General, Definitions, Basic Applications

Basic Units and Definitions

The fundamental units in electrical systems are:

• Current ($I$): The flow of electric charge (electrons) through a conductor. Measured in Amperes (A).
• Voltage ($V$): The potential difference between two points. Measured in Volts (V).
• Resistance ($R$): The opposition to the flow of current. Measured in Ohms ($\Omega$).
• Power ($P$): The rate of doing work or transferring energy. Measured in Watts (W).
  • $P = V \times I$
  • Also defined as Energy Transferred / Time (Joules per second).
• Frequency ($f$): The number of cycles per unit time for alternating current. Measured in Hertz (Hz).
• Work: Measured in Joules (J).

Ohm's Law

Ohm's law states that the current passing through a conductor between two points is proportional to the voltage across the two points.

$V = I \times R$

• Voltage is proportional to Current.
• If voltage increases (with constant resistance), current increases.

Circuit Protection

Devices are installed to protect circuits from overcurrent, which is a situation where current exceeds the safety rating of the conductor, potentially attempting to generate excessive heat and causing an electrical fire.

Fuses

A fuse is a thermal protection device containing a strip that melts ("blows") when current exceeds a specific value.

• Function: Protects cables and components.
• Replacement: Must be replaced with a fuse of the correct amperage, voltage, and type (fast or slow blow).
• Incorrect Rating: Installing a higher-rated fuse allows overcurrent to persist, creating a severe fire risk. In a healthy system, an incorrect fuse causes no immediate issue, but offers no protection during a fault.
• Current Limiters: High-amperage fuses (often for TRU outputs) designed to withstand short-term overloads but break on sustained faults.

Circuit Breakers (CB)

A resettable device that combines the function of a fuse and a switch.

• Thermal CB: Relies on a bi-metallic strip heating up and bending to trip the massive latch. Suitable for small, prolonged overcurrents.
• Magnetic CB: Uses an electromagnet to trip immediately upon high current surge. Best for fast response.
• Resetting Rules:
  • If a CB trips, it may be reset once after a cooling period (if necessary).
  • If it trips a second time, it should not be reset again (indicates a permanent fault).
  • Repeated resetting damages the system and increases fire risk.
  • In flight, a tripped CB is usually only reset if the system is essential for safety.

Static Electricity and Bonding

Friction between the aircraft skin and air particles generates static electricity.

• Bonding: The connection of all aircraft metal components with flexible wire strips.
  • Purpose: Ensures equal electrical potential across the aircraft (zero voltage difference).
  • Prevents: Sparks between components (fire hazard) and electrical noise.
  • It also provides a return path for current (earth return).
• Static Wicks: Conductive rods on trailing edges.
  • Purpose: Dissipate static charge back into the atmosphere.
  • Failure: Missing wicks can cause radio interference (static noise on comms) and loss of communication.

AC Power Standards

Modern large aircraft typically use Alternating Current (AC) generators.

• Standard Values: 115/200 Volts, 3-phase, 400 Hz.
• Frequency: 400 cycles per second. High frequency allows for lighter generators and transformers (efficient for aerospace).

Logic Gates

Logic circuits allow complex decision-making in systems.

• AND Gate: Output is 1 only if ALL inputs are 1.
• OR Gate: Output is 1 if ANY input is 1.
• NOT Gate: Inverts the input (1 becomes 0).
• NAND Gate: AND followed by NOT.
• NOR Gate: OR followed by NOT.

Electromagnetic Interference (EMI)

Current flowing through a wire creates a magnetic field that can induce unwanted signals in nearby wires (crosstalk/interference).

• Shielding/Screening: A conductive braid (mesh) wrapped around the wire.
• Function: Blocks EMI from entering or leaving the wire.
• Result: Prevents interference with other systems (e.g., radios).

Capacitors and Inductors (General)

• Capacitor: Stores energy in an electric field. In AC circuits, current leads voltage. High frequency reduces capacitive reactance (current increases).
• Lorentz Force: Force exerted on a charged particle moving perpendicular to a magnetic field.