One thing to watch out for when putting a commutating diode on
a solenoid or relay coil is the fact that it will not loose its
magnetic field as quickly as it did before the diode was added.  This
is because the commutating diode for all practical purposes, feeds
that energy back in to the coil such that it keeps itself energized
for a fraction of a second while the magnet dumps its stored energy
through the now forward-biased diode.

        I was made aware of this as a small boy when I connected a
relay with normally-closed contacts such that it buzzed when energized
from a battery.

        The buzz or chatter was normally fierce with nothing across
the coil.  If you touched the coil, the shock was painful and there
was enough voltage released in the field collapse to light a neon bulb.

        If I put a diode in reverse across the coil, the chattering
turned to a faint buzzing sound and the high-voltage spikes along with
AM radio static were gone.  It was obvious that the relay couldn't
release quite as fast as it used to.  If split-second timing is
important, remember that the solenoid will energize as quickly as
before, but it may hang up for a split-second before releasing.

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