Why is a diode OR-ing arrangement used when multiple power sources are present?

• A. It averages outputs from all sources to create a stable supply.
• B. It prevents backfeeding between sources and allows the highest available source to power the loads while isolating others.
• C. It regenerates DC power from AC.
• D. It filters out noise from each source.

Answer: (B)

Diode OR-ing lets multiple power sources share the load while keeping them isolated from each other. Each source has a diode in series to the common load, so current can flow from the source to the load, but not from one source back into another. This means the load is supplied by whichever source currently has the higher voltage; if one source drops, the others continue to power the load automatically without backfeeding into the weaker sources. It's a simple, automatic way to provide redundancy and prevent each source from feeding into the others.

Keep in mind the forward voltage drop of the conducting diode, which reduces the load voltage by roughly 0.2–0.7 V depending on diode type (Schottky diodes have lower drops). This needs to be accounted for in the design.

Other options don’t fit because averaging outputs, rectifying AC, or filtering noise aren’t the goals of diode OR-ing.