Electrical Fundamentals
Electrical fundamentals
This domain tests the DC and AC theory an AME uses to diagnose aircraft circuits: Ohm's and Kirchhoff's laws, series and parallel networks, magnetism, inductance and capacitance, AC impedance, power, transformers, and rectifiers. Expect numerical questions and questions on what a meter should read in an isolated or live circuit.
Core rules
- Ohm's law: V = IR; power: P = VI = I²R = V²/R. Use consistent units.
- Kirchhoff: currents into a node sum to currents out; voltage rises equal voltage drops around a loop.
- Series: same current, resistances add, voltages divide. Parallel: same voltage, conductances add; two resistors R1R2/(R1+R2).
- In AC, impedance Z combines resistance and reactance: X_L = 2πfL, X_C = 1/(2πfC); |Z| = √(R² + (X_L − X_C)²). Aircraft AC is commonly 115 V at 400 Hz, so reactance is strongly frequency-dependent.
- Transformer action: V_s/V_p = N_s/N_p; current ratio is inverse for an ideal transformer. A transformer does not change frequency.
- Capacitors block DC and pass AC; inductors pass DC and oppose changes in current. Time constant τ = RC or L/R.
Worked example
A 28 V DC bus feeds a 56 Ω heater and a 28 Ω lamp in parallel. Branch currents: 28/56 = 0.50 A and 28/28 = 1.00 A, so total current is 1.50 A. Equivalent resistance is 28/1.50 = 18.7 Ω, matching 56×28/84. Total power is 28 × 1.50 = 42 W. A 5 A breaker is adequate for the load current, but it must also protect the smallest wire in the circuit; load current alone does not set the protection rating.
Traps
Mixing rms and peak AC values; forgetting that 400 Hz changes reactance; assuming a transformer changes frequency; adding parallel resistances directly; using a meter on the wrong range; and calculating power from a nameplate rating without checking the actual supply voltage.