Schematic Symbology

A schematic uses symbology to reveal the anatomy of a system. On a road map, many of the symbols are lines to indicate roadways. But of course, a single black line that portrays State Route 522 doesn't resemble the appearance of this highway as you drive along it! You need only know the fact that the line symbolizes State Route 522. You can make up the other details in your mind. If you always had to see pictorial drawings of highways on paper road maps, those maps would take up thousands of times more space than the folded-up papers that you keep in your vehicle.

On a well-produced road map, you'll find a key to the symbols. The key shows each symbol and explains in simple language what each one means. If a small airplane drawn on the map indicates an airport and you memorize this fact, then each time you see the airplane symbol, you'll know that an airport exists at that particular site as shown on the map. Symbology depicts a physical object (such as an airport) in the form of another physical object (such as an airplane image).

A road map contains many different symbols. Each symbol is human engineered to make sense in your mind. For instance, when you see a miniature airplane on a road map, you'll probably know that this location has something to do with airplanes, so you won't need a detailed explanation. If, on the other hand, the mapmaker used a beer bottle to represent an airport, anyone who failed to read the key would probably think of a saloon, not an airport! Because a map needs many different symbols, a good mapmaker tries to make sure that the symbols make sense.

Logical thinking will only take you to a certain point in devising schemes to represent complicated things, especially when you get into the realm of electronic circuits and systems. For example, a circle (or sometimes an ellipse) normally forms the basis for a transistor symbol, a silicon-controlled rectifier (SCR) symbol, and an electric outlet symbol. Additional symbols inside the circle reveal which type of component it represents.

In the olden days of electronics, engineers used circles containing various electrode symbols to represent vacuum tubes. (Sometimes they still do!) Transistors have evolved to replace vacuum tubes in most situations, so the schematic symbol for a transistor also starts with a circle. Electrode markings go into the circle as before, but transistor elements differ from tube elements, so transistor circles contain different markings than tube symbols did. Transistors perform many of the same functions as vacuum tubes did (and sometimes still do!) so their symbols look somewhat alike, but they're far from identical.

Inconsistencies arise in schematic symbology, so electronics-related diagrams can get a lot more sophisticated and subtle than any road map you'll ever see. For example, you can portray an SCR as a circle with a diode symbol inside and an extra line coming out of it. But an SCR performs a function that differs from what a tube or transistor does. An electric outlet can serve as another example. It doesn't work anything like a tube or transistor or SCR, but the basis for the symbol is a circle, just like the circle for a tube or transistor or SCR.