imsai 8080 power switch

[dwight elvey]

>From: "Lee Davison" <lee at geekdot.com>
>
>Hello Dwight
>
> > Please try at least one of two experiments. Either actually measure
> > with an inductive load or do a simulation. For a transformer, zero
> > cross is the worst time to start.
>
>Why? What do you expect to happen? At zero crossing there is no volts
>and no current will flow. The current will start to increase as the
>voltage increases and continue to do so until the next zero cross point.

It isn't the current at the turn on that is the problem. It is that the 
curtrent
builds in the inductor for the full half cycle instead of the 1/4 cycle that
happens once the inductor has reached the continuous state. Normally at the
continuous state, current is just reaching 0 at the peak voltage, for a pure
inductor. If one turns on the voltage at this point, the current will follow
the normal continuous cycle current.
I know it seems counter intuitive but think about it. For the pure inductor,
at the peak voltage, the inductor has no stored energy and can it would be
as though it was already at its normal cyclic state if one started the 
voltage
at this point in time.

>
> > Don't confuse steady state action with the first cycle. The response
> > is not the same.
>
>Agreed. At the first cycle there is no energy stored in the inductor so
>switching it on at zero cross gives a circuit with no stored energy.

But that first half cycle is pushing current into the inductor for not just 
1/4
cycle but a full half cycle.

>
> > With an inductive load, this is at 90 degees, or peak
> > voltage. This is also the best time to turn it on.
>
>If you turn on an inductor at peak voltage you will get a magnetization
>offset that may take many cycles (approx LR) to decay. This gives a peak
>current that is much greater that that from switching on at the zero
>cross point and is clearly not the best point to turn it on.

If it were a pure inductor, as stated above, the magnetic field that
is created by the current in the inductor would be following the normal
current and therefore the normal magnetic field. It is the zero cross
starting that would put the current offset into the inductor that would
have to decay over time.
Remember, the inductor has zero magnetic field when the current is
zero, this happens at the peak voltage in the normal cycle so starting
at this time would cause the least current offset. It is only at the peak
voltage that one can start the signal without causing a current
offset that you describe.
Dwight

>
>Lee.
>