section iii schematics cells in series and · pdf filesection iii schematics cells in series...

SECTION III

SchematicsCells In Series And Parallel

In Section 2 the ideas of potential difference andcurrent were introduced in the discussion of a cur-rent electricity source such as the voltaic cell. Inthe course of his work, the avionics technicianwill come across many current electricity sourcesboth electrochemical, such as the cell, and elec-tromechanical, such as the generator. The basicprinciples discussed in these sections can be ap-plied to all.

In this section we will begin to stress the prac-tical side of our subject. While the earlier sectionswere necessary to develop a theoretical base, thefollowing sections will feature more practical ex-amples and exercises of the type the avionicstechnician will encounter in his daily work.Therefore, it is recommended that the reader at-tempt to answer the self-test questions and try allexperiments. These self-test questions require aworking knowledge of elementary algebra and ex-ponentials, such as may be obtained from themathematics textbook in this same AvionicsTechnicians Series. The experiments for this sec-tion require only the most common electroniccomponents and a multimeter, which is a basicmeter movement equipped to measure voltage,current, and resistance.

As a basis for the practical material to follow, thissection begins with a discussion of DC and ACsources followed by an introduction to schematicand wiring diagrams. These are the conventionalways of describing how sources of current elec-tricity and the components to which they are con-nected may be arranged. Also defined are openand closed circuits. The section concludes with adiscussion of how cells may be connected to formbatteries like the familiar 12-volt storage bat-teries used in automobiles. In this discussion, theconcepts of series and parallel, aiding and buck-ing, and resistance are introduced.

A. DC and AC Sources

The direct current (DC) source, whether it is a cellof the sort discussed in Section 2, a group of cellsconnected to form a battery, or an electro-mechanical device, has two output poles, one ofwhich is always positive, the other alwaysnegative. In general, the potential differenceacross the source is made to remain as nearly con-stant as possible.

The alternating current (AC) source, which will beconsidered in greater detail in a later book in thisseries, is an electromechanical device with twooutput poles, each of which regularly changesfrom being the positive pole to being the negativepole. When a standard AC source device isstarted, there is no potential difference betweenthe two poles. As time elapses, the potential dif-ference between the poles increases to a max-imum, with one of the output terminals servingas the positive pole, and the other as the negative.After maximum potential difference is reached, itbegins to decrease, finally reaching zero, (i.e., nopotential difference between the poles). At thispoint, the polarity of the output terminalsreverses and what was the negative pole becomesthe positive pole, with the former positive polebecoming the negative pole. The potential dif-ference again builds up to a maximum (this timewith the electric field direction reversed) and thenfalls back to zero. Then the polarity of the outputterminals again changes, returning to its initialstate. This whole process of events is called onecomplete cycle of an AC source. The entire cyclerepeats itself 60 times per second in Americanstandard house current and 400 times per secondin most airborne AC supplies.

In this book, we shall deal with the electrical ef-fects produced by connecting various electronicparts, called components, to terminals or poles ofa dc source. For the most part we will ignore theeffects that might be noted at the moment the

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components are connected. Instead, we will con-centrate on those effects each system shows after"settling down" to what are called steady stateDC conditions. This does not mean that theswitch-on or so-called transient state effects areto be ignored. Rather, it will be easier to discussthem along with the electrical effects observed incomponents connected to AC sources in anothervolume of this series.

B. Pictorial, Wiring, and Schematic Diagrams

At the beginning of our exploration of avionicsequipment, it is best to agree on the type of mapswe will be using and how we will interpret them.These maps are indeed necessary, for there is noway that the winding conduction paths andmasses of components in even the simplest elec-tronic equipment can be usefully described usingwords alone. In fact, we might modify the old say-ing to: "a picture may be worth a thousandwords, but a map is beyond price."

Just as there are specialized maps for pilots,motorists, weathermen, and politicians, there area number of different kinds of diagrams used inelectronics, depending upon the requirements ofthe user. The simplest to read is called the pic-torial diagram. It may be either a line drawing(sometimes with shading to emphasize shapes), oreven a photograph of a piece of equipment. Thenames, part numbers, or what are called referencedesignators are shown, either printed on the com-ponents or connected to them by thin lines calledleader lines.

Reference designators, which generally consist ofa combination of letters and numbers, are used touniquely identify the component or assembly towhich they refer. For this reason, referencedesignators are "stackable", that is, a fullreference designator identifies the major as-sembly, sub-assemblies, and, finally, the compo-nent. Let us assume that a particular piece ofequipment, a cabinet housing a transmitter in acontrol tower, contains two power supply units,and that each power supply unit contains threeidentical sub-assemblies. In this case, thereference designator of the entire cabinet could beHFT 1 (high frequency transmitter number one)and the two power supplies could be called PS1and PS2. The full reference designators of the twopower supplies in the cabinet would then beHFT1PS1 and HFT1PS2. Similarly, if the threesubassemblies are called Al, A2, and A3, the

reference designator HFT1PS2A3 uniquely re-fers to the third subassembly of the second powersupply of high frequency transmitter number 1.The method may seem cumbersome at first, andeven a little silly, but it is much simpler and muchless confusing to tell a technician to "interchangeHFT1PS1A3 and HFT1PS2A3" than it would beto say the same thing without reference desig-nators.

In general, then, a pictorial diagram provides uswith information concerning the overall ap-pearance of a unit, the shapes, relative sizes andlocation of components, interconnecting wiresand cables, and so forth. It usually names thecomponents in such a way as to permit crosschecking with a parts list. However, it does notprovide any clue to the function of any partwithin the unit or system. An additional form ofpictorial is sometimes seen, the assembly or partsblow-up. It is a curious sort of diagram, showingeach individual component or mechanical part ofthe unit connected to its eventual place on a mainassembly or chassis by a dotted line. The overallappearance of this drawing is that of a unit thathas had an explosive charge go off inside it. Inspite of its bizarre appearance, this sub-type ofpictorial diagram is of particular value for show-ing otherwise hidden components or assemblies.It may, if carefully done, greatly help in thedisassembly or assembly of the equipment. SeeFig. 3-1 for examples of pictorial diagrams.

A second kind of map or diagram used in elec-tronics is the interconnect or wiring diagram.This diagram is meant especially for those whomust wire or interconnect equipment. In general,the wiring diagram uses squares or rectangles torepresent the various components or units involv-ed, and is detailed only in specifying the wiresand connection points. As might be expected, thecolors of the insulation on the various wires, wiresizes, and cabling details are given.

For simple units, the pictorial diagram is oftenmade to serve as a wiring diagram (as shown inFig. 3-2). Again, as in the case of the pictorial, noeffort is made to show the function of the com-ponents. The main purpose of the wiring diagramis to describe the electrical interconnections ofthe various components that make up a piece ofequipment.

The third kind of map with which we shall deal inthis book is a functional one, the schematic

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Courtesy of Wilcox Electric

Fig. 3-1 A — Pictorial diagram of a printed circuit board showing some components. B — Assemblydrawing and parts blow-up.

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Courtesy of Wilcox Electric

small portion of the entire unit. B — GraphicFig. 3-3 A -- A sample schematic diagram showing asymbols used in schematic diagrams.

diagram. The schematic diagram is an arrange-ment of standardized graphic symbols for thefunctions of the parts of a piece of equipment. Thesymbols are connected by solid lines representingthe electric current or potential paths betweencomponents (these lines sometimes represent ac-tual wires, but sometimes they do not; as long asan electrically conductive path exists betweentwo points on a schematic diagram, a line isdrawn between them).

Once we become familiar with the conventionalsymbols and layout of a schematic diagram (Fig.3-3), a surprising amount of information aboutthe function of the unit becomes immediatelyavailable. The basic graphic symbols for variouscomponents are given in Fig. 3-3. These will berepeated and new symbols introduced as needed.

In general, the reference designators and func-tional quality magnitudes (the resistance of

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section iii schematics cells in series and · pdf filesection iii schematics cells in series...

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