Re: boom

Splendid explanation!

Haven't used those formulas sense the Vacuum Tube Days.

Used to use the collapsing current of D.C. relay coils by
recirculating the current through a diode (effectively shorting out
the relay). The relay would hold closed several seconds before
dropping out. This simple and cheap delay trick proved handy in many
applications.

Thanks

Tom M.

--- In CAD_CAM_EDM_DRO@y..., "Bevan Weiss" <kaizen__@h...> wrote:
> Keeping in mind that you can express the back emf caused by a
collapsing (or
> building) magnetic field as
> EMF = -L*di/dt;
>
> This means that the voltage produced (EMF) will always oppose the
change in
> current. Whether the current is increasing or decreasing. The
only real
> time that we have to worry about this however is when energy is
stored in
> the magnetic field of the inductor (aka winding). The energy
stored in
> magnetic field of the inductor will be
> E = 1/2*L*I^2;
>
> Thus when the magnetic field is made to collapse, via disconnecting
the
> current source (either mechanically via switch or electrically via
> transistor) the energy will be expended in trying to keep the
current
> flowing, this will cause a large voltage to be produced which will
be
> proportional to the rate at which the current was reduced.
> Generally a transistor will cause the current to reduce more
gradually than
> a simple switch will, although the switch will also bounce more and
thus
> take a longer time to finally settle in the off condition.
>
> In either case the driving circuitry should be protected from the
emf (to a
> certain extent) by the proper use of a snubbing circuit.
Disconnecting the
> windings whilst current is flowing through them (in either the hold
state,
> or whilst moving) is not a good idea, although I'm sure we're
already
> decided on this. The electrical switching (via transistor) should
be done
> in such a way that the the reverse voltage produced by the winding
is
> lessened, and wont cause damage to the driving or switching
circuitry.