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Appendix C — The Hardware Level: A Headlight Driver

Three designs for one malfunction, and what each actually buys.

A headlight FET driver is analyzed across three designs for the same top event — both headlights out. One FET is an indefensible single point; splitting into two cuts the risk roughly threefold but moves the limiter to the shared supply and controller; a cyclic sense feedback leaves the aggregate unchanged yet is what makes the redundancy real. Redundancy and diagnosis answer different metrics.