Sprayberry Academy of Radio - ND-10 - Review of Fundamental Principles

7/28/2019 Sprayberry Academy of Radio - ND-10 - Review of Fundamental Principles

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REVIEW OF FUNDAMENTALPRINCIPLESLESSON ~ D - 1 0


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COPY.RIGRTED l t l lBY SPRAYBERRY ACADEMY OJ' RADIO

CHICAGO, ILLINOIS(I ' ll 4-54 2000


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The abo'e photograph shows mot or th e parts uod in th e Sprayberry Tele,ioion receiver. l t Islntuostlntr to note thal ro r th e U


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The one formula to remember isOhm's law. Later in this lessonare some hints on firmly implanting it in your memory. Otherformulas can be looked up when youhave need for them. But Ohm'slaw will be your constant companion-and indeed, an old and valuedfriend. One without which you, asa practical serviceman, could notdo a fast, efficient (and therefore,profitable) job.No one would deny that radioTV is a complicated subject. Nevertheless, as you progress further inyour course you will notice howeach and every part neatly dovetails with the rest.Even items which aren't tooplain at one time will later fall intoplace. They will fit so perfectlywith other facts that you will findyourself thinking, "Why, of course,i t just coullln't work any otherway."

Let us, then, return to the fundamental facts concerning electricityand matter-making sure that youhave these important facts clearlyin your mind.MATTER AND ELECTRICITY

number of electrons as protons.Protons and electrons are bits ofelectricity, positive and negativerespectively. They also have massor weight. Altho their electricalcharges are equal but oppiJdie,their masses are. Wtl difl'erent.The relatively heavy proton forulSthe center or nucleus of the atom.In all atoms except hydrogen, thenucleus also contains neutronsthe uncharged particle. The planetary theory of matter suggeststhat the electrons encircle this 'sun'like tiny planets.You may have heard or read ofother atomic theories than this one.However the planetary atomictheory is su1ficient for our purposes. We can safely leave themore complicated onea to the nuclear scientists.

ELECTRONS:BOUND AND FREEIn some materials the outer electrons are \'ery difficult to removeentirely from the parent atom.'l'hey can be displaced a bit, butstill stick pretty close to home.These substances are our insula-tors.

The atomic theory of matter Others, especially the metals,merely means that all substances have a much looser hold on someare made up of a comparatively of their electrons. Even in thesesmall number of different atoms. conductors there is considerableThese atoms are alike in being variability in their grip on theircomposed of p r o t o ~ s , c!ectrons and electrons.neutrons-and unhke m the num- The majority uf substances areber combined in different atoms. neither perfect immlators nor goodThe atoms, then, are the building conductors. We might call themblocks of Mother Nature. the 'ln-betweens.' From variousThe simplest atom is that of hy- mixtureA of these in-betweenA, ourdrogen. I t consists of one proton radio parts manufacturers make.upand one electron. Heavier atoms resistors running from a fract10nhave greater numbers of protons, of an ohm to many millions ofplus some neutrons. I f the atom ohms.is not charged, it has the same Static electricity is a condition

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where some of the available "free"e l e c t r o n ~ are removed from somer r o m ~ . and temporarily attached toother atom::;. Friction is one wayto do this-and, in fact, is done ona magnificent scale by mo\'ing airmasses in the earth's atmosphere.B, the Jaws of at.traction and repulsion between electric charges, thesetransferred electrons are continually trying to return to their normal state. In consequence we havethe spectacular displays of lightning and thunder as the energyused to mo\e the electrons is dissipated when they r-eturn to theiinormal positions- and, incidentally, ''static" in your radio.

P R O T O ~ S A..'\'D ELECTRONSIt might be well to point out thatthe terms "positive" and "negative" as referred to protons andelectrons have one meaning-andtuff. but oppositP in character. They might just as wellha,e heen called "A" and "B,"alpha and omega, or even male andfemale. In ''iew of the laws of attraction and repubion, the latterterm:; mig-ht make a lot more :wnse.Since the positive and negativenomenclature is no'v standard, youwill have to go along with it. Justkeep in mind what it actually signifies: o p p n s i t c n e . < ; . ~DIRECT CURRENT

There are of course ma11y sourcesof electric current. Devices :mchas the thermocouple you are notlikely to encounter frequently. Batteries and generators - at leasttheir theory of operation-will beof more practical enryday use.Furthermore, you wj]J remem-

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WATER F= WATER WATER F= SOURCE PUMP WHEELt {lOAO) IF"LOW

....... /

...eLE.CTFUCAl. -lNERA,-OR.- 0 . . -LICTP.O" PUMP

l.OAOLOADFlOW-

FIG. Iber that you must have a completecircuit in one way or another. Theconducting paths may be wires,liquid solutions (electrolytes), gases or even vacuum. Most tubes,for example-a subject you willtake up in your next lesson-have avacuum for part of their conducting path.Since moving electrons constitute an electrical current, theyleave the source from the negativeterminal. Flowing thru or aroundthe circuit. they reenter the sourceat the positi\'e te1minal. Of courseuilhin the source, the flov is fromplus to minus-else there would nothe the required complete circuit.We might think of a generator asan electron pump-pumping elect r o w o ~ around a circuit as a waterpump s h o v e ~ drps of water thrua pipe. Note Fig. 1 for the relativecomparison.

MEASUREME:t\TSI f we are going to work withsomething we can't Hee, we mustbe able to measure it in some way.And in order to measure it, wemust have standards of amounts.The coulo;nb is a eertain quantity


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WATER FALLINO TWAOUGH&.AAGii P IP . DCV&LO

~ R U S U A I . AT LOW.IlI N O T O ~ l &TUIIIINa,

I = : ~ 1 ~ t OADILICTRICAL. GINI.IlATOilDI.Y&lON YOI.TAG& TOI'OitC& Till. FL.OW 01' C:UIIAENT TNIIOUGH l l!St5TANCEllleACfANC& OR IUI'eDANCI! 'LOAN .

FIG. 2of electrons-as we refer to a gal-lon of water. The fact that thisnumber of electrons runs over 6 ~million million million is of no moreconcern to us than the fact that agallon of water contains severalthousand or million drops.The volt is the pressure like the"head' of water at a water wheelor turbine. See Fig. 2. This electrical pres..c;ure is also referred to asemf (electromotive force) or potential. But the volt is the unit usedfor measurment.The ampere is quantity per unittime. That is, it tells us how-much,how-fa;;t. \'e would say a pipecarries so many gallons of waterper minute or second. Electricallywe say coulombs per second. NoteFig. 3. The ampere is the specialname for this rate of flow.omn

If we incre,apresaure, - . . - ~ ~ . o ~ ~ - o u = - c=iiliiill6WATER PIPE

aLECTAICAL. CONDUc:TOR100 COIJI.Ota P&R SECOND

FIG.3 4

water-more gallons per minutethan with the lower presslU'e. Sim-ilarly, a greater voltage forces agreater flow of electrons. And as awater pipe has friction, so does anelectrical circuit offer resistance tothe flow of the 'drops' of electricity.Here again we have a specialname and unit to measure the 'elec-trical friction.' This is the ohm.It is the amount of resistancewhich permits one volt to push acurrent of one ampere thru it.Ohm's law is derived from thesefacts: Increasing the voltage oremf increases the intensity of thecurrent in proportion to the in-crease of voltage. Similar resultsoccur when the resistance is de-creased.This relationship is expressed inthe formula, E divided by R equalsI; or E divided by I equalR R, and Eequals I times R.

.itAll thrlee tomis ue ufiuralbr thesame formula. They are -. . .J.yarranged differently to give the 1111;oknown quantity from the two tbatare known. Consequently if yotr;!HI.._can remember any one of the threeforms. it is easy enough to changeit to the one desired. Figure 4 givesa diagram for easily remembering


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all three. Place your finger over thequantity desir e d - the unknown.The remaining two indicate the operation-d ivision or multiplication.TilE WATT-POWER

You are probably quite familiarwith the 1catt as a unit of power.You know that a lamp has to consume a certain number of watts inorder to produce the amount oflight you need. Or you may use a~ m a l l e r or larger soldering ironaccording to the amount of heat-power-necessary for the job athand.In other words the watt consumption is the rate of doing tvo1k.In mechanical devices-even electric motors-power is measured inhorse-power. 7J6 watts is equivalent to one HP.Watts are how-much, how-quick(ampere!\) at what pressure(vo'ts). You \ ~ i l l encounter powertransformers rated in VA (voltamperes). This is another way ofsaying watts since the product ofthe voltage and current (in amps)equals the power in watts. Atransformer, however, is not designed to dissipate power. Ratherits job is to transfer it from onecircuit to another. Hence the VArating is preferred to watts. Figure' i l'hows how a wattmeter would beconnected to measure power consumption of an electrical circuit.Stated as a formula, E times Iequals power in watts; wherein Ei:- in v o l t ! ' ~ , I in amps, and P inwatts.There are two alternate forms.We know from Ohm's law that Edhrided by R equals the current. I fwe substitute ~ for I in the power

f)

IN P'UT

WATTMETERMEASURII-IG1000 WATTSPEP HOUR

FIG.o

OUT PUT

formula, we have E times ~ or Esquared, divided by R equals power.Similarly we can substitute forE. Ohm's law again tells us Eequals I times R. Substituting inthe power formula, IRI or PR=Por I squared times R equals P.

This last form is very important.You will see and hear it referred tomany times as the 12R losses. Itmerely means power loss; that is,the heating effect of the currentflowing thru a resistance.As a consequence when you arereplacing a component such as aresistor, you must pick the new resistor to fit two requirements. ( 1)The ohms value must be the same(within permitted tolerance), and(2) the power rating must be atleast as much as the maximumpower that it will be expected tocarry.You can see that the two ratings have no direct connection witheach other. You can buy a 100 ohmresistor, for example, which willsafely carry 1 watt, ten or a hundred. For economic reasons youwill not use one very much largerthan necessary.

GRAPHSThere is an old saying attributedto the Chinese, "One picture isworth ten thousand wotds." Likemany old sayings, it makes sense.And it is exactly why we usegraphs.


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9876

2I

4A

IvI IIV vr;VeL

l2n.Iv vvv/ /

/ v/ r--~ --

f.n._ ~. /. / v

/ r Jlll!6'- ~. /v ~ . / J/ l--I - ------ ~- -

~ V - ~ -6 0 ( 2 3 -1 S G 7 B 9 10 II 12 /3 /4 IS 16A/1PERES FIG.6A graph is a pictorial represent- ALTERNATING CURRENT

ation of two quantities. For exam- Alternating current (AC) ex-pie, time and charging current of a plains itself in a general way. Theelectrons as units do not move outcondenser. of one terminal of the source, thruBy measuring the current at var- the circuit and back to the otherious intervals we compile a column terminal for every cycle of move-or list of figures. From this we ment. However, the effect of thisplot our graph. Time is most often movement is made evident all alongplotted on the horizontal lines. The the circuit. Energy is passed byother quantity goes in the vertical the movement of the electrons fromdirection. atom to atom. Thus one electronTrue, we could look at the figures will move to a nearby atom and thisand see what was taking place. But action will free another electronit would take close inspection, plus from the same atom to move on tosome calculations, to realize how another atom. Thus as individualsfast the current changed. And also electrons may not actually movehow that change itself varied. In but a very short distance. See Fig.addition, if the graph is construct- 7. I t s the effect that a moving elec-ed with any sort of accuracy, it will tron brings about that is importantshow the values between the acto- ATOMS IN WIR CONDUGTORally measured ones. Therefore a cr 0+ 0+ O.O.O+O+O+O+D-D-0

11. " ' , o i l "raph not only shows us the situa-tion at a glance-it actualJy givesmore infonnation than the list offtnres themselves l See. Fig. 6 for ~ of a simple graphcurrent are used

ELECTI2UNS MOVE FROM 1 0 2 , A N O T I . I E ~ LMOVES FROM 2T03,ANOTI-IER FllOM3.,...ETC.NOTE THE FlllST ELECTRON STAVSAT 2,8UT IN MOVING IT CAUSES AM()\1MENT OF OTWERS FROM t,3,4,f.TC.,UPTOI2

FIG.7


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and not how far an electron actually m o n ' ~ that is of greatest importance.Such action gi\'us AC propertiesthat DC does not have. \\hen ACreverses a second time its effect isback whrc i t started. It has ineffect completed a full circle orcuclr.. For comparison purposes wedidde a cycle into 360 degrees justlike a circle. But more of thatwhen we consider phase relations. N 'OBLONG LENGTHOf MAGNET IC

IRONFIG.8The next thing to note is howoften or how frequent the completetever,al or cycle occms. This is

the fn (}IJr ncy of the AC. It is measured in cycles per second (cps). Weu ~ u a l l r drop the "per second" in STAND-:;peaking or writing. It's '1000kilocvcles' or 'one megacycle.' I t isa l w a ~ s understood that we refer tocycle's per second. If, perchance,we mean otherwise, we so state.

rived from the way they look on agraph.MAGNETISM

wave form' is an expression you Magnets have fascinated manwill run into many times. It refers ever since the discovery of the lodeto ,our old friend, the graph. When stone or natural magnet. Evera j,articular current or voltage is curious, man soon found be couldplotted on a graph, its shape or produce artificial magnets."aye form is shown. I f the picture I t must have appeared akin tothus presented looks like the busi- magic to find a suspended bar orness edge of a saw, what is mol'e needle turn until its poles or endsnatural than to refer to that type pointed roughly north and south.as a " ~ a w - t o o t h " wa\'e form. But it provided a very simple andIn lesson ~ - 2 you studied a cir- logical way of differentiating becuit for switching a DC current onand off to produce a square wave tween the two poles: a North (see}{-form. Such a wave form is used ex- ing) and a South (seeking) pole.tensively in TV circuits. The actual Refer to Fig. 8.:


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' 'pression has arisen and we speakof 'magnetic flux.'In this N to S business you willnote a similarity to the historically'assumed' flow of electricity. Be-fore there was any evidence to thecontrary, electricity was assumedto flow from positive to negative.

MAGNETIC FIELD ABOUT A BAR MAGNET Further experiments eventually.There are two reasons for this. showed i t to be the other wayF!rst, to help us in forming a mind- around in strictly metallic conducptcture of what is going on. Sec- tors. In a gas or liquid there mayond, it simplifies calculations of be electrons flowing in one direcmagnetic fields to assume a certain tion and positively charged ionsmagnetic field bas a definite num- flowing in the other direction.ber of lines. Transformer design- So it may be with the directioners use this method. Lesson ND -19 of a magnetic field. The pointing"P T ' N S . hwer ransformers" will show from to may be rig t--it mayyou bow this system is used for be wrong. At present there is nopractical cases. way to tell. Perhaps some dayThere are two important things some scientist or technician (possito remember in regard to these bly you) will come across positivelines: (1) That such a procedure proof of the truth of this matter.is a representation . That is, you The strength of a magnetic fieldcannot actually get 'between the varies according to the Inverselines.' And, (2) the lines are con- ~ q u a r e Law like the intensity oftinuou.r;. The latter statement im- hght. I f we measure its strengthplies that north and south poles al- at some certain distance, say oneways exist in pairs. It is impos- i n ~ h , at twice as far, 2 inches, itsible to have one pole without the will belA, as strong. Figure 10(a)other. shows the general case of a bar

Experiments show a difference magnet. In the case of Fig. 10 (b)in the direction of magnetic lines. we have theN and S poles close toAn example is the polarity of the each other. Within the gap the Involtage induced in a conductor verse Square Law does not apply.moving across a magnetic field . I f Here the field strength is fairlythe poles are reversed or inter- constant. Only on the edges doeschanged, the induced voltage is re- it taper off. Such an arrangementversed. has many applications - radio

Magnetic lines are assumed to speakers is a good example (see' ~ o w ' from North to South (out- Lesson ND-18, "Speakers").s1de the magnet) . Point N to S ELECTROMAGNETISMW?uld be a better expression. Note t ctFig. 9 and the direction of arrows Fortunately we are not res ~ Itn a magnetic field of constant ed to natural magnets for r n a ~ ; : !s. e n l g t t ~ ' there is no movement or use of magnetic energy. ~ o f l e l dctrcu a Ion . N rth electrons have a magnetic )liseve eless, the ex- about them. Magnetis:rn from t

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As you will not be too closelyconcerned with motors, it is un-necessary to give a great deal ofspace to the many fine points ofmotor theory.THE GENERATOR PRINCIPLE

Like the motor the generatortakes advantage of the forces exerted on a moving electron in a magnetic field. In the former we passa current thru a wire--a series ofwires, in practice. From the in-teracting magnetic fields we getmachanical motion. In the generator we apply mechanical motion,and the interactingfields cause electron movement.Obviously there is a great similarity between motors and generators. In fact it is quite possible tohave one machine act as either.Generally this is not done for reasons of efficiency. A motor is sometimes braked to a .sudden stop byusing it temporarily as a generator. The supply voltage is disconnected and the motor leads shorted-either directly or thru a resistor.Its rotating momentum is quicklyconverted into heat.

EMF PRODUCTIONMoving a conductor thru a magnetic field f o r c e ~ the electrons toflow towards one end. There mustbe a vo!tage difference between thetwo ends. You can readily see thatthe stronger the field and the fasterthe conductor is moved, the greaterthe v o l ~ g e produced. That is, thevoltage J.s proportional to the number of hnes of force cut and thspeed of cutting-100 million line:cut p ~ r second produces one volt

dyne-about 36 millionths of anounce) is said to radiate 4r "lines."Now physically this is a lot of nonsense. We have .Ueady said thatthe lines are continuoa; We canhave no parts or fractionsAs mentioned J ) N \ 1 ~ , . this particular number set-up ts for calculations only. But there it is extremely useful. Design calculations

a l w a y ~ refer to these lines, imaginary or no.In this connection it might bewell to emphasize that the expres;;ion "Jines cut" means cnt across-not snipped in two.We have occasion in radio to refer very frequently to voltages andcunents of sine wave form. Theyare produced in a number of ways.You will be closely concerned withthe electronic method- t he vacuumtube oscillator. Your course takesup this subject short ly- LessonND-14.To insure that you understandwhat is meant by the term let usreview the sine wave as p;oducedmechanically.When a conductor is revolved ina m a g _ n e ~ i c . field, the voltage induced In tt ts at a maximum whenthe conductor cuts straight across

~ h e flux-interRects it at 90". Themduced emf is at a minimum-zeroI when it is traveling exactly paral-el 0 the flux. Of course the angleof mteresection here is also zero.At any point between these ex-

r e m e s , the voltage at that instantIS proportional to the sine of thtangle the conductor is making withthe flux. Remember that the conductor is in constant motion-otherwise there would be no emfproduced at all. Figure 12 i l l ~ s t r a t e s these condlbons. The small arrows indicate10

Inctdentally a unit magnetic 1(one of such a stl-ength th . po epels an identical I at It remeter away with po: one centi-a orce of one


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I 4' III IIT II II I' II I 6 1 180 210I I I 0 J O " ~ I I S \I I I I ' II I I II I \ II I I I I I ' II I \ II I I \ II I I I I ,I I I \ ,I ' ~ 'I II I IM4GME'r1C IFI*L1 II I I I I I I I

FIG. 12the cllrecticm of the conductor atvarious points. At point 1 it is~ s p i n at 6. At point4 i t is 90 At others i t can be seenthat the angle from the horizontalliDe is the same M&'le the conduc-tor mouent makes with the tlux.In a r ip t angle trJaD8Ie the sinevalue of a11 aDele i l lie ratio of thestde oppasite the angle to the by-putatase (the side opposite tbe 90angle).

I I bSINE. OFANGLE AT A ~

with relation to the other. To pro-duce a voltage in a conductor, allthat is required is a ( r e l a t i v e l y ~changing field. So, if a current isvarying, its magnetic field will bevarying. Any conductor in thatfield will have a voltage induced init. This is the principle of induc-tion (induced emf) in general. Itis the principle on which all transformers function.

ELECTROSTATIC LINESOF FORCEhe rpm of the revolving conduc-tor must be absolutely constant ina uniform magnetic fteld to produce We assume a magnet bas a fielda true or pure siDe wave. of energy around it. Since electricActually it makes no difterence cbargea attract or repel each other,whether the conductor moves there must be some sort of energyacross the magnetic field, orwheth- fteld about electrons (and protons) .er the fteld moves aci'OIIB the con- As this field is different from thatdQCtor. All that is required is reltl- around a magnet we give it a dift i t ~ e motion-that one or both move ferent name: eiectrostatic field.11


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Here again we represent the energyfield by lines.Like the magnetic field an electric field gets weaker the fartherfrom its source. In fact, it followsthe same rule: The Inverse SquareLaw. At twice a given distance itsstrength drops to 1/ 22, or 1A. ; at 3times the distance, its strength is1 82 or 1 9.If an electric charge-such as anelectron - is comparatively farfrom any other charges, the linesrepresenting the electrostatic fieldradiate outward in straight lines.Like the spokes of a wheel-but inall directions. Figure 13 (a) showsan electron with its electrostaticlines of force. At some distanceaway is another similar charge.Since the strength of their fieldsdrops off rapidly with the distance,there is little interaction betweenthe two fields. Figure 13 (b) showsthe two charges much closer. Here

the fields have measurable actionon each other. We might considerthat the lines of force act as leaf:Springs. The closer the two approach, the greater the force pushing them apart.I ' : ,,I /" I ,' I ,- - - ~ ____, ' - - ~ ~ ~ , ~ - - -)';


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Rf R2 R3 R4 R5Et E3 E4 + - E5

lE2 + - E6

-E- fiG . 14plete cJrcuft, the electrons can eon- materials are then rated as havingtinue to ftow. more or less resistance than copper.So we see that there are similari- In most substances increasingties and dUfereuce.s between elec- the temperature increases the retrie and magnetic fields. Each is Ristance. There are exceptions torepresented by lines of force. this, but they are not ordinarilyNeither when at rest--static un- very important in radio-TV servicvarying in space or time-has any ing. Usually when the change is inetfect on the other. Either varying the opposite direction, it is small.in space or time produces the other. Sometimes a definite amount ofIn the production of one by the resistance is wanted. You will enother, the two are always at right counter this in using vacuum tubesangles to each other. for amplifiers. Here a resistorBESISTANCE often is the 'load' in the plate cir-cuit. A material of relatively high

specific resistance is used. Most ofthe materials are also designed tohave a low temperature coefficitmt.This last is the percentage ofchange in resistance as the temperature changes. The temperatureeoefficieut may be either positivsor ugczti11e-tbat is, the resistancemq ~ ., or clottm respectivelywith WwiGBB in temperature. InI ' ntr!JJia f1l J.,..tb aad d j r - - certain frequmcy sensitive circuita,eter are eat ap to compare the ;; one part with a poeitive iempalasistance of uteat mat.arJaJt. JD tun coemateat J8 atebed with oneyour work the ataDdarcl Ia the DdJ.. ha'V'hqr a neptlve ooeffhdent to blfoot-a leqth ot wire one miJ G'8lll8 tM ~ ot the elllaft.

(1 1000 in.) in dfa=ettJ aDd oae ~ a t .. , . , . . . a Mfl-foot long. Theactul r.ilhnce of ~ at ~ Js nota mil-foot (in ohms) for a apeefftft M Uttleu conven-substance is its P B t J i ~ r-- . . JeilP1 re: N'evertbela, BOmfJAs copper is the most wide) uiied l'ililsta:uce wiD remfn. It may bematerMJ for conductors, it is some- so small that it can be Ignored. Or,times used as a standard. Other altho small, i t may have to be taken

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into account. Here again we mightrefer to the designing of powertransformers. The actual voltageinduced in a given secondary winding must be sufficient to supply itsload-say the filament or heater ofa vacuum tube - as well as thevoltage loss (voltage drop) in thewinding and connecting wiresthemselves.

her: We learn to walk before welearn to run.We know resistors connected inseries must be car rying the samecurrent. As noted, each has itsown voltage dr op ( IR). There isthat much less voltage available toforce a current thru the rest. Sothe total resistance is the sum ofthe individual resistances.

Rr-R1+ R2+ Ra, etc.If a single resistor is connectedacross a battery, it will have onlyone polarity: the same as the battery. If two resistors are in seriesacross a voltage, the junction pointof the two is positive with respectto the other end of one resistor, andnegative with respect to the otherend of the other resistor. This po-larity of its own, must be connectedto match the polarity it is across.Voltages connected to tubes musthave the right polarity as well asthe right amount. You will getmore details on the latter soonLesson ND-13.Electrons flow away from a morenegative point towards a more pos-itive point. Think of the old sav-ing, "Water seeks its own level."Whether any pmticula r point isp ~ s i t i v e or negative depends on1111th what point it is compared.

Paralleled resistors have thesame voltage across each. Eachwill carry its own separate currentas figured from Ohm's Jaw. Thetotal _current will be greater thanany Individual current. The ef-fect as far as the voltage is concerned is as if it were conneehdto one resistor of lowei' value thllilthe lowest in the combination:R.r= ___ _1 1 1-+-+ - tc

The principle of voltage drops isone you will use very often. Its application is really simple. All youhave to remember is that the voltage source (voltage rise) mustequal the voltage loss (voltagedrop). All of which is logicalenough. I f you apply a voltagewhether from a battery, generatoror whatnot-to a resistor or aseries of resistors, that voltage canbe measured across the resistanceas a whole or as the sum of the individual voltages across the individual resistors. You can prove thisby assuming a value of voltage forE and resistor values for Rl toR6 in Fig. 14. In all cases the voltage drops across the six resistor,;will equal E regardless of the ac-tual values in ohms of the resistors.A point to keep in mind is that ifthe voltage itself has resistance (asin practical cases it always has), a!Jart of the voltage will be dissipated in the source. In many cases thiswill not matter much. For maximum power transfer from one circuit to another it does.At this pointin your course it is of little concern.We mt!ntion it here only to makeyour course as complete and correct as we can. As we have told.rou b;fore, your Sprayberry courseIs designed to take you step by stepthe knowledge of radio-TV. I fIt ~ e m s to you now that things aregomg rather slowly, just remem- . Rt R ~ Ra. e tThJ.S formula 'boils down' to a shor14


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.c;ut in the case of only two resistors.rbeir product is divided by theirsum:

You can take a greater numberthan two with this last formula bysolving for the first two, using thatvalue with the third, and so on . Usually this will involve more figuringthan by using the first formula.Series - Parallel combinationshave to be worked out a section ata time using the appropriate formula for each section.

CONDUCTANCE

hot, it would be wise to use a largersize than otherwise. If you knoweither the voltage or the current, itis easy to find what power the re-E ~sistor has to carry. -R or 12R

gives the answer.The actual value of a resistor isnever-well, hardly ever-what isprinted on it or what the color codetells you it is. Most circuits arenot at all critical as to exact values.In the usual case 207o, plus or minus, is quite close enough. If a setmanufacturer specifies a closer tolerance, he probably has reason forit. Otherwise you might as welluse the less expensive type.As indicated by its name, conductance (G) is the opposite of re- In regard to the color code, mostsistance - mathematically speak- resistor manufacturers put out aing, its reciprocal: pocket size color card that's handy1 1 to refer to. For occasions whenG==R and R= G you don't have one here's a tip that

Its unit is the mho. may help you to match color toThe "G" value of a conductor number. Black, appropriately, isshows how well or how nwch it put first for 0. Brown follows as 1.conducts. Conductances in paral- Next come the real colors and theylel are added to get the total con- fol!ow the order of the spectrumductance. This is on the same prin- the rainbow. If you can recall howciple as adding separate currents to the rainbow blends one color intoget the total current. the next, you can count right down :~ - G + G 2 + G etc. Red, 2 ; Orange, 3; Yellow, 4;We now aee the reason for the Green, 5; Blue, 6; Violet, 7. We'refonanlQt reaieto.ra iu,paraUel. All still two numbers short, so we tackwe are,cJolg la e a c h r ~ on Gray, 8 and White, 9.sktance tat.D eondudaJice - Otitrls Ohm's law is a very useful relato mhos. Then adding the separate tion. However there are times whenG" G2, Ga. etc., gives the total c6n- you must be careful about its appliductance. The final step recon- cation. I f you are dealing with a\'erts GT to its equivalent R.r. 'pure' resistance_ no reactance-Watt or power rating has been Ementioned before in this review. y -=1 appUes in AC as well as in

You probably have it well in mind. DC circuits. If the chocuit has someJust be sure a replacement is large reactance, either inductive capacienough. I f it is located in a spot tive or both, the formula ~ t u s t bewhere there is little ventilation, or modified before it is valid. Thisnext to some other part that runs modification is not excessively15


18/32

complicated. It is only necessaryto combine the resiltanee aad reactpee In one value, the impedance(Z). Tbia foam ot the equation,Bz =I, appllee to AC.)o 1: ;! :f

FIG.I5We eaJenlete reaetance, induethe or eapaeltive, in terms of itseqahalaat naiataace. This is ther ' 1 ce that would give the samecurrent for a given voltage.Similar methods are reneraUy will have an emf induced in it. Theaaed in giving the value of aa AC changing field may be producedvoltqe (or current) . Since the from a DC source by interruptingAC Ia eontinuaUy changing, what the current. Figure 15 shows an ex.Ia ita effective value? The equlva- ample. Such as the ignition coil inlent amount is figured in terms of a car, or the vibrator power supplythe power it produces. U a given used in car radios. Or the changingAC voltage produces the same heat- field may be produced from AC.ing effect-11R loss as a certain Since the primary coil is in aDC voltage, it has the same effec- changing field as well as the sec-tive value. This value of an AC ondary, if any, it too will have anvoltqe is termed the RMS value. induced voltage.'l'bla Ia a rather teelmical mathe- The actual amount of voltagematieal term meaning root-mean- produced in any conductor depends

aquare-the aquare root of the av- on the amount of flux linkage witheraae of the squares of the inatan- the conductor. For this reason ataneoua values of that voltage. The core of magnetic material is usedproceseee of arriving at it are of no in power and audio transformers,concern to us. The peak value of an and to a lesser extent in IF and RFAC voltage is 1.414 (actually v2) transformers.times ita rma value. And the rms The strength of the field or mag-

( 1 ) netic ftux is determined by the totalvalue is 0.707 v2 times its peak current flowing in the same direc-value. tion. That is, if a certain numberYou will be introduced to the rea- of amperes is flowing thru a coil,sons for making these chaDgeS- the amperes times the number ofnns valuea to peak values, and vice turns determine the magnetoMO-versa-fn Leason ND-13. This lea- ti'Ve force (MMF). This correson deals with radio tube amplifi- sponds to EMF in electrical cfrcation. AB a tube uses both AC and cults. The field thus establishedDC voltqes, there are times when will encounter more or less resist-we must change from one value to ance from the material in the f i e l ~ .the other. In magnetic circuits this quabty I:!JND known as reluctame. The opposite,y UCED VOLTAGES or reciprocal, of reluctance is per-tor : will recall that any conduc- meabi.lit11 So in electrical circuits c h a ~ n g magnetic field we have resistance or conductance.

16


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In J2lagnetic circuits we have reluctance or permeability. The !l1aestablished corresponds to the current in electrical circuits. It isadvil&ble to remember that in theease of magnetic circuits there isno actual flow as in the c a . < ~ e ofelectrons in electrical circuits. Nevertheless it remains a good comparison. In practical applications itseldom makes any difference whichway we think of it.

INDUCTANCE IN ACCffiCUITS

maximum, the currentmum. When the voltage Is zero,the current is zero. When the voltage reverses its direction, the current does likewise. Such a currentand voltage are in phase. A graphof their values thruout a completecycle would show that their directions, maximums and minimumsoccur at the same time. I f he graphof any currents or voltages showthat their maximum values, etc., donot occur at the same time, thenthey are not in phase. Figure 16shows examples of voltage acrossIt makes no difference whether R, c and L units which are out ofDC or AC is applied to an inductor phase with one another. The-Lenz's law still holds true. I f a amount out of phase is figured oncurrent of sine wave form is flow- the basis of the complete cycle--oring thru a coil-an inductor-the circle-of 360 degrees.induced voltage, caused by the fluxchange, must be largest when the INDUCTIVE REACTANCEcurrent is changing the fastest. Since an inductor reacts against

Also it will be the least-zero-- an AC current, we measure its rewben the current is passing thru actance in terms of how large a reits maximum value in one direction sistor would restrict the current toor the other. This is because at the the same value. In other words wecurrent's largest value it is momen- measure its reactance in ohms. Ref4rilr neither increasing nor de- membering, of course, that altho increeeing. The tlux change, then, such a case we would have the sameia zero; the induced voltage must value of current, the phase rela.W, be zero. Consequently the in- tions would not be the same.dated voltage lags behind the cur- The formula for XL (inductiverent by a quarter cycle, 90 degrees. reactance) shows that the reacAnd since the induced voltage must tance increases with frequency:oppose the voltage actually produc- XL=2rfL. This is reasonableW tbe current, the two voltages enough when you recall that theaust beopposite or 180 degrees out f d d Itof phase. The total result is that magnitude (size) o an m uce votheearrent lags the applied emf by age depends on the amount of tluxto and the induced emf lags the and the speed of cutting. In an ACeaueut by another go. current the speed of cutting de

'l'hia is a good place to review pends on the frequency. The higherideas behind cycle and phase the frequency, the higher the speedhips. I f we apply an AC of cutting. The higher the induced,to a pure r e s i s t a n ~ the or back emf, the greater the reaechange just as the volt-When the voltage is tance.17


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~ ...., .... ,l + ,0 I~ ~ ' I \' I \.... "') - ' I \l .. '.._. .. ...., .

+ I0 k------,f------:.'-------,1--c\ \- ' , I ~~ , R ' ~ . D

- T IME - + fl0.16INDUCTIVE REACTANCE adding the two shorter sides of aAND RESISTANCE right triangle to find the fongesr,If a reactor and a resistor areconnected in series with an AC volt.age, the resulting current still lagsthe applied voltage. But not by90 . The chart given in LessonND-7 shows how much. This chartgives the phase angle for the ratio

side (the hypotenuse). You willrecall that it is the square root ofthe sum of the squares of the sides.Stated in words it can be a littleconfusing. In a formula it is notonly much shorter but also easierto see the operations, or calculations.of . ( XL )eactance to res1stance R ~ E T = v ' E R 2 + E L ~I f you measure or calculate the andvoltages across the inductor and Z= v',.,R..,.....,..+ ... .-,.:!.the resistor, you will find that they Z is impedance, R the effective readd up to more than the applied sistance and XL the inductive reacvoltage. Therefore you cannot add tance--all in ohms.the two voltages by straight arith- RESISTANCE OF A COILmetic (algebraic addition is the No matter what kind of materialp!'OJ)8r name). It is necessary to we use for winding a coil, it will

ae *vector addition.' This may have some resistance. o n s e q u e n ~ yaound very complicated, but it is it is impossible to have a 'pure' m-merely the correct mathematical ductance in a coil. Neverthelessfor values at right we can calculate its perfon;nance~ .. m . You are prob- by considering it as a pure mductl.:."U t'be method of tance in series with some resistance

lB


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ELECTRIC LICI-IT

the frequency is increased a neweffect makes itself felt. This isknown as 'skin effect.' I t is causedby ~ e l f - i n d u c t i o n . The center ofa Wire has a greater flux linkagethan the outside-hence a greaterback emf. As the frequency in

~ r e ~ s e s , less and less current flowstnside the wire. For example at100 megacycles-in the prP.RentFM band-the current flows onlyin . he outer 0.00035 of an inch.Skm effect, then, results in reducing the area available to the current. The wire has a greater aPparent "DC" resistance.Other causes of coil losses-decreased efficiency-are radiationdielectric and hysteresis losses. '-the resistance of the coil. In In a well designed coil these lastmany applications the actual resist- will not cause much trouble. Inance will be so small in compari- general, the 'Q'-Qualityofacofi-son with the inductive reactance depends on the ratio of inductivethat it can be ignored. reactance to resistance (including

Fl G. 17

MUTUAL INDUCTANCE skin effect).I f two coils are near enough to ~ L L = Q .have their flux affect each otherthe interaction is termed mutuai Note that the "Q" of a con is not

i n d ~ : t a n c e The total inductance to be confused with coulomb afor two such coils connected in Quantity of electricity.series may be either more or less SHIELDINGthan the total inductance of the There is no complete, absolutetwo separately. "More or less" be- shielding of magnetic fields. Th'ecause the connections can be so best we can hope to do is reducethat the coils either aid or oppose them to a point where any interao-eacb other. tion is too small to affect n,..,

COn. EF.lt'ICIENCY circuits.No man-made device is 100 ro For RF and IF coils it is neeJlel'fect. We always have some loss. sary to use a non-magnetic, lowInductors are no exception. Inmost resistance material such_ fJ coppereases) the largest loss is the liB or aluminum. incldenta ly, theOSs-h low ~ r ~ e n t in-th eat loss of current flowmg cluclil krrf joinia in the sbleld.

ru resistance. Figure 17 shows a J'Or AF and power frequencyCO!ntnon example. sbielding Is easter. In theFor. comparatively low AC be- Coil lace their use of mapetlcquenc1es a coil's resistance f i about first Pbelp keep the ftux whei'e itequal to its DC resistance. But as cores 819


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is wanted. Secondly, a d d i t i o n ~ !~ h i e l d i n g may be done by magnetic -l Cl T1 CJJt illfmaterials. - .1' 2 I C3-CONDENSERS -: - ltwo J 5MFoA condenser is a device that has T._____ .....____...-J_the property of capacitance. Allthat is required is two insulated f I0.18conductors. I t may be the elabor- an electric field. Any dielectric beate construction of many inter-con- tween the plates will make it easiernected plates or foils-or it may be to establish the electric tield-thattwo simple wires. is, easier to charge with a particu-

The term "insulated conductors" lar quantity of electricity.may be slightly misleading. A coil, DC APPLIED TO CONDENSERSfor example, has capacity between If a battery is connected to a conturns-this in spite of the fact that denaer, current will flow into i t unthere is a connection from one turnto the next. yet there is a voltage til the voltage equals that of thedifference between the two turns battery. I f at a given voltage onecondenser stores a larger quantityand this gives rise to capacitive ef- of electricity than another, the firstfects. In fact there is a capaci- has a greater capacity. Note Fig.lance between turna, between 18.groups or sections of turns, etc. I t The unit of capacity is the farad.all adds up to a distribtttetl capac- It ia cteftned by the formula,ity which acts like a condenser in QIE-C. Q is Jn coulombs, E inparallel with the coil. Sometimes volta, C in farads. This is a largedesigners take advantqe of this unit for radio work, so we usesituatioD. They uae the distributed microfarads and mlcromicrofarads.capacity to tune the coiL You are more likely to encounterFundamentally capacitance is this formula ae Q OE. This is beabiUty to store or bold electrons. cause you usually know the voltageWe could compare this quality to a appUed and the else of the condenwater tank's ability to hold water. aer. A tblrd form IaQIC E.In view of the incompressibility of As mentioned before, u y dielecwater a dollar comparison would trie will iJX:I zue the ltDriDc capacuse compre?'ed air to ftll the tank. ity of the condenser. The amountThe greater the pressure, the more of increase over air is the dielectricair in the tank. Similarly the constant of the material.greater the voltage on a condenser, An interesting point in the chargthe more electrons it holds. . ing of condenRers is how long it

S t r l ' c t l ~ speaking, in the ordi- takes. I f a circuit could have no" 1 t resistance. a condenser could benary condenser we pump e.ec rons charged (or dbchargcd) in ! i t e ~ -onto one plate and nff nf the other allY no time at all. smce there willplate. Thus one plate has a surplus be. some resistance pregent. theof electrons while the other has a process takes a definite amount ofdeficiency of electrons. One plateis then charged n e g a ~ i v e l y , , the t i r ; , ~ time constant (in seconds)other positively. This ~ v e s nse to e20


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DIAPNRAGM

PISTON

FIG.l9le the product of the resistance (inohms) times tbe capacity (in farads)-RO=t. This is the time required for an uncharged condenserto reach 63% of any applied volt-age. Or i f discharging, to lose 63%of emf. lfmHar action.Jiatier tan thru an in-Here 1M time oonstantequals the inductance divided bythe resistance: L!R=t. As usualthe quantities are in the funda-mental units of henries, ohms andseconds.Returning to condensers, it is im-portant to keep in mind that thepurpose of a c o n d e n ~ e r is to storeelectrons-regardless of for howshort a time. That is, the currentflows in and out of a condenser, butnot thru.

Figure 19 gives us a mechanicalcomparison. The air tank has athin diaphragm dividing it in twosections. When the piston is movedup, the air pressure is greater in~ h e top of the tank. I f the pistonIs pushed down, the pressure will~ e c r e a s e in the top half-increase

~ ~ t h e . bottom half. In each instancea ~ r Will move thru the connecting?"Pes until the pressure is equal-Ized.

~ condenser has a comparativeacbon. When an emf is connected

to it, electrons will flow thru theconnecting wires-onto or off of agiven plate. When the voltage dif-ference of the two equals the applied voltage, current flow ceases.(This assumes a DC voltage is be-ing used).I f he applied emf is high enoughto puncture the dielectric, a breakdown results--current flows thru.In Figure 19 if the piston exerts toomuch pressure, the air tank diaphragm will break. In the case ofa condenser such a breakdown maybe temporary or permanent, ac-cording to the type of condenserand the nature of the dischargecausing the breakdown.It is essential, then, to make cer-tain that the voltage rating of acondenser is at lea,st as great as themaximum voltage to be impressedon it.CONNECTING CONDENSERSIf two or more condensers areconnected in parallel, any voltageacross one must also be across therest. The voltage charges eachcondenser in proportion to its size.St.udy Figure 20 for this effect.The total charge is the sum of thecharges of all the condensers. Theresultant or total capacity is thesum of the individual capacities:

CT=C1 C ~ + C a , etc.I! condensers are connected insenes, the same current must flowinto each in a given amount of time. ~ E : I ~ I :...+ 'I Ct

PAaAL.Lal. CONDIJISI!RI NAVIi 5AMiiVOl.TAG& Ac.ROSS THIM If THiiiR C N "CITY VALUES Alii.I! DIPFI!Ili!NT TWI!IRCHARGES WILL Ill! OIP'I'IRENT.1

FIG .2021


24/32

--+ ao - -aNSilRSH IUI!S CONN,.C:T . . - CNARGINGMAYa $AMI. AMOUNT OFcuaaeNT f'I.R UNIT OF TIME .

FIG. 21The Q (Quantity of electricity)must be the same in each eon?enser. Using the third form as g1venbefore, -% =E, we can see that ifone condenser is one microfaradand another in the series is two microfarads, the first will have twicethe voltage the second has. (If thisis not clear at the moment, take avalue for Q. Take two differentvalues of capacities. Use the equation to find the voltage across each) .The voltage across condensers inseries varies inversely with theircapacity. The smaller the condenser, the higher the voltage. So becautious if it is necessary to substitute condensers connected inseries. Refer to Fig. 21. If you aresure your condensers are of highquality-have low leakage-it issafe to apply the above formulas.Otherwise it is best to cheek theactual voltages with a good voltmeter. Even then keep a good margin as leakage values are likely tochange as the condensers are in use.As a general rule it is more costlyin money as well as in space to useseries connections except for special reasons.

The formula for the total capacitance ofcondensers in series is similat to tDe one for resistances inl'alilW:

c.l' - I1 + 1 1c -+1 c2 c3, etc.RATE OF CHANGEThe graphs show LeND8 h nm sson- !\ ow how the \"oltage andcu r rent change as a condenser ischarged or discha1ged. In each instance the change is rapid at firstgradually tape1ing oft until t h ~change is practically zero. In otherwords the slope or . ~ t e e p n e s s ofthe graph is a measure of therate of change (either of current orof voltage). This is plain enough

from the graphs.We notice further that when thevoltage across the condenser ischanging the fastest (maximumrate of change), the current in thecondenser is maximum. When thevoltage is not changing (zero rateof change), the current is zero.After a condenser is charged to aparticular voltage, the current willbe zero as long as that voltage doesnot change. I f a voltage of different value is connected across thecondenser current must flow intoor out of the condenser-accordin.gto whether the applied voltage 18larger or smaller than the voltageoriginally on the condensers. Whenthe two emfs are equal-no changeon the condenser-current flowstops. It-All this occurs because the v ~ h eage of a condenser is caused by. f the twoelectron difference o isplates. And that difference nt.caused by electron flow---:curreweFrom these consideratlons truearrive at an important fact-;,.enefor AC as well as DC : The cu t;bein a condenser is dependent 0!ss itrate of change of voltage acrtseJf.- not the actual voltage 1

~


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When the rate of change is high, a other than the start of a voltagelarge current is flowing into or out pulse (positive or negative), theof the condenser. 'Vhen the rate of cttrrcnt values will 'wobble around'change is lower, a 1>maller current for a few c y c l e ~ . Then they settleRows. If the rate of change is zero, down to the steady state values.none flows. The former are called transients asAC APPLIED TO CONDENSERS they do not last very long. Theyare important in circuits dealingAs you probably have noticed, with pulses-including some TVcapacitive effects are the opposite circuits. Their application is chieflyof inductive effects. This is because in design and engineering. We needa coil functions on current-stor- not consider them too closely here.ing energy in a magnetic field; a I t may be of inte1cst to note thatcondenser functions on voltage- the underlying cause of interferstoring energy in an electric field. encc from motors, electric signs

If an AC voltage is connected to and such like, comes from trana eapacitor, the applied emf is sients. Altho the device is operatchanging. To detennine the cur- ing on power frequencies, the tranrent in the condenser in relation to sients produce radio frequencies.~ ~ _ 9 } ) H e d voltage, we must exam- To the receiver this is noise-either. . a ate ofchange. Assuming a audible or visible.stne wave, a graph of the applied CAPACITIVE REACTANCE'oltage shows a rapid increase at The interaction of a condenser'sthe beginning of the cycle, tapering current and voltage cause it to reoff until it reaches a maximum val-ue at goo or %. cycle later. At this act against the applied voltage. Thepoint the voltage change is momen- capacitive l'eactancP- is measured intarily zero. From previous consld- ohms. It is the same amount as aerations we know that the current resistance which would give themust also be (momentarily) zero. same current value (neglectingTherefore the condenser voltage phase relationships). Xc (capacimust be equal and opposite to the tive reactance) decreases with in-applied voltage. That is, current creasing frequency. This again ishas been flowing into the condenser the opposite of XL which increasesin the same sense (positive or neg. with increasing frequency. Asative) as the applied voltage. If might be expected, the formula iscurrent has been flowing in and fin- practically the reciprocal of theallv 'trickled off' to nothing as the formula for inductive reactance:voitage reached ma.ximum, it must X 1lead the applied emf by * ycle- c=271'fC go. Here again we see the oppo- CAPACITIVE REACTANCEsite character of capacitors and in- AND RESISTANCEductors. Yr. lags E; l o leads E. I f I f a condenser and resistor arethere is no resistance in the circuit, connected in series across a volt

the phase difference is 90. age, the current thru the combina-The foregoing applies to 'steady tion still leads the voltage. But itstate conditions. If the emf is con- will not be the full 14, cycle. Thenected to the circuit at any time greater the capacitive reactance in23


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comparison to the resistance, thecloser the phase angle is to 90" The greater the resistance, thecloser to o. The value of any proportion can be estimated from thechart in Lesson ND-7-the samechart used for a coil and resistance.The only difference is that in a capacitive circuit the current leadsthe emf in an inductive circuit thecurrent 'tags the emf. The principleof the chart is that the tangent ofthe phase angle is the ratio of thereactance (inductive or capacitive)to the resistance. (A table of trigonometric functions also would givethe value of the phase angle).Adding the voltages across thereactance and the resistance to arrive at the total voltage is done byvectors, or right angle addition.The same system is used to add thereactance and the resistance fortotal impedance.

E T = , = E . - n - + - EandZ=vR2+Xc!!

must also be different.a b ~ e in tvvo forms:

RXcZ - - : : - ~ ~VR'+Xc2or,

I t is a'VIail-

RZ== - ~ = = ; ; ;' ' 1+ 4.,-2f 2C:R1Remember what we have saidbefore regarding formulas in general. I t is neither necessary noradvisable to memorize all theseformulas. Understand their usehow to supply them. When yn1have need of a particular one, lookit up.CONDENSERS WITHPOLARITY

Electrolytic condensers are sonamed because of the liquid (electrolyte) used in their construction.The so-called 'dry' electrolytic isdry in the same sense as a dry cellor dry battery. The electrolyte isin the form of a paste instead ofan actual liquid.Because the dielectric is formedafter the construction of the con-

I f a condenser and resistor are denser-by application of a DCconnected in parallel, the total cur- voltage-the same polarity must berent will again be between 0 and applied in use. Reversal o ~ the90 degrees ahead of the voltage. polarity tends to destroy the dlelecAs the phase angle of the conden- tric, ruining the condenser. Elecser current is 90 and the angle of trolytic condensers must thereforethe resistor current is o, the two be used on DC. In this case "DC"must be added vectorially to get the really means unidirectional. Ittotal current : change in value-have a pulsatmgh =vlu2+ Ic2 character, or be mixed with an AC. component-as long as the a ~ t u a lThe phase angle can be obtained polarity does not reverse. E1therfrom the previous chart using the the container for the condenserratio of either _ie , or R . The will be marked indicating t h ~ cor-

R Xo rect polarity, or the leads w1U beparallel connections cause the Iat- color coded-quite often both.~ r ratio to be the inverse of what The big advantage of e l e c t r o ~ y -tt would be for a series circuit. tics is the large amount of capac1tyThe formula for total impedance possible in a given size condenser.

24


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________________________________________'fheir use is extensive in AF' and\fer circuits-subjects to be tnk' : up in your next few lessons. In

~ u c h circuit-. the comparathelyhiP leakage current is of littleconsequence.At RF frequencies these condens e r . ~ usunll.r are not efficientenough. However, less capacity isrequired. So we use condensershaving other dielectrics-paper,mica. ceramic, etc.Some non-electrolytic capacitorsha\e a notation on the case showing which lead is connected to theoutside foil. The usual practice isto connect this lead to "ground"tbe chassis-or to the side of thecircuit closer (electrically) toground. The outside foil then actsas an electrostatic shield, preventing or reducing capacitive couplingbetween circuits.

CONDENSERS INELECTRONICS

~ D - 2 0 . Coupling condensers areused to couple energy from one circuit to another. Blocking condenst>rs are primarily for preventingthe passage of DC while permittingthe AC part to pass th1u. By-passcondensers are provided to give anAC current a low resistance patharound a grid bias resistor for example. Most such connections provide more than OJHl of these actions.The last three types-coupling,blocking, and by-palling-will betaken up in the next few lessonsdealing with vacuum tubes foramplification.So you cau sle'e that very interesting parts of your course arecoming up-actual applications asused in all radio-TV s c t l ~ . Thisis why we are using this lesson fora general review. In order for youto easily understand the importantthings to follow, you must be thoroly grounded in theRe fundamentals that we use to explain the op-

The uses of condensers might be eration of other circuits.classified as tuning, neutralizing,filtering, coupling, blocking and by- With the exception of tuningpassing. condensers, condensers are for pro-The first you will recall from the viding a low reactance path for AC.The AC may be in a specific range,last lesson on resonance. Neutral- such as AF or RF frequencies. Inizing condensers are used to cancel either event it offers a high reacthe effects of an inherent, or built- tance to DC. If connected in a cirin, capacitance. With modern tubes cuit containing pulsating DC-DCand circuits this type of connectionis not nearly as common al:i it used modulated with AC--it acts to sep-to be. At this time it is u ~ e d more arate the two.in tran:.:mitters than in recehers. Condenser losses can be summedThe remaining four types are up as either series losses or para\"ery :;imilar in many ways. The llel l o s . c ~ e s . So called because theynames come from the primary pur- act like the losses that would resultpose for using the condenser. Fil- from a perfect condenser with ater condensers, for example, are resistance in saies or in paraUet.used to smooth out the variations Parallel losses result from dielecof a rectifier. Your next lesson in- tric leakage. Series losses can betroduces you to these circuits with caused by the DC resistance of theadditional information coming in connecting wires and conducting

25


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path within the condenser. Themost important loss is dielectricloss. This is a series type losscaused by Lhe heating effect of thechanging electl'ic field in the dielectric. It corresponds to hysteresisloss in iron-core coils.While it is true that electrolyticshave a high leakage, they are usedin circuits that make this loss relati\ely unimportant.RESOXANCEThe reactance caused by a coiland that caused by a condenser areexactly opposite in effect. The coil

retards the current, holding it behind the voltage by 90". The condenser retards the voltage holdingit back of the current by 9o. (Ofcomse we are assuming no resistance in either case).It might be well at this point tomention that these current andvoltage shifts are phyaically rea l -not merely mathematical. we mustuse mathematical terms to describetheir actions and to calculate values.Suppose we connect an inductorand a capacitor in series anu applya voltage (AC) across the combination. Since they are in series, thesame current must flow thru eachpart U ~ i n g theil common currentas our reference, we see that thecoil p u ~ h e s if,q voltage ahead of thecurrent. The condenser pulls itsvoltage behind the current. 90 degrees in opposite directions makethe voltages across the coil andcondenser 180 degrees out of phase.When one is in a positive sense theother is in a negative sense. Consequently the two effects balanceeach other more or less.

one exactly cancels the voltath th . Th' . ge ofe o er. ts ts resonan Thfrequency at which the c o t ~ l e . te. . t reac s1n one way JUS as much as thd Se . e t . e conen I J a,c s m the opposite way.. Hence for resonance the inductlvf: ~ e a c t a n c e must equal the capacitive reactance: XL=Xc. Using

t ~ e formulas for these two quantities,2r.fL 2 r . ~ C

I f we solve this equation for f, thefrequency, we arr1ve at an oft usedexpression,f- 12r.VLC

From this formula we can seethat the frequency of a resonantcircuit depends on the product ofthe inductance times the capacitance-Land C being the only variables. So we can match any valueof L with a certain value of C togive a certain frequency. Or wecan match any value of C with acertain value of L. Stated anotherway, we can tune a circuit to resonance by varying either L or C. Inyour radio servicing you will en-counter both types. In the formerthey are usually referred to as'slug tuned'. The inductance isvaried by changing the p u ~ i t i u n ofa slug of magnetic material in acoil. Capacitive tuning is donewith variable air condensers.

The impedance of a series circuitof L and C is very similar to thatcontaining one of the two. Sincetheir effects are opposite, the tworeactances ate subtracted (algebraically) to get the net reactance:

At one frequency they exactlybalance each other-the voltage of AtZ=,IR2+ (XL-Xc)

resonance, X,,=Xc and Z=R.26


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pARALLEL RESONANCEsuppose we now connect a coiland a condenser in parallel and ap...., an AC voltage across the combfhation. The inductance and capacitance must still follow the Jawsfound for them. But, the effectsare very different due to the different connections.In a series connected circuit, thetwo reactances balance each other-cancel out entirely at resonance.This leaves only the resistance forthe total impedance. But in theparallel circuit at resonance, thetwo reactances work together toproduce a high impedance. Thelower the effective DC resistance,

M---- , ...p

c

' 'X " REPRESENTS INOUC&OVOLTAG& FROM PRIMARY COIL(p)

FIG. 22the higher the impedance. monly applied to both series andThe applied voltage sets up a parallel circui ts.current in each reactance. The In examining a circuit diagramvalue being the emf divided by the it sometimes is not immediately apreactance (assuming no resist- parent whether an LC hookup is inance) . The current in the coil is series or in parallel. If an externalE emf is appJied aeross the coil andx;:- and the current in the con- across the condenser, it obviouslyE T _ is parallel. U the external emf isdenser is Xc. But IL and .LO are applied across one terminal of the

180 degrees out of phase with each coil, one terminal of the condenserother. What is more natural than and the other two terminals of coilfor the two currents to 'short' or and condenser connected to each11ow together. This produces a other, it is equally obviously a sehigb circulating current even tho ries connection. The circuit thatthe line current-total current into may look parallel but actually isthe system-is low. The energy series is the case of the emf beingof the circulating current is stored induced in the coil. The inducedin the system, shifting periodically voltage is distributed along all orfrom the magnetic field of the coil a portion of the turns of the coil.tao-tile ~ c field of the conden- I t acts like a number of miniatureser. The Oilly energy lost is due to generators in series with a num-12R losses plus any energy removed ber of smaller coils. And all theseby radiation or coupling to another in series with each other and thecircuit. condenser. Figure 22 illustrates theThe parallel combination is some- voltage "rises" in the coil. Anotherti01es referred to as an anti-reso- viewpoint of this diagram is that,nant' circuit. But the usual term, if current is to flow, there is onlytuned or resonant circuit, is com- one path: Around the circuit.

27


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RESONANCE CURVES matic frequency controlI f we vary the frequency of a cuits.voltage across a series LC circuit, The general shape of t h e ~ r -the current thru the circuit will rent-versus-frequency graph of aalso vary. When the frequency is parallel LC circuit resembles the

equal to the resonant frequency of I-vs-f graph of a serie::; circuit upthe combination, the current will side down - and vice versa, ofbe at its largest value. At this course. Similar shaped graphs canpoint the impedance is equal to its be obtained if the frequency is conresistance. At any other frequency stant and L or C varied. Thatthe circuit is out of resonance. is we tune the circuit by changingEither L or C will offer more reae- either the inductance or the capacitance than the other. At lower tance.than the resonant frequency Xc The "Q" or efficiency factor ofwill be larger; at higher frequen- an unloaded tuned circuit is the Qcies, XL will be the larger. The of the coil. It has practically all oftotal impedance is their difference the DC resistance. This applies- the net reactance-plus the cir- equally well to series or parallel circuit resistance (added vectorially). cuits. I f extra resistance is added,As the frequency is changed the it is termed loading. This may becurrent will gradually build up to done by coupling power into ana maximum (resonant frequency), other circuit, or by putting a rethen drop off again. Note that sistance across (or in) the LC comOhm's law applies, even in AC cir- bination. This flattens out the curcuits, if we substitute Z for R-and rent frequency graph. The circuitare concerned with the steady state is less sensitive and less selective.values. As mentioned before, tran- The latter term means that itsients are another problem and passes a wider band-spectrum-need not concern us. of frequencies.

It is worth noting that below res- In some cases the highest selec-onance the current is capacitive: I tivity and sensitivity is the objectlead8 E. Above resonance the cur- desired. So the circuit Q is maderent is inductive: I lags E. comparatively high. In other cir-

ln a parallel LC circuit we have cuits a wider band is required. Thisthe lowest current at the resonant is usually accomplished by puttingfrequency since the impedance is a resistor across the tuned circuit.then at its highest. Off of reso- FM and TV circuits need this treatnance either L or c offers less re- ment-the latter have to pass aactance. The total current (IT) very wide band of frequencies, 4 tomust increase. 6 megacycles.Below resonance, h is inductive We hope this review has clarified(lags E). Above resonance IT is any doubtful points in your mind.capacitive (leads E). ' If you don't feel sure you under-Th stand these fundamentals, i t woulde rapid change of current- b 1J t f 1voltage phaae relationship on each e we o re er to the previous es-side of resonance f 1 . sons. They naturally give moreIS use u In auto- details than is possible here.

28


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,tgain we wish to emphasize the\ail.le of frequent, and. if pos::;iblc,regular reYiew. It w11l pay youbig dividends in the long run.)fake sure you thoroly understand each lesson. Each one is care-

fully designed by our long years ofexperience to take you along theroad to a sound knowledge of electronics. You will be surprised athow Huceeding lessons seem to geteasier and e a : : ~ i c r .

These questions are designed to teat your knowledge of this Jesson. Read themo ~ e r first to see if you can answer them. I f you feel confident that you can ,then write out your answers, numbering them to correspond to the q u e s t i o n ~ ; .If you are not confident that you can answer the ques tions, re -s tudy the leRsonone or more times before writing out your answers. Be sure to answer everyqur,.tion, for if you fail to answer a question, it will reduce your grade on thi t.lesson When all questions have been answered, mail them to us for g radinr .

QUESTIONS1. What general electrical formula ia usetl mOfrt. in Radio-TV repairing?2. State the most commonly used formula for power t-.3. In a e s res istance circuit. what factor ia the um e throughout tlte eirc:ult14. What is the relatiGIIShlp between c:aaduetaaee and realataaee?5. Does impedance apply to A.C. or D.C. c:ireuita?6. What elfec:t does resis tance have on an inductaac:e?7. What is "skin elfeet" of a wire winding, aad how don It dec:t the U.pedane.

of the c:oil78. Why ia it nec:eI IU J ' to take the .,oltage ratinl' of a c:oadenHr into c:oaalderation 79. When aaintr electrolytic c:ondenaen, why muat polarity always be observed?

10. In a resonant cireu.it. what is the relationship between X and X 7(, 0


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Sprayberry Academy of Radio - ND-10 - Review of Fundamental Principles

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