims, F. M. (1990). Engineer's Mini-Notebook: Science Projects. Fort Worth, TX: Radio Shack, Inc.Mims, F. M. (1984). Engineer's Mini-Notebook: 555 Timer IC Circuits. Fort Worth, TX: Radio Shack,

Inc.Radio Shack. (1972). Electronics Data Book. Fort Worth, TX: Radio Shack, Inc.


17/18

Page 17

APPENDIX A

Table A-1. Ohm's law equations for direct current (DC) circuits.

KnownValues

To CalculateI

To CalculateR

To CalculateV

To CalculateW

I & R I * R I2 * R

I & V V / I V * I

I & W W / I2 W / I

R & V V / R V2 / R

R & W ( W / R )0.5 ( W * R )0.5

V & W W / V V2 / W

NOTE: I = current, R = resistance, V = voltage, W = power.

Table A-2. Ohm's law equations for alternating current (AC) circuits.

KnownValues

To CalculateI

To CalculateZ

To CalculateV

To CalculateW

I & Z I * Z I2 * Z * cos

I & V V / I V * I * cos

I & W W / I2 W / ( I * cos )

Z & V V / Z ( V2 *cos ) / Z

Z & W ( W / (Z *cos )) 0.5 ((W * Z) / cos ) 0.5

V & W W / (V * cos ) (V2 cos ) / W

NOTE: I = current, Z = impedance, V = voltage, W = power, (theta) = phase angle.


18/18

Page 18

APPENDIX B

TROUBLE-SHOOTING GUIDE: ELECTRONIC MEASUREMENT

SYMPTOMS POTENTIAL CAUSES REMEDIES

1.00 No signal. 1.01 Signal source (SS) is not turned on. Check power plug and powerswitches; check fuses.

1.02 Measuring device (MD) is not turned on. See 1.01.1.03 SS controls are set out of range of MD. Reread instructions; consult

SS manual; vary positionsof SS controls.

1.04 MD controls are set out of range Reread instructions, consult(too sensitive or insensitive). MD manual; vary positions

of MD controls.1.05 Incorrect connectors are being used. Check connector type and "sex"1.06 Connector polarity is incorrect. Verify that contacts are properly

mated (e.g., ground connected

to ground).1.07 Cables are not attached to proper points Reread instructions; trace routing

(ports or terminals). of signal: verify that outputs"feed" inputs, not other outputs.

1.08 Cable, connector, or adapter is "open" Gently jiggle cable or connector;

or shorted. test continuity with ohmmeter;replace defective units.

1.09 Other. Ask for help.

2.00 Intermittent 2.01 SS is unstable. See 1.03.signal. 2.02 MD is unstable. See 1.04.

2.03 Cable, connectors, or adapter is "open"or shorted. See 1.08.

2.04 Other. Ask for help.

3.0 Wrong signal. 3.01 SS is improperly adjusted. See 1.03.3.02 MD is improperly adjusted. See 1.04.3.03 Cables are not attached to proper See 1.07

points (ports or terminals).3.04 Other. Ask for help.

4.0 Signal present at 4.01 See 1.0. See 1.0.some points, but 4.02 Other. Ask for help.not at others.

5.0 Signal is distorted 5.01 S S output level is set too high. See 1.03.(visual or auditory). 5.02 SS output signal is incorrect. See 1.03.

5.03 MD input sensitivity is too great. See 1.04.5.04 SS and MD are improperly matched in function. See 1.03 and 1.04.5.05 Cables are not connected to proper points See 1.07.

(ports or terminals).5.06 Other. Ask for help.

Electronics - Electricity Basics

Documents

Transcript of Electronics - Electricity Basics