Drafting for Electronics--Schematic Diagrams

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INTRODUCTION

It is important to communicate ideas effectively through schematic diagrams. A schematic diagram shows the function of a circuit with little emphasis on its physical characteristics. This diagram has been called the most important drawing for use in equipment production, testing, and analysis. The schematic drawing describes all circuit functions and values of components, using standard symbols in most cases. (See Section 8 for a review of standard symbols.)

Although the schematic diagram shows the inter connections between electrical components, it should not be confused with the wiring diagram. In the wiring diagram (fully discussed in Section 13), components are arranged showing their physical relationship rather than their functional relationship. Often, in a wiring diagram, the parts are drawn as pictorials rather than as graphical symbols. The wiring diagram is used, as its name implies, in the actual wiring stage. It can be very difficult to analyze because the physical relationships are so complex.

The primary goal of the schematic diagram is to show the functional relationships between parts, allowing the circuit to be easily analyzed. Generally, the schematic, like block and logic diagrams, is laid out to be read from left to right with the input coming into the upper-left and the output going out the lower-right sides of the drawing, although a “top to bottom” flow can be used when a circuit does not flow naturally from left to right.

The very first conception of an idea drawn by an engineer is usually in the form of a schematic diagram, although it is often a very rough sketch. Once the engineer has completed the ideas for the design, it becomes the drafter’s responsibility to prepare a finished schematic diagram. A drafter must be aware of all accepted drafting standards used in industry and in individual company practice. Frequently, the engineer’s sketch will not be prepared to meet these standards. Schematic diagrams, like many other electronic diagrams, should be drawn to standards set up by such groups as ANSI, JIC, and IEEE.

The schematic diagram is the master drawing and is used in the preparation of wiring diagrams, printed circuit board layouts, mechanical layouts, parts lists, and other associated drawings. Basically, three elements are included in all schematic diagrams: graphic symbols rep resenting the components; reference designations de tailing component values, tolerances, or other ratings: and the interconnections between all components. These elements make up the original layout of the drawing and are clarified in the final drawing. The final drawing must be checked to ensure that these elements are clear and accurate.

FIG. 1 Typical engineering sketch to be redrawn.

LAYOUT

Most of the time it is impossible to simply redraw a sketch provided by an engineer. It would be a simple task if all that was needed was to take the layout and put it in standard symbols with straight vertical and horizontal lines. Unfortunately, many sketches have to be completely rearranged to provide a finished diagram that is not only functionally correct but also easily interpreted and, just as important, esthetically pleasing. Drawings that are not pleasing to the eye will be difficult to work with. Circuit components must be well spaced around primary or active components. Lines should be horizontal or vertical, with crossover lines minimized.

The drafter may have to make one or more trial sketches to be sure that this balance is achieved. The use of grid paper is strongly recommended for trial sketches and for the final layout. Whenever major re arrangement is necessary, keep one thing in mind: any changes or alterations that are done must not affect the technical correctness of the drawing. You must be careful not to alter the electrical function of the circuit.

FIG. 1 shows a freehand sketch that could have come from an engineer’s pad. The sketch was pre pared with little regard for drafting practice because the engineer was concerned primarily with the electrical function of the circuit. The correct drafting techniques were left for the drafter.

If you received this sketch, many things would be required for you to prepare the finished drawing. No reference designations are used for any components. The transistor symbols are incomplete. It might be a good idea to prepare another sketch to see where some crossovers could be eliminated, how vertical and horizontal alignment could be achieved, and how better spacing could be maintained. FIG. 2 shows how some of these problems were corrected, and FIG. 3 shows a final drawing.

A drawing can be laid out in a number of different ways. For a simple circuit, visually divide the number of components in half. Components associated with input functions should be arranged on the left half of the page. and those associated with the output should be on the right half. Input signals flow through the circuit in various stages. Components that complete a stage should be aligned vertically. Horizontal alignment is used for the relationship of one stage to another.

FIG. 2 Trial sketch and possible alignment from original sketch.

FIG. 3 Final drawing completed from trial sketch.

A more complicated circuit is normally made up of sections or functional units. These units are arranged in an order, allowing for left-to-right signal flow. Each small group of components representing a functional unit follows the same spacing and alignment procedures used for a simple circuit. Keeping this in mind will help prevent the more complex circuits from becoming over whelming tasks. A complex circuit is nothing more than the sum of the individual parts.

Do’s and Don’ts

A schematic diagram must always show clearly and precisely the circuit design and function, as well as the relationships among the various components. The following drafting practices will help you prepare final drawings:

1. Use medium-weight lines for all symbols and inter connection lines.

2. Ensure adequate space between parallel lines for notes, reference designations, and so on.

3. If two wires are to be connected, use a heavy dot at the junction to indicate a connection. Using the dot for a connection will leave no question or cause for confusion.

4. Avoid crossovers whenever possible. If it is not possible to avoid a crossover, indicate clearly that the line is a crossover and not a connection. The older method of jumping over or forming a half-circle is considered obsolete and time consuming. The method of dot/no dot, as shown in FIG. 4, will help eliminate confusion. (For a detailed discussion of the handling of connections and crossovers.)

5. Avoid diagonal lines and curves. Besides representing the electrical function of the circuit, the schematic represents the capabilities of the drafter. Long diagonal lines or curves upset the balance and symmetry of the drawing and detract from the overall pleasing quality.

FIG. 4 (A) Connections; (B) crossovers.

6. Avoid long lines; use interrupted lines instead (covered later in this Section). The use of long lines requiring a number of changes of direction can be very confusing and require much time to draw and interpret.

7. Ensure a balance of lines and blank spaces in the final drawing. If large open or blank areas can be found on the drawing or if portions of the circuit or groups of components appear to be very crowded, the best layout probably has not yet been found.

8. Show signal flow from left to right and top to bottom. For more complex circuits you may have to use more than one row or column. The general flow in this case should start in the upper left-hand corner and finish in the lower right-hand corner of the diagram.

9. Try to ensure that all technical data are correct in order to present the most accurate and highest- quality drawing possible, even if you are not responsible for checking the accuracy of the schematic.

10. Check and recheck the drawing for completeness and accuracy at all steps of the preparation. This includes all intermediate drawings that are completed to make the final layout easier.

The Final Drawing

After all spacing and layout have been determined, the final drawing can be prepared. All graphical symbols should be constructed as described in Section 8. The use of templates like the one shown in FIG. 5 is highly recommended to ensure that all symbols are uniform and consistent. This will also save a lot of constructiontime. If dry transfer symbols are available, as illustrated in FIG. 6, they can also be used to save time and produce a uniform symbol. When dry transfer symbols are used, the interconnecting lines and additional symbols should be done in ink to avoid the drastic difference in appearance that would occur if pencil were used to draw the connecting lines.

FIG. 5 Template used to draw graphic symbols on schematic diagrams. (Courtesy Berol RapiDesign)

Since the majority of the lines are horizontal and vertical, a grid underlay is very helpful in the preparation of the final drawing. Grid paper is used to draw the original or additional trial sketches. The finished drawing, however, is prepared on either plain vellum or fade-out grid paper. Using a grid underlay gives the advantage of having a grid without having the final drawing on a grid paper. If the same size grid is used for the sketch and the underlay, it is easy to transfer the spacing and placement of components and connecting lines from the trial sketch to the finished drawing.

Interrupted Lines

When the simple circuit was drawn in FIG. 7, it contained a single voltage value and was depicted as the symbol for a battery or voltage source. Frequently, in the more complex circuits, a number of different volt ages are supplied at different points in the circuit. All these voltages might be supplied by the same functional unit, called a power supply circuit. The use of interrupted lines helps reduce any confusion that might arise if long connection lines were used to make all voltage connections to the power supply circuit.

A typical power supply is shown in FIG. 8. This supply is drawn separate from the rest of the circuit, and the termination points are given labels. Labeled points, wherever they appear in the circuit, are in reality connected to the power supply. The power supply circuit is generally found at the lower left of the drawing be cause it must be kept away from the signal path part of the circuit.

When long connection lines extended from the power supply throughout the drawing would cause clutter and confusion, interrupted lines should be used. Make certain to indicate clearly the destination of all interrupted lines by using letters, numbers, or unique symbols. The unique symbols in FIG. 9 include ground, plus or minus signs indicating voltage polarities, terminal bars, jacks. and plugs. If a number of lines will go to the same place, they are frequently bracketed and noted as in FIG. 10.

The labels used for interrupted lines may be en- - closed in triangles, circles, or other shapes. This helps call attention to the fact that an additional connection is made and may help to locate that connection else where on the drawing.

ANSI Y32.2

FIG. 6 Dry transfer aids: stick-ons or PUPPETS (Provided courtesy of Bishop Graphics, Inc.)

FIG. 7 Simple circuit showing all connections and standard graphic symbols for all components and voltages.

Switches

Some switches are quite easy to represent in a circuit. The single-pole, single-throw (also called SPST), single- pole, double-throw (SPDT), and the double-pole, double-throw (DPDT) are a few of these simple switches. They are used frequently in control circuits and are discussed in Section 14. Other types of switches can be very difficult to represent on a drawing. Rotary switches can be especially confusing. A rotary switch rotates a movable contact connected to a center shaft, completing a circuit by touching the stationary contacts around an insulating material. A pictorial representation of a rotary switch is shown in FIG. 11. The rotary switch is found frequently in test equipment and is used to select meter function, scale, or range.

To add to the confusion, rotary switches can contain multiple layers, sometimes called decks or wafers. In general, when multiple layers are used, the movable contacts or wiper parts are all connected to the common center shaft, and all the contacts are changed simultaneously. Since the different layers have no electrical relationship, but merely a mechanical one, it is possible to represent each section individually on the drawing. Because each section is individually represented, it must maintain the same relative position. In other words, if the terminal next to the mounting screw is called terminal 1 for the first layer, then all other layers have the terminal closest to the same mounting screw labeled terminal 1. Unlike some components and integrated circuits, rotary switches often do not come with numbers on the contacts. They can be numbered arbitrarily. Therefore, whenever a switch is drawn, the relative position to mounting screws and other parts must be made clear. Additionally, some terminals may not be used or even physically there. In this case a space is left where the terminal would be. To avoid confusion, all terminals should be numbered in sequence, whether used or not. The switch in FIG. 12 shows space for lug 8 even though it is not in use and in this case is not even shown in the diagram. The direction of rotation (clockwise or counterclockwise) should also be included and noted on the switch detail, as well as how the switch is to be viewed—from front or rear side.

FIG. 8 Typical power supply.

FIG. 9 Unique symbols used for interrupted lines: (A) earth ground; (B) chassis ground; (C) plug; (D) jack; (E) plus or minus for voltages; (F) terminal connectors.

FIG. 10 Grouped lines going to a common destination.

FIG. 11 Pictorial presentation of two-layer rotary switch. (Reprinted by permission of HEATH COMPANY)

Switch Tables

To help users understand the changes that occur when the switch is rotated, data are frequently presented in a table near the switch. As illustrated in FIG. 13, the table indicates which position the switch is in, what function is being done, and what connection is being made. Normally, the switch is drawn in the de-energized, or OFF, position. In the case of rotary switches, the No. 1 position is frequently shown.

FIG. 12 Numbered switch lugs, whether used or not. (Reprinted by permission of HEATH COMPANY)

FIG. 13 Switch table showing relationship between functions and positions.

The decision to show the switch as separate layers or as an integrated version can only be made after a trial layout is prepared. If the switch is shown all in one area and the functions that it controls are scattered throughout the circuit, then long connection lines result. Even though interrupted lines help reduce the clutter and confusion, they are still more confusing than the short connection lines. FIG. 14 shows the best way to illustrate scattered functions, which is to split up the layers and locate them in the portion of the circuit that they control.

Templates are available to help in the layout and correct spacing of the switch contacts. A typical switch template is shown in FIG. 15.

Whenever a mechanical connection occurs between moving parts, dashed lines are used to show the mechanical linkage. For multiple-layer rotary switches, however, this mechanical linkage is hardly ever shown, be cause it is understood that all layers rotate with a common shaft. Every layer has a label identifying what view it has (front or rear) and what its relationship is to the other decks.

FIG. 14 Switches shown in layers for representing connections to each wafer or deck.

FIG. 15 Template used in drawing of switches for schematic diagrams. (Courtesy Berol RapiDesign)

FIG. 16 shows the schematic of a VOM or multi-test meter. The individual layers of the switch have been separated and are located in the portions of the circuits where their electrical connections are being made. They are all identified as parts of SW1 and include additional letters A, B. and C for different wafers or layers. In addition, each wafer is identified as a front (F) or rear (R) view. Notice that each section of the switch is shown with the same orientation, and the mounting screws are identified to ensure proper location of terminals. Instead of a switch table, inset diagrams and notes are included on the drawing showing rotation, position, common connections, and identification of wafer locations on the actual switch.

Figures 17 and 18 show additional representations of switch connections. The switch in FIG. 17 is similar to the one in FIG. 16. The major difference is that this meter includes five wafers, showing both front and rear views of each wafer, in the rotary switch. Inset diagrams identify the knob position, deck locations, and the relationship of position No. 1 to a strut or mounting screw. FIG. 18 displays the switches in a straight-line method even though the switch is a rotating switch. The circuit for this meter ( FIG. 18) is not terribly complex. and this layout is easier to construct.

Scale and Grouping

Schematic diagrams are not drawn to any particular scale. The size of the final drawing is determined by the number of components, connections, and notes that must appear on the drawing. The size of the symbols used to represent particular components does not reflect the physical size of the component itself. Therefore, a larger diagram does not necessarily reflect the need for a larger physical circuit, only a larger number of components.

FIG. 16 Schematic of meter with switch layers located in the functional areas. (Courtesy Triplett Corp.)

FIG. 17 Meter with five switch decks. (Courtesy Triplett Corp.)

FIG. 18 Rotary switch represented with straight lines. (Used with permission from Radio Shack, a division of Tandy Corporation, Fort Worth, TX 76102)

For a drafter, the preparation of the schematic diagram is the opportunity to fit together all previously learned skills. Drawing a schematic includes good drafting skills, line-weights, lettering, and symbols, as well as use of reference designations.

The schematic diagram layout is very similar to the block diagram layout. The circuit can usually be divided into functional units or blocks. These blocks are divided into areas of the page. FIG. 19 shows a schematic with each of the functional units identified with a heavy outline surrounding it. For manufacturing or production purposes, these identifying blocks can add clutter and confusion. For the preparation of a printed circuit board (PCB) or multiple PCBs, the identification of the functional units can be an aid. (For a more complete presentation of PCB design and layout. The identification of functional units is a definite aid in the analysis of the total circuit operations.

FIG. 19 Layout and placement according to functional grouping. (Courtesy of Motorola, Inc., Semiconductor Products Sector)

Space Estimations

Starting with primary components such as transistors, you can make space estimations for the final drawing. After you are satisfied that the general placement has been achieved, you can determine final placement. Typically, the transistor is enclosed in a circle /s in. in diameter when the grid size used is eight squares per inch. Using this size as a reference, you can determine spacing for all components, lettering, and notes.

FIG. 20 shows good spacing for notes, reference designations, and other marks around the components. Lettering should be no less than 1/8 in. when transistor enclosures are /s in. if the grid used is eight squares to the inch. Sometimes a smaller-sized grid is used for less complex circuits. A common size of smaller grid is ten squares to the inch. When this size of grid is used, lettering can be as small as .100 in. When a reduction of 2 to 1 is desired, .100 in. is the minimum lettering size that should be used.

FIG. 20 Adequate space around components for lettering.

In general, the overall size of the drawing is not limited. Using the larger squares (eight squares to the inch) will present a drawing that can be reduced and yet will not be too small to read. When using the ½-in. squares, you can make resistors as large as 1 in., or eight squares, whereas capacitors will probably be four to six squares, as shown in FIG. 21. A space of at least five squares should be left between components for reference designations, as shown in FIG. 22. If your trial layout makes the lettering appear crowded or confusing, try another layout with better spacing before you finish the drawing.

FIG. 21 Common number of squares for components on an 8 x 8 grid.

FIG. 22 Spacing of at least five squares between components on 8 x 8 grid.

Diagonals

In most cases, avoid using diagonals. It is impossible, however, to eliminate them totally, and in some applications the use of diagonals is appropriate. For example, a circuit called a bridge circuit is normally constructed with diagonals so that it looks like a diamond. This configuration promotes quick recognition for analyzing the circuit function.

FIG. 23 Diode bridge rectifier in diamond shape.

FIG. 23 shows a typical full-wave rectifier bridge. The corners are constructed at 90°, and the square that is formed is rotated to appear as a diamond shape. Since this particular type of circuit is a popular configuration, the shape is as important as the components within the drawing.

There may be other times when diagonals or crossovers simply cannot be avoided. When diagonals must be used, an angle of between 45° and 90° and consistency are most important. If you must use a crossover, be sure that the viewer can quickly recognize it as a crossover and not a connection. The use of saddles or lines that appear to jump over other lines is not recommended practice. The dot method discussed earlier should be used.

Inset Diagrams

Keep in mind that one of the more important qualities of a schematic is clarity. When inset diagrams are to be included, these small illustrations will be set apart from the main part of the diagram. They must be placed so they do not confuse the main portion of the drawing. Frequently, the inset drawing may be a pictorial representation of the component or switch assembly, as shown in Fig. 24. For assemblies of multiple-layer switches, this pictorial can be quite helpful for later preparations of wiring diagrams or assembly diagrams. On some schematics the lead connections for transistors or arrangements of pin connections are included as inset diagrams. Since the schematic diagram functions as a master drawing in the preparation of all other drawings, the lead identification will be useful later. When the inset diagram is used for lead identification, a simple outline of the bottom of the component or terminal strip is usually adequate, as illustrated in FIG. 25.

FIG. 24 Inset diagram showing lug numbers and switch orientation.

FIG. 25 Inset diagram showing component bottom for lead identification.

Alignment and Placement

The placement or orientation of the graphic symbols on the schematic has no relationship to their actual physical location or arrangement within the system. In terms of electrical function, there is no correct right side or left side of any symbol. Standard practices have been developed and should be followed to make it easier to interpret the diagram. For example, by convention the ground symbol is normally pointed down and is located near the bottom of the drawing or at least near the bottom of a functional unit. If a number of transistors are used in a circuit, they should all be arranged in a consistent manner. In addition to having similar orientations, components should be aligned horizontally and vertically. The flow paths or interconnecting lines should be as short as possible, showing the most direct routes without diagonals or curves. FIG. 26 shows a portion of a larger circuit and maintains good horizontal and vertical alignment.

FIG. 26 Sample of good horizontal and vertical alignment. (Courtesy Joint Industrial Council)

Spacing dimensions will undoubtedly vary with individual company practice. If no specific rules are listed in drafting room manuals, the following specifications can be used. Components should be at least .25 in. from corners or crossovers, as shown in FIG. 27 (A). There should be a .5-in, minimum between two components located on the same horizontal or vertical line [ 27 (B)]. These are only suggested minimums, and more space may be required depending on the placement of reference designations or other notes and identifiers.

The most important characteristics of good spacing and placement are consistency and clarity. In addition to being functionally or electrically correct, a good schematic diagram should be esthetically pleasing to the eye.

The size of the drawing paper that is required is determined by the number of components that can be found in the circuit. Table 1 shows the size of drawing paper that should be used according to usable space when the average size of the graphic symbols is 2 in. The usable space on a drafting sheet is the amount of drawing area that is left after borders, title blocks, and notes are placed on the sheet.

When space is required for revisions and parts lists, only about half of the number of symbols can be placed on each sheet size. Table 1 can be used only for estimating the size of paper required for a drawing. For complex circuits that include many interconnecting lines, fewer symbols may be counted in the same amount of space. When diagrams are subdivided into functional units, more space is required, so there are fewer symbols per paper size. When components appear too close together or lettering may be too crowded, a larger size of paper should be used. If reductions are standard practice or company policy, then additional space allowances must be made so that the drawing will remain clear and legible when reduced.

FIG. 27 Space between components and corners or between components: (A) minimum .25 in. (6.35 mm); (B) minimum .50 in. (13 mm).

TABLE 1 Estimated Number of Components That Can Be Placed on Standard-Sized Papers

CONSTRUCTION OF THE DRAWING

For more complex circuits, first divide the circuit into functional units. Lay out each of these units in a trial sketch to help determine the overall required space. Check the trial sketch for any alterations that can be made to eliminate crossovers or crowding. If necessary, make another trial sketch. To prepare the sketch, start with symbols that will be aligned horizontally in the finished drawing. These will normally be the active components, such as transistors. If special attention is given to the placement of the transistors in the sketch, like the placement in FIG. 28, additional components can be easily connected to the transistors, as shown in FIG. 29. Then add the interconnection lines, as shown in FIG. 30.

When going from the trial layout to the finished drawing, you can make slight alterations without making an additional trial sketch. Make light pencil construction lines on the paper to establish the general overall shape and placement of the finished drawing. These lines will give you a last chance to make small changes in placement or spacing.

Skills are acquired after much practice. Layout spacing, placement, and alignment, as any other skill, will become easier as you gain more experience with complex circuits.

The Finished Drawing

FIG. 28 Placement of active components first.

FIG. 29 Additional components in relation to transistors.

FIG. 30 Connecting lines with interrupted lines identified.

If necessary for photo-reproduction, the finished drawing maybe done in ink, with special appliqués, or with other dry transfer methods. The appliqués should be placed on the drawing first. After they are applied, if the final drawing appears cluttered or crowded, these symbols can be removed with very little difficulty. After all the transfer symbols have been placed on the paper, additional symbols should be added. These symbols should be drawn with a standard template and ink. When all components have been drawn or placed on the finished drawing, the interconnection lines should be added. The final step is to add reference designations and other notes.

You may follow these steps when preparing a schematic diagram:

1. Obtain the original sketch and check for accuracy and all data.

2. Make a sketch on grid paper with trial spacing and alignment to estimate paper size and space requirements.

3. Make any alterations or changes necessary on the trial sketch. This might include making additional trial sketches.

4. When satisfied with the circuit layout, place the finished drawing paper over a grid underlay of the same grid size as the sketch. Lightly place construction lines on the drawing using the grid spacing as the placement guide.

5. Begin the finished diagram by placing dry transfer symbols on the paper or by drawing the individual component symbols with the aid of templates.

6. Finish the schematic diagram by adding all flow lines, terminal markings, labels for interrupted lines, notes or reference designators, and other lettering.

7. Check the finished drawing for technical accuracy and clarity before submitting it as complete.

DRAWING VERIFICATION

Checking a schematic diagram is every bit as important as the initial preparation of the drawing. A print is made of the final drawing so that it may be marked on as the checking process is done. The best method of checking is a line-by-line, symbol-by-symbol check. The final drawing should be checked against the original sketch. Any questionable details must be made clear. Obviously, it is better to eliminate any misunderstandings long before the finished diagram is prepared. You can use a colored pencil or marker to indicate the portions that are being checked off, tracing over a copy of the finished drawing and the original sketch as you check it. If you find any errors, clearly mark them in red so that they may be fixed on the final drawing. In addition to checking interconnecting lines and connection points, check the following items:

1. Component symbols

2. Reference designations, including values, tolerances, and polarity markings, if needed (Do reference designations clearly identify the correct symbol?)

3. Identification of numbered terminals or other leads

4. Title blocks, including spelling and punctuation

5. All additional notes added to the drawing, including correct spelling, technical accuracy. and placement away from crowded areas

Once the checking is completed, the marked-up checking print will be returned to the drafter so that corrections can be made to the finished drawing. After corrections have been made, another checking print should be made, and the process should be repeated. When the checking process produces no errors, the finished drawing is initialed and dated. Then the required number of prints is made. The finished drawing will be the master for associated drawings.

FIG. 31 Drawing of a circuit on a CAD system. (Courtesy California Computer Products, Inc.)

FIG. 32 CAD library. (Courtesy Prime Computervision)

ELECTRONIC SYMBOLS IN CAD

In traditional drafting methods, each symbol, or graphical representation of standard parts, must be drawn individually using a template or other construction methods. The use of CAD allows the drafter the opportunity to draw the symbol once and store it in memory for recall later. Once this image is created, it can be scaled, rotated, or mirrored ( FIG. 31) and used in any circuit drawing. When attributes and properties are stored with the symbol, parts lists can be automatically generated and other calculations can be performed. The collection of stored symbols is called the symbol library ( FIG. 32).

Creating and Defining Symbols

Drawing a symbol begins with the formation of the connect nodes (base points), which must be created as permanent points, or handles that are used to place and position the symbols. The symbol origin is specified and used as the reference point for future placement of the symbol. The attributes are text notes used for additional information about the symbol. These notes can be collected into a disk file and processed by applications (software programs) designed to automatically generate parts lists. The attributes can be a very complete description of the symbol, including component, style, designation, capacity, code, cost, and company or vendor.

Symbols are stored as an individual block or subpart of a larger graphic. New symbols can be created by nesting together individual symbols. Once a symbol is created, it may be recalled, changed, and restored. This modified symbol may be saved as a new symbol similar to an existing one or may simply replace the old symbol. If a symbol in the symbol library is changed, the drawing

may be updated with the new version by simply executing a command. There is no longer any need for many tedious hours of redrawing entire schematics to update one symbol or figure. When the update command is used, only the active drawing is changed. All other drawings remain unaltered.

CAD-GENERATED DIAGRAMS

In general. the creation of an electronic diagram ( FIG. 33) on a CAD system is very similar to preparing the diagram manually on a drafting board. The primary difference, when using a CAD system, is the capability to recall instantly all the needed symbols from the library and place them on the drawing at the desired location and in proper position. A schematic diagram provides a means of capturing, concisely and accurately, all the data required to describe a particular circuit. As such, it forms an essential basis for any electronic design project and is used throughout the design process from development, through printed circuit board layout, to inclusion in service handbooks.

Diagram symbols can be edited by using the standard editing commands of the CAD system. Symbols can be moved or reoriented if necessary. Symbol attribute text can also be added, deleted, or modified. Interconnects can be added, rerouted, or deleted as required. When two interconnecting lines cross but are not intended to be electrically connected, the old method of adding a semicircular bridge is no longer used. Instead, the dot/no dot method is preferred. An interconnect may also be broken, if desired, without affecting its electrical continuity.

Diagram Data Retrieval

CAD systems provide the capability to store and later extract information such as part number, material, vendor. and cost. These data will always accompany the part when it is used on a diagram and can easily be tallied; a report can then be generated by the system. The use of CAD and symbol libraries eliminates the need to constantly re-create the same information.

If attributes are attached to the standard symbols, data can be extracted and used to control NC/CHC equipment, including drilling machines, board profilers, automatic component-insertion machines, and automatic test equipment (ATE). CAD systems today allow for the complete design cycle with drawings, simulation, design documentation, parts listings, and engineers’ reports.

FIG. 33 Electronic diagram.

REVIEW QUESTIONS

1 What is the primary purpose of the schematic diagram?

2 Discuss the primary differences between a schematic diagram, wiring diagram, and a pictorial diagram. For what would each be used?

3 What is a switch table and for what is it used?

4 Why are reference designations required on schematic diagrams?

5 What size drawing paper would be required for a circuit with 100 different components?

6 Why are inset diagrams used on the schematic?

7 What kind of information is usually included in the notes found on schematics?

8 When are diagonals used and why are they recommended in some cases and avoided in others?

9 When would interrupted lines be used?

10 Who has primary responsibility for the correctness of the final drawing?

11 Why is it important to try to have input on the left and output on the right?

12 When does the schematic have sections that are enclosed in boxes and what purpose do the boxes serve?

13 Why can multiple-layer switches be drawn in pieces in various parts of the circuit?

14 Why are power supplies drawn separate from the main circuit?

15 How many trial sketches are necessary before the final drawing can be done?

16 Why are trial sketches made?

17 Explain the dot/no dot method of showing connections and crossovers.

18 What is the physical relationship between the size of the drawing and the size of the actual circuit?

19 What items should be checked before the drawing can be considered finished?

20 Why are graphic symbols used on schematics instead of pictorials?

21 In drawing the schematic, what parts should be done first and why?

22 What could be considered the primary advantage of using a CAD system to generate the final schematic?

23 Does the use of a CAD system eliminate the need for trial sketches and other preliminary placement considerations?

PROBLEMS

When the following problems ask for a trial sketch or preliminary placement sketch, complete the exercise on grid paper. (Ten squares to the inch is recommended.) For drawings that are to be completed as the finished drawing, complete them on plain drawing vellum. (Use of a grid underlay is recommended.) Complete all graphic symbols or components using templates. Lettering should be done by hand; practice good lettering techniques, which were covered in earlier Sections. For adequate practice some exercises should be completed in ink, with Leroy lettering and Leroy symbol templates, as assigned by your instructor.

1. Draw a trial sketch of the circuit shown in the figure for PROBLEM 1.

PROBLEM 1 Rough hand-drawn sketch.

2. Replace the boxes shown in the figure for PROBLEM 2 with the following components:

1. DC battery, 9 V
2. 100-fl resistor
3. 200-fl resistor
4. NPN transistor
5. DC battery, 15 V

PROBLEM 2 Simple transistor amplifier circuit.

3. Given the original sketch in the figure for PROBLEM 3, draw a new trial layout.

PROBLEM 3 Rough sketch.

4. For the circuit drawn in PROBLEM 3, prepare a finished drawing complete with parts list.

5. Make a check print of the finished drawing completed in PROBLEM 4 and demonstrate how to perform the final check using the line-by-line, component-by-component method.

6. On a B size sheet, make a schematic diagram of the circuit shown in the accompanying figure.

PROBLEM 6 Power supply.

7. On a B size sheet, make a schematic diagram of the circuit shown in the accompanying figure.

PROBLEM 7 Discrete flip-flop.

8. On a C size sheet, make a schematic diagram of the circuit shown in the accompanying figure.

PROBLEM 8 AC power input power supply output.

10. For the two-stage RC coupled amplifier shown in the figure for PROBLEM 10, the following components are missing:

1.50-uF ceramic 9. 24K
2. 68K 10. 6.2K
3. 22K 11. 8.2K
4. 5.6K 12. 10-pF ceramic
5. 12K 13. 50-p.F electrolytic
6. 50-btF electrolytic 14. plus 20 V
7. 10-jiF ceramic 15. ground
8. 75K

Both transistors are NPN 2N3904.

Redraw the schematic diagram, including the components listed. Place reference designations beside every device and include a note that says all resistors are tolerance of 10% and ½ W.

PROBLEM 10 Two-stage transistor amplifier.

11 Redraw the circuit in PROBLEM 11 and modify it to include interrupted lines for the ground, positive voltage, and offset horizontal alignment for the transistors.

12. (Advanced problem) From the printed circuit board in the figure for PROBLEM 12, develop the schematic diagram for the power supply. Use standard graphic symbols and note all reference designations and component identifiers.

PROBLEM 12 Power supply printed circuit board.