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SCHEMATIC DIAGRAMS

The functional relationships of electronic elements are of primary interest to all technical

personnel in the electrical field. Graphical representation of these relationships is best shown in

schematic diagrams. The schematic diagram is the hallmark of the design room of the electronics

industry. It is a diagram that shows the connections and functions of a circuit arrangement by

means of graphical symbols. It does not show the physical relationships of the components

within a circuit.

Such a diagram makes it possible for a person schooled in electronics to trace a circuit with

comparative ease. For this reason it is used for:

design and analysis of circuits

for instructional purposes

for trouble-shooting.

Standards that govern preparation of Electronic Schematic Diagrams.

Two main standards control the preparation of elementary diagrams. One is the American

Standards publication ASA Y32.2-1954, "Graphical Symbols for Electrical Diagrams”. This

standard lists the approved symbols to be used in circuit drawings. The other standard is ASA

Y14.15-1958, "Electrical Diagrams," which concerns approved practice in drawing schematic

diagrams.

A working knowledge of the symbols used in such drawings is necessary for all persons engaged

in the preparation and reading of schematic diagrams. The symbology of some of the elements

most commonly found in electronic circuits follows.

The battery. A single-cell battery, shown in Figure 6.1a. The horizontal line represents the path

of the signal, or electrical impulse, and is not a part of the battery symbol itself.

The longer of the two lines of the symbol always represents the positive terminal. The shorter

line is made about half as long as the long line. Additional long and short lines may be added to

show a multicell battery, as shown at the right side of Fig. 6.1b. Plus and minus signs may be

placed adjacent to the appropriate symbol lines. They might, if shown, prevent the reader from

confusing the battery symbol with others of similar shape.

Resistors

Figure 6.1. Symbols for batteries. (a) Single cell. (b) Multicell

with polarity added.

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Two approved symbols for the resistor are shown in Figure 6.2a. The zigzag symbol is used in

the radio and television fields, whereas the rectangular block is widely (although not exclusively)

used in the industrial electronics field. The zigzag symbol, made with

a 60º angle between adjacent lines

should have only three points on each side, unless extra taps or other special features

make the showing of more "turns" advisable.

The capacitor. The straight line drawn perpendicular to the signal path is made about the same

size as that of the longer line of the battery symbol. However, the capacitor symbol is sometimes

drawn about twice the size of the battery symbol in actual practice. The curved line should

always be made with a compass or template, and its size relation to the straight line should be

that shown in Figure 6.3. Also shown in the same figure are symbols for an adjustable and a

shielded capacitor. Good drafting practice should be observed in the drawing of the dashed lines

of the shield. Comers should be full and complete, and the dashed lines should not touch or

intersect the signal path.

Other symbols, are sometimes employed to represent capacitors. The older symbol, superseded

for many years by that shown consists of two straight lines.

Transistors

1. Arrowheads shall be of 45º included angle. They shall be filled and about half their length

away from the semiconductor region symbol.

2. The emitter and collector symbols shall be drawn at approximately 60º to the semiconductor

region symbol.

3. The line enclosing the device is for recognition purposes and its use is recommended. .

4. The elements of the transistor may be rotated 90º from the position in which they appear in

figure 6.4a to 6.4b. This practice often facilitates the layout of a schematic diagram.

Figure 6.3 Symbols for capacitors (a) General. (b) Adjustable or

variable. (NOTE: The shaft of arrow is usually drawn at a 45º

angle.) (c) Shielded.

Figure 6.2 Symbols for resistors. (a) General. (b) Variable or adjustable.

(c) With adjustable contact. (d) Tapped.

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Size of symbols.

A guide to drawing sizes is furnished by another American Standards publication, ASA YI4.15-

1958, "Electrical Diagrams." The suggested drawing sizes of several symbols are shown in

Figure 6.5

In very large drawings the symbols may be made larger than shown here, and in very small

drawings they may have to be made smaller. Other symbols should be made to match those

shown in Figure 6.5 in so far as size is concerned.

Principles that apply to the preparation of electronic schematic diagrams.

The following 12 items give a general coverage to the layout and drawing of electronic circuit

drawings.

1. Medium-weight lines are used for symbols and connecting leads or wires.

Heavy lines may be used where emphasis of a component is desired.

2. A diagram should be arranged to give prominence to main features.

3. Uniform density is desired so that there are no congested areas and a minimum number of

large white areas.

4. Very long lines or interconnecting leads are to be avoided as much as possible.

Figure 6.5. Drawing sizes recommended by ASA (values are in inches).

(a) Capacitor. ( b) Contact. (c) Resistor. (* This figure appears to be too small for

ease of drawing. In many drawings it is likely to be in the 0.20- to 0.30-inch

range.) (d) Inductor. (e) Relay coil.

(a) (b)

Figure 6.4 Symbols for transistors

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5. Lines are usually, and preferably, drawn vertically or horizontally, but may be drawn in other

directions when circumstances so dictate.

Turns, or "bends," should be kept to a minimum.

6. A diagram should be arranged so that it reads functionally from left to right. In other words the

source, or input, is at the left, and the output at the right.

In complex drawings, the input is placed at the upper left, and the output at the lower right, if

possible.

7. A complex drawing may be laid out in layers, with each layer reading from left to right or

downward.

8. Parallel lines are grouped together, usually in groups of three. Groups should be separated by a

space twice as large as the space(s) between lines within a group.

9. Connecting lines may be interrupted, where necessary, provided proper identification is added

at the points of interruption.

10. A drawing should be arranged so that adequate space is available near each symbol for

proper identification.

11. Attention must be given to the spacing of lines, sizes of symbols, and size of lettering if a

drawing is to be reduced for publication.

12. A schematic diagram should have a consistency, or uniformity, of spacing throughout, and

present an over-all picture of symmetry.

Layout of a stage.

Circuits primarily consist of components laid out in a vertical or horizontal arrangement.

As much as is possible, major components should be placed in a vertical or horizontal

line.

Spaces between these lines can be made equal, for the sake of appearance.

Light vertical and horizontal construction guide lines can be drawn on which the

centralized symbol will be placed.

This spacing must be large enough to permit the inclusion of the largest symbol between

parallel vertical lines.

It must also be large enough to allow sufficient room for any lettering that may have to

be placed within the diagram.

After the network of construction lilies has been drawn, the remaining symbols can be

added.

All resistors should be drawn to the same size, and all other symbols that appear more

than once should also be drawn to the same size.

Each symbol should be centered in the span of wire on which it is to be located.

Transistor circuit layout.

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The nature of transistor symbols is such that they are often not located on the same common

center line, even though they are in the same linear progression. The power regulator circuit

shown in Figure 6.6a illustrates this situation. The transistor symbols could have been placed on

a common horizontal line, by using the method pictured in Figure 6.6b. However, a number of

electronics firms, apparently do not think that much is gained by aligning the transistors in this

manner because many diagrams of transistor circuitry exhibit the same general alignment as

shown in Figure 6.6a. This feeling is probably the result of a certain opposition to the extra turns,

or bends, that appear in Figure 6.6b.

Arrangement of circuits.

Placing and positioning the elements of a known circuit arrangement in a schematic diagram

sometimes presents difficulties. There are circuit arrangements for which it is impossible to draw

all lines horizontally and vertically, thus making it necessary to draw some lines at other angles.

There are other circuits which, at first glance, appear to require this approach which can be made

entirely with horizontal and vertical lines if enough thought is given to their layout.

The typical flip-flop circuit shown in Figure 6.7a is drawn exactly as it was published. This is

one of those rare examples where not all lines are drawn vertically and horizontally. Note the

two lines in the center which cross where a crossover loop has been drawn to emphasize the fact

that the two diagonal lines are not connected at the crossover. Figure 6.7b shows how the circuit

drawing can be rearranged, and a schematic diagram drawn with all lines horizontal and vertical

and the crossover loop omitted. This necessitated rotating the transistor symbols 90º. Many

engineers prefer the diagonally drawn lines for a circuit such as this, because they believe that

the layout of Figure 6.7a more clearly shows the circuit operation than the layout of Figure 6.7b.

Figure 6.6. Transistor circuitry. (a) Shunt regulator circuit

diagram shown as published. (b) Method by which

transistor symbols may be aligned.

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Several large manufacturers of electronics equipment continue to use such loops, along with dots

showing connections. Although this practice is not recommended in the standards, the

combination of loops and dots certainly leaves no doubt as to whether there is, or is not, a

connection where two lines are drawn across each other. Many commercially available reduced

television diagrams would be hard to read if loops were shown, however.

Laying out a multistage circuit.

The steps described before apply to fairly simple circuits. They can often be applied to segments

of more complex circuits, and should be put to use within multistage circuit drawings whenever

possible.

Some transistor circuits have the transistors of the main circuit in a straight horizontal line This

arrangement is sometimes referred to as cascade, or linear, projection. Also, the components

between the transistors are so positioned that the transistors symbols can be positioned with

equal, or nearly equal, spaces in between them. Utilizing the above facts, initial layout of the

elementary diagram can be started.

1. About 2 or 2½ inches below the top border, a construction center line can be drawn

across the sheet. If transistor symbols of approved size are placed on this center line, there will

be enough space above this horizontal construction line for other components, which project

above the transistor and also for identification lettering.

2. A draftsman's sketch may be made, depicting the signal flow before any work is done on the

actual sheet of detail or drawing paper. Important dimensions can be placed on such a sketch if

it is not made the size of the final drawing.

3. Space for interstage coupling (transformers or capacitors) should be provided. The lateral

spacing of this transformer must allow for the lead which drops from the primary and

secondary of the transformer.

Figure 6.7 Variations in the arrangement of a flip-flop circuit.

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Some persons prefer to space all transistor equally wherever practicable. This concept provides a

nicely balanced drawing, unless there is a decided difference in the number of elements or lines

between some transistor as compared with others. If not, the diagram will have certain areas with

more white space than others, and may not be pleasing to the eye.

4. Now, all the remaining symbols may be placed on the diagram. Care should be taken to use

the correct symbols, to center them between connections (or crossovers), and to allow

adequate room for identification.

Reference designations.

Symbols of all replaceable parts should be labeled with an identification letter and

number.

Capacitors, coils, and resistors are designated by the capital letters C, L, and R,

respectively followed by the number of each element.

The numbers for each part are assigned in order, from left to right, and down, if more

than one are on the same vertical line.

These designations, should be located as close conveniently possible to their respective

parts.

A reference may be above, or below, or on either side of its element. But preferably

above it.

Transistors are labeled Q, X, or T (not to be confused with a transformer)

Integrated circuits are labeled IC

It consists of the letters followed by a number the latter often,

but not always, following left-to-right sequence as in the case of capacitors and resistors.

A transistor or IC may be assigned the designation a function below the type number if

desired.

Thus, the complete designation of the transistor would appear, as follows

Q1

2N6004

These letters are entirely upper-case-as is nearly all reference lettering on a schematic diagram

and they are often made larger than the other lettering in a circuit diagram.

Numerical values of resistance, capacitance, and inductance.

In most schematic diagrams the value of each resistor, inductor, and capacitor is indicated. The

value is placed directly under the reference designation, both being adjacent to the symbol.

However, some companies show the values in a table, or key, below the completed diagram. In

the diagrams shown so far in this unit, these values have not been shown, partly to avoid

complicating the appearance of the drawing, and partly because without the explanation which

follows, their inclusion in preceding diagrams might tend to confuse the reader and detract from

his concentration on the problem at hand.

The main objective in selection of values is to maintain a standard of uniformity within the

diagram using as few zeros as possible.

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In Table 1 the range of values for a given element and the unit recommended are listed.

Examples of reference designation and values.

Figure 6.8 shows a two-stage amplifier completely lettered (referenced) according to the

procedures outlined. In accordance with accepted practice, the resistance values that are in ohms

(R2, for example) do not have a letter or word after the value. Some companies use the symbol Ω.

in such instances.

Figure 6.8 shows a two-stage amplifier completely lettered.