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Ohm's Law and Basic Circuit Relationships

Every electrical troubleshooting job you'll ever do comes back to how voltage, current and resistance relate to each other.

E = I × R (Voltage = Current × Resistance)

Rearrange it and you get the two other forms you need to know cold:

• •I = E / R (Current = Voltage ÷ Resistance)
• •R = E / I (Resistance = Voltage ÷ Current)

Electrical power, in watts, is P = E × I (Power = Voltage × Current).

Series vs. Parallel Circuits

• •Series circuit: the same current flows through every component. Total resistance is the sum of the resistances (R_T = R1 + R2 + R3...). The voltage drops add up to the source voltage.
• •Parallel circuit: every branch sees the same voltage. Total resistance is less than the smallest single resistance. Total current is the sum of the branch currents.

Capacitance and Inductance

Capacitance is a component's ability to store energy in an electric field. It's measured in farads (F). A capacitor resists a change in voltage. It fights sudden voltage changes, lets AC through and blocks DC once it's charged.

Inductance is a component's ability to store energy in a magnetic field. It's measured in henries (H). An inductor resists a change in current. That's why inductive loads like motors and relay coils throw a voltage spike when you switch them off.

Measuring Voltage, Current, Resistance and Continuity

Reading Electrical Circuit Diagrams

Aircraft wiring diagrams use standard symbols for switches, relays, circuit breakers, diodes and solid-state logic devices. To find a fault, you need to trace current through the schematic: start at the power source, follow the path through each switch and protective device to the load, then back to ground or return.

You'll see solid-state logic devices (AND gates, OR gates, NOT gates or inverters) in modern avionics and engine control schematics. An AND gate's output is high only when all its inputs are high. An OR gate's output is high when any input is high.

Battery Inspection and Servicing

• •Lead-acid batteries: check the electrolyte's specific gravity with a hydrometer. A fully charged cell usually reads about 1.275 to 1.300. A low reading means the cell is discharged or sulfated.
• •Nickel-cadmium (NiCad) batteries: don't use a hydrometer. You find the state of charge from voltage under load and capacity testing, not electrolyte density. NiCads can go into thermal runaway if a shorted cell isn't caught.
• •Terminal corrosion: some corrosion around the terminals is a normal result of the chemical reaction. On lead-acid batteries, clean it with baking soda and water. Inspect the battery box and vent lines too. The vents have to stay clear so gases can escape overboard.