"Olin Lathrop" 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.

Thank you for the explanation. It certainly makes sense, but I honestly 
never thought about it this way. I really did think that the regulator would 
sink the excess current, so the amount it could sink would depend on how 
much current the regulator can supply.

I just checked one of our schematics, that uses this type of protection on 
an RS232 line that goes into a logic gate. There is a pretty big resistor in 
series with this line (10K), so I'm sure the rest of the circuit can eat 
whatever excess current gets in through this resistor.

Vitaliy 

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