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Self-induction back-voltage - This is back-voltage produced by self-induction. This induced
electromotive force opposes the change in current, restricting it if the current is increasing and
enhancing it if the current is decreasing.
If the rate of change of the magnetic field in a solenoid (relay solenoid, solenoid injector,
ignition coil, inductive sensor for rotation detection) the self-induction back-voltage can reach up
to thousands of Volts. The magnitude of the back-voltage mainly depends on the inductiveness
of the coil and the rate at which the value of the magnetic field is changing. In electromagnetic
executive mechanisms the value of the magnetic field changes the fastest when the field fades
away after a quick shutdown of the powering voltage. In some cases the self-induction effect is
undesirable and precautions are being made in order to reduce it or to remove it. But some
electric circuits are designed to produce the maximum self-induction back-voltage, for example
the ignition system of gasoline engine. Some ignition systems can produce a back-voltage of
self-induction up to 40kV– 50kV. Such voltages can be easily measured with an automobile
oscilloscope by using capacitive pick-up.
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