A schematic is a way to record an idea. Schematics are drawings or diagrams that are used to communicate electronic ideas to others. You may have never thought about it, but anyone who understands electronics can look at your schematic and understand your idea, no matter what language they speak!
Schematics are a universal “language” for recording and sharing electronic ideas.
Here’s the official definition from IPC-T-50 (Terms and Definitions):
Here is a brief introduction to how schematics work, with some useful tips and guidelines to keep in mind as you design them. Let’s start with a simple example. Anyone who understands basic circuit theory can explain the following schematic:
A very simple schematic diagram, consisting of a battery, a switch, and a lamp.
The building blocks of schematic diagrams are called symbols, which represent a component or function, and the lines between them represent electrical connections. 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. 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.
The letters BT, S, and DS in the schematic shown above are labels that help identify the component type. These letters are called “class designators”, however, few people use that term because once the components in each class are numbered, it becomes the “reference designator”. For example, if more than one resistor is added to a schematic, they will be labeled with reference designators R1, R2, R3, etc.
Each component in the design will have a unique reference designator.
Different symbols are used for different component types, but keep in mind that different symbol graphics can share the same class designation. There are many different types of transistor symbol graphics, but all transistors will typically be assigned a reference designator that begins with the letter “Q”. There are many different types of transformers, but they will all use the letter “T”. “C” for capacitors, “R” for resistors, “L” for Inductors, etc.
Symbols for Integrated Circuits, Modules (like power supplies) and Sub-Assemblies (like daughter boards) are usually drawn as a rectangle with pins assigned to any or all four sides of the rectangle (usually inputs on the left, outputs on the right, sometimes power connections at the top and ground connections at the bottom). Many components have so many pins that the symbol has to be divided into multiple parts. Thousands of components share this symbol style and most integrated circuits use the class designation “U”.
(For trivia fans, the origin of the reference designator "U" meant "Unrepairable")
If you need to learn more, a comprehensive list of symbols and reference designators can be found in the publications IPC-2612 and IEEE STD 315
Values and Attributes
In the example above, there isn’t enough information to know what kind of circuit is being described. The components are identified by symbol type but there is no information about what the specific components should be. Consider the fact that there are many different types of batteries available, but nothing in the schematic suggests which battery would be best. The circuit shown in the example above could be:
- a 12V car battery, a lever switch and a headlamp, or
- a simple flashlight with a size AAA battery, or
- a laser to destroy the moon
The same basic circuit can refer to different applications, so a schematic must provide more information. The symbols must be assigned attributes to narrow down the possible components that can be used. 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 not used on schematics because it is not available in all character sets, so a resistor with the number 100 next to it would be defining "a hundred ohm resistor". Other types of components are identified with different units. For example, the value for capacitance is expressed in Farads (F), and the value of Inductance is expressed in Henrys (H).
Several additional attributes can be added to symbols, like Tolerance or Power Ratings, to more accurately define the type of component needed for the design.
In addition to visible attributes for each symbol, the Altium Designer software can take advantage of hidden attributes such as manufacturer part numbers, cost, simulation data or revision history.
Beyond simple documentation of a circuit, schematics created with attributes that can be leveraged by other software systems become extremely powerful tools.
The International System of Units (SI)
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, we use an International System of Units to abbreviate values.
SI units you might see on schematics are:
The SI system of units has prefixes for the powers of 1000.
More about SI Units
- Symbols are written in lowercase unless they are greater than 1000 (kilo)
- Symbols are not pluralized. For example, 10uF should never be written as 10uFs
- According to SI there should be a space between the units and the number, but many CAD systems do not follow the standard, so you might see it both ways, i.e. 10 uF or 10uF
You can get more information about the International System of Units (SI) from the National Institute of Standards and Technology (NIST).
Ports and Power Ports
There is one more category of symbols you need to know about: symbols that don't have reference designators.
The symbol types in Altium Designer® called Ports and Power Ports don't represent components, they represent electrical connections. These symbol types can reduce a lot of the clutter that would be created if every connection had to be shown. Adding all the power and ground connections would make the drawing more difficult to interpret. Instead, we can use Power Port symbols. Anything connected with the same Power Port symbol, even across multiple sheets, is assumed to be connected.
The circuit below is equivalent to the one shown previously, but with more detail and less clutter:
Using ports helps designers make more sense of the schematic, and removes clutter.
A Few Schematic Design Guidelines
Here are some more “good practices” to make your schematics more effective:
- When you are creating symbols, place pins on a 100 mil grid. This will make it easier to connect wires and buses at the schematic level. It is possible to use a metric grid for symbols and schematics, but then any symbols imported from other sources (like manufacturer databases, for example) will have pins placed on a grid that is not compatible with yours, and will have to be modified before being added to the library.
- Circuits should flow from left to right and top to bottom. Try to keep inputs on the left and outputs on the right, and try to keep positive voltage supplies above and ground symbols below.
- Design schematic sections in functional blocks. Symbols should be placed on the page to help understand the circuit, not to show the physical location of the part on the board layout.
- All text should be horizontal to avoid confusion. Do not allow text to overlap, and do not draw lines or wires that cross through text.
- Draw the circuit cleanly and don't be afraid to leave empty areas on the sheet. Don't try to fill the whole sheet.
- The first sheet should contain a title block in the lower right corner. It needs to show a Drawing Number and Revision Level, but should also include a Title and the name of the company or designer who created it.
- Taking the time to implement these guidelines will make your document more useful and easier to understand for others down the line who will be using your schematic for various purposes.
For More Information
There is more to learn about schematic entry using Altium software.
Keep checking the Altium blog for a more in-depth article on this subject, but in the meantime, there are some related resources you can explore:
- The Altium Community hosts several Online Videos showing Schematic Features HERE.
- Working with Schematics in Altium Designer (presentation).
- The Altium Online Documentation system has a step-by-step tutorial of a simple design from beginning to end.
Altium Designer puts the design power you need into your hands and you can learn even more about utilizing advanced circuit schematic software with intuitive tools. Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium.
About the AuthorMore Content by Jack Olson