Schematic Circuit Design Rules

The Engineering Web

Here is a collection of rules to follow when making circuit schematics, printed circuit boards, parts lists, and assemblies.


Each circuit design requires the following elements for successful configuration management

  • Schematic
  • PCB
  • Parts List
  • Assembly = PCB + Parts List

Schematic

The Schematic of the circuit design shall have the following requirements:

Drawing Rules

  • Read Appendix E of the 2nd Edition of the "Art of Electronics" by Horowitz and Hill.
  • The schematic diagram shall not use four wire junctions. This screams amateur.
  • The schematic diagram shall use well known units of measurement including:
    • Amps for electrical current - abbreviated capital A
    • Farad for capacitance - abbreviated capital F
    • Henry for Inductance - abbreviate capital H
    • Ohm for resistance - abbreviated with the Greek letter capital omega Ω
    • Watt for power - abbreviated capital W
  • The schematic diagram should use acceptable SI prefixes when denoting units of measure.
    • Example 55pF for 55 pico-Farads.
  • The schematic diagram shall not capitalize any SI Prefix unless there is a 0% chance it could be mistaken by technical and non-technical staff alike.
    • Reason: SI prefixes are case sensitive.
    • Example 55pF is 55 pico-Farads (x10E-12), but 55PF is 55 peta-Farads (x10E15)... a very different number.
    • It is not acceptable that pico-Farads is the obvious value to an electrical engineer, electrical technician or electrical assembler;
    • Those who purchase parts and handle administrative tasks will take you literally.
  • The schematic diagram should lead decimal values with a zero and a period.
    • Good Example: 0.1uF
    • Good Example: 0.047uF
    • Bad Example: .1uF
    • Bad Example: .047uF
  • The schematic diagram shall not contain a space between the numerical value and the SI unit
    • "28k" is preferred to "28 k"
    • Reason: It is easier to parse values in various post processing needs when the values and their scaling units are combined with no "white space" between them.
    • Note when writing papers and other documents that are simply written words, you should adhere to good English practices and separate the numerical value and the scaling unit with a single space.
    • The description in your Bill of Material shall follow the same rules for English writing and place a single space between the numerical value and the SI scaling value.
    • The scaling value shall be combined with the Unit.
  • Reference Designators shall be composed two strings: an alpha prefix and a numerical suffix. The prefix shall be one or more uppercase letters, A-Z. The suffix shall by an integer number.
    • Reason: Keeping this simple is paramount to allowing your assembler to easily know where you want which part.
    • There have been some reports of schematic packages adding long strings to parts which are common to several repeated sheets within a schematic. Altium Designer 6 is one example. This behavior shall be regarded as a schematic capture program defect and should not be acceptable to proper schematic documentation. The fact that OrCAD Capture and its DOS ancestors have been handling complex hierarchy schematics for decades with no unusual issues suggests modern software design packages are attempting to subvert well understood practices to accommodate mal-designed software. Buyer beware.
    • Example: C3 - Capacitor #3
    • Example: R156 - Resistor #156
    • Example: TP45 - Test Point #45
    • Chart of common prefixes:
      • B - Batteries and Cells
      • C - Capacitors
      • D - Diodes, Tranzorbs, Zener Diodes
      • I - Inductors
      • J - Connectors
      • P - Connectors (P is not usually used on PC Boards)
      • Q - Discrete Transistors, BJT, FET, MOSFET, etc.
      • VR - (NOT PREFERRED) Potentiometers and other variable resistance devices.
      • R - Resistors, Potentiometers, etc.
      • TP - Test Points
      • U - Integrated Circuits

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