Why is there no voltage induced into the secondary winding (in high-voltage step-up transformer) when the ignition switch is closed (OFF). When the current flows in the primary winding of the coil a magnetic field is generated. This interacts with the flux from the magnet. Assuming that the ignition switch is closed, the flow of alternating current created by the interaction of the two magnetic fields will flow to the ground. Will the flow of alternating current in the primary (while the ignition switch is closed, OFF position) induce a current in the secondary? If it does not, why not?

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    $\begingroup$ Can you edit to make it relate to aviation a little more please? As it currently stands it might get a better response on EE.SE $\endgroup$
    – dalearn
    Jul 1, 2018 at 14:33

1 Answer 1


I assume you're talking about an aircraft mag. Mags are never really OFF off. Instead of ON/OFF, it's more like ON, and "on but not on enough to work".

With the ignition switch OFF, grounding out the primary, the alternating current generated in the primary as the mag spins flows to ground, and it does induce a voltage in the secondary as the current alternates, but this voltage is much too low to be able to jump a plug gap. So the mag is effectively off if not totally off.

To get a spark, the flow of alternating current in the primary needs to be fully cut off by the opening of the points within the primary to create a field collapse strong enough to generate sufficient high voltage to be induced in the secondary.

  • $\begingroup$ It makes perfect sense. The amount of the voltage induced in the primary coil is determined by the rate at which the flux changes (as well as the number of turn of wires in the coil), therefore a collapse of the magnetic field provided by opening points will result in a high induced current. If a reciprocating engine were to run at extremely high RPM, would it provide a rate of flux change high enough to induce a high voltage in the secondary if the ignition switch were to be closed - OFF position? $\endgroup$
    – Simone
    Jul 1, 2018 at 19:01
  • $\begingroup$ Perhaps but I'm guessing the RPM would have to be extreme and the mag would blow up anyway lol. $\endgroup$
    – John K
    Jul 1, 2018 at 23:29
  • $\begingroup$ @Simone I doubt you could turn a piston engine fast enough for the magneto to still make a spark when the switch is off. For the alternation to emulate the collapse time from points opening the engine would need to turn twenty or more times normal operating speed -- call it 50,000 RPM. There'd be nothing left of the engine by the time you got the propeller hub turning that fast. $\endgroup$
    – Zeiss Ikon
    Jul 31, 2018 at 19:12

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