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Connecting the Dots: Schematic Diagrams

This article originally appeared in the December 2012 issue of The PCB Design Magazine.

Over the years, we have developed ways to control the flow of electricity for many useful purposes. These techniques can be recorded and shared by using diagrams called schematics. A schematic diagram is a convenient and informative method for documenting electronic circuitry.

The Schematic

Anyone who understands basic electronic theory can explain the following schematic, no matter what language he speaks:

The basic building blocks of schematic diagrams use a set of standardized symbols to represent different component types. In the schematic shown above, the symbol next to the BT represents a battery, the symbol next to the S represents a switch, and the one next to DS represents a display or a lamp. The letters BT, S, and DS are labels that help identify the component, and these labels are called reference designators.

Anyone who knows how to interpret the schematic can build the circuit it represents by connecting a battery, a switch and a lamp together with wire or any other form of conductive material.

In addition to reference designators, symbols are also assigned numbers to differentiate components of the same type. For example, if more than one resistor is added to a schematic, they will be labeled R1, R2, R3, etc.

NOTE: For you trivia fans, the reference designator “U” originally meant “unrepairable.” 

When you are ready to learn more, a more complete list of symbols and reference designators can be found in the publications IEEE-STD-315 and IPC-2612.

Values and Attributes

In the example schematic, there is not enough information to know what is intended. The components are identified by a reference designator (the letters BT, S and DS), but there is no information about what type or what size the components should be. Consider the fact that there are many different types of batteries available, but nothing in the schematic above suggests which battery would be best.

The circuit shown above could be:

• a simple flashlight with a size AA battery, or
• a 9V battery, a doorbell button and a buzzer, or
• a 12V car battery, a rotary switch and a headlamp

Applying the same basic circuit to these different applications illustrates the fact that in the real world, a schematic must provide more information. A schematic must include attributes to insure that appropriate components are selected. The same resistor symbol can be used for thousands of different kinds of resistors, so to be useful it must declare the value, expressed in ohms. The ohm symbol is usually dropped because it is not available in all character sets, so a resistor with the number 100 next to it would be interpreted as "a hundred ohm resistor." Other types of components are described with different terms; capacitors are differentiated by their value in farads (F), inductors are differentiated by their value in Henrys (H).Many additional attributes can be added to symbols, such as power ratings or tolerances, to more tightly control the type of component needed for the design.

In addition to visible attributes for each symbol, many CAD systems can also assign hidden attributes such as manufacturer part numbers, cost, revision levels or simulation data. Although a discussion of various CAD capabilities is beyond the scope of this tutorial, sophisticated libraries can be developed with them, and schematics created from them can become very powerful tools.

International Units

Attributes can have a wide range of values, from very small to extremely large. To avoid filling diagrams with long repeating strings of zeros for values like 1,000,000,000 or .0000000001, the world has converged on an International System of Units.

The SI units you are likely to see on schematics are:

Some examples:

• a .000027 Farad capacitor would be expressed as 27 microFarads, written as 27 uF
• a .05 Henry inductor would be expressed as 50 milliHenrys, written as 50 mH
• a 10,000,000 Ohm resistor would be expressed as 10 megaOhms, written as 10 M (most CAD systems don’t use the ohm symbol)

You can get more information about the International System of Units (SI) from the National Institute of Standards and Technology (NIST). They maintain a very good website and the document that describes SI in detail can be found in the Bibliography (online publications and citations) as “Special Publication 811.”

Schematic Design Guidelines

The schematic provides enough information to select appropriate components and connect them together during the circuit board design phase. Schematics also provide a basic level of documentation for electronic products, and become useful tools during design reviews, testing and debugging, field service, technical manuals, etc.

Here are some guidelines for creating good schematic diagrams:

• Lines connecting symbols together represent electrical connections between components. Lines that cross each other are NOT connected.
• A connection dot is used to show connections between lines that touch, but it is better not to have four wires that connect at a point (don't have lines that cross AND connect). In other words, a connection dot will typically have three line segments attached to it.
• In a complex circuit, if every power and ground net were shown connected together with lines, the diagram would be too cluttered, and more difficult to interpret. Instead of showing them connected with lines, we use GLOBAL SYMBOLS. Anything connected with the same global symbol, even across multiple sheets, is assumed to be connected together.
• If other connections need to be made without connecting lines together (across multiple sheets, for example) net names can be used. Any net labeled with the same net name is connected.
• All text (pin numbers, net names, polarity, values and attributes) should be horizontal and clearly labeled to avoid confusion. No overlapping text, No lines crossing through text.
• Circuits should flow from left to right and top to bottom. What this means is, try to keep inputs on the left and outputs on the right, and try to keep positive voltage supplies above and ground symbols below.
• Try to design schematic sections in functional blocks. Symbols are placed on the page to facilitate an understanding of the circuit, NOT to show where they are physically located on the board or to show which connections should be long or short.
• Don't try to fill the whole sheet, and don't be afraid to leave empty areas or open spaces.
• Align similar circuit symbols vertically or horizontally. You might not have time to make everything as beautiful as you wish, but schematics should be easy to read and unambiguous. It may be helpful to use a grid if your CAD tool supports it.
• Pin numbers should be on the outside of the symbol graphic. Net names can be placed inside the graphic if practical.
• If sheet connection symbols are used, they should be placed on the extreme left edge or extreme right edge of the page.
• Depending on the conventions of the company or customer, you may have to provide additional information such as an IC Power Table, or a list of spare gates. These are often placed on the last sheet.
• The first sheet should contain a title block in the lower right corner. At minimum it should contain the Part number, Title, Revision Level and the name of the person who created it. Titles should be distinctive enough to differentiate the design from other similar designs. (You don't want to end up with dozens of designs named "Amplifier," for example). Subsequent sheets may have a reduced title block, but must contain a sheet number and some method of identifying it as belonging to the design (at least the part number and revision level).

Final Advice

None of these guidelines are set in stone, and no two companiesdo things exactly the same way. Try to learn your customers’ preferences as quickly as you can. If you are new, the first thing you should do is study some of their existing drawings. Some companies use "D" as the reference designator for LEDs, while others use "LED." Some use a little circle for test points, but some use that symbol for voltages. Some use a rotated connector symbol for voltages. It doesn't really matter as long as it is clear and consistent. After you have some experience you can suggest ways to standardize if you think it will help.

Finally, if you move to a new company and they do everything differently, don't bother saying, "Well, at my last company, we did it this way." Chances are, they won't care.

Jack Olson, C.I.D.+, has been designing circuit boards full-time for over 20 years. He would like to thank the United States Navy for teaching him the basics.