Schematic Design
Schematic design is the stage in which the circuit's electrical connection diagram is drawn component by component. The concept block diagram turns here into real resistors, capacitors, integrated circuits and reference designators; every component value and voltage rating is decided at this step.
The schematic is independent of the board's physical layout, but all of the production documents derive from it: the bill of materials, the wiring diagram and the test plan come directly out of the schematic.
- Reference designators: codes such as R12, C7, U3 are the same both on the schematic and on the board; this is the first thing looked for during fault finding.
- Rating margin: a 25 V capacitor is not used on a 24 V line. In practice, choosing components rated for at least twice the supply voltage is a common safety margin.
- Protection stage: reverse supply, overvoltage and transient surges on the input pins are explicitly handled in the schematic; they are not added in the field.
When the schematic is complete, review by a second engineer is standard practice. The typical errors looked for in that reading are a controller input left floating, a missing pull-up resistor, a diode drawn with the wrong polarity, and a decoupling capacitor not placed at the supply pins. Board layout is not started before the schematic is approved.
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