FAA Aviation Mechanic Powerplant (AMP)Ignition and Starting SystemsMedium
Inside a magneto coil assembly, the secondary winding consists of many turns of very fine wire wound over the primary winding on a common iron core. What is the primary function of this secondary winding?
- ATo limit the current flowing through the primary circuit during engine cranking
- BTo provide mechanical support for the rotating magnet assembly
- CTo step up the collapsing primary magnetic field into the high voltage needed to fire the spark plug
- DTo store electrical charge that suppresses arcing at the breaker points
Show answer & explanationAnswer & explanation
Correct answer: C. To step up the collapsing primary magnetic field into the high voltage needed to fire the spark plug
The magneto coil acts as a step-up transformer. When the breaker points open and interrupt primary current, the collapsing magnetic field induces a very high voltage in the many-turn secondary winding by mutual induction, which is then routed through the distributor to fire the spark plug.
Why the other options are wrong
- A. Current limiting in the primary is not the secondary winding's job.
- B. Mechanical support is provided by the coil core/frame, not an electrical winding function.
- D. Charge storage to suppress arcing is the function of the condenser, not the secondary winding.
Magneto Coil (Transformer Action)
The magneto coil uses primary and secondary windings on a common core to transform low-voltage primary current into the high voltage needed to fire spark plugs.
- Primary winding: few turns of heavy wire, carries low-voltage current.
- Secondary winding: many turns of fine wire, produces high voltage via mutual induction.
- Voltage is induced when the primary circuit is interrupted by the breaker points.
Memory trick: Few thick turns in, many thin turns out — high voltage shout.