To implement a two-input AND function in relay logic, how should the input circuits be connected and when is the output energized?

• A. In parallel; energized if any input is active.
• B. In series; energized only if both inputs are active.
• C. In parallel with one input inverted; energized if either input is inactive.
• D. In series with an inverter on the output; energized if exactly one input is active.

Answer: (B)

In relay logic, an AND condition is created by wiring the input paths in series. The output coil will energize only when current can flow through both input paths, which happens when both inputs are active (their normally open contacts close). If either input is inactive, the path is opened and the output cannot energize. Wiring inputs in parallel would give an OR function—the coil lights when any one input is active. Adding an inverter or placing the inverter on the output changes the function away from a simple AND. So the input circuits must be in series, and the output is energized only when both inputs are active.