What is the max milliamperes the interlock circuit shall draw from the aircraft's 28 VDC circuit?

• A. 5 milliamperes
• B. 50 milliamperes
• C. 20 milliamperes
• D. 100 milliamperes

Answer: (C)

In an interlock circuit, the signal path is kept deliberately low-power so it won’t tax the aircraft’s 28 VDC bus. Limiting the current to a small value ensures enough energy to register the signal while avoiding voltage drops, heat, or unnecessary load on the electrical system. Twenty milliamperes is chosen as the maximum because it provides a robust enough signal for the sensing/logic inputs across typical wiring and contact resistances, yet it minimizes power consumption and potential impact on other equipment on the same bus. Higher currents (like 50 or 100 mA) would waste power and risk larger voltage drops; too little current (like 5 mA) might not reliably drive the input under real-world conditions. Therefore, twenty milliamperes is the appropriate maximum.