How to Read a Schematic

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Contributors: jimblom
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Name Designators and Values

One of the biggest keys to being schematic-literate is being able to recognize which components are which. The component symbols tell half the story, but each symbol should be paired with both a name and value to complete it.

Names and Values

Values help define exactly what a component is. For schematic components like resistors, capacitors, and inductors the value tells us how many ohms, farads, or henries they have. For other components, like integrated circuits, the value may just be the name of the chip. Crystals might list their oscillating frequency as their value. Basically, the value of a schematic component calls out its most important characteristic.

Component names are usually a combination of one or two letters and a number. The letter part of the name identifies the type of component – R’s for resistors, C’s for capacitors, U’s for integrated circuits, etc. Each component name on a schematic should be unique; if you have multiple resistors in a circuit, for example, they should be named R1, R2, R3, etc. Component names help us reference specific points in schematics.

The prefixes of names are pretty well standardized. For some components, like resistors, the prefix is just the first letter of the component. Other name prefixes are not so literal; inductors, for example, are L’s (because current has already taken I [but it starts with a C…electronics is a silly place]). Here’s a quick table of common components and their name prefixes:

Name IdentifierComponent
RResistors
CCapacitors
LInductors
SSwitches
DDiodes
QTransistors
UIntegrated Circuits
YCrystals and Oscillators

Although theses are the “standardized” names for component symbols, they’re not universally followed. You might see integrated circuits prefixed with IC instead of U, for example, or crystals labeled as XTAL’s instead of Y’s. Use your best judgment in diagnosing which part is which. The symbol should usually convey enough information.