Vitaliy wrote:
>> Note that the current able to flow through the series resistor must
>> be less
>> that the supply current if the supply rail is not to be "pumped up"
>> by ths system. This is almost never a problem, but has causght some
>> people out in special situations.
>
> Brrrr... come again?

If the off-board signal goes above Vdd, then the circuit looks like the
signal thru a resistor thru a diode to Vdd.  This dumps current onto Vdd.
Regulated power supplies generally are only designed to dump the necessary
current onto the output to keep the voltage constant.  They are not designed
to sink current to keep the voltage constant.  You therefore have to make
sure that the rest of the circuit is drawing at least as much current from
Vdd that the external signal is dumping onto it.  If not, the Vdd voltage
will rise above the regulation threshold, possible damaging parts and
causing other problems.

In this case this is not a issue since we're only talking about clipping
transients on the line caused by inductive ringing.  These will be short and
of limited energy.  The total average current will be very small to the
point it can be ignored unless you are doing something very unusual.  Any
one single spike that gets clipped has so little energy that the bypass caps
alone will limit the Vdd rise to a miniscule amount.  It would take many
clipped spikes to make a significant rise in Vdd.  That gives the rest of
the circuit time to consume the extra charge as normal operating current.


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