PCB Reference Designators
Complete guide to component labels on schematics and circuit boards
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Understanding Reference Designators
Reference designators are short codes used to identify components on schematics and PCBs. They follow the IEEE 315 / IEC 60617 standards, though variations exist.
Naming Convention
Prefix (R) — Component type (Resistor)
Number (101) — Sequential ID, often indicates location (1=first digit of sheet/area)
Suffix (A) — Optional: sub-component (e.g., part of a resistor network)
Common Numbering Schemes
- Sequential: R1, R2, R3... — Simple, assigned in order
- Sheet-based: R101, R102... R201, R202... — First digit = schematic sheet
- Location-based: R_A1, R_B3... — Grid reference on PCB
- Functional: R_PWR1, C_FILT1... — Describes circuit function
Historical Notes
Some designators have interesting origins: Q for transistors came from "semiconductor" since S was already taken by switches. U for ICs originally meant "Unrepairable" — you couldn't fix them, only replace them. Y for crystals represents their oscillating "Y" frequency response curve.