> On May 27, 2006, at 4:48 PM, Spehro Pefhany wrote:
> You can swap out the diode, in the current position, for a TVS or
> beefy zener, and the circuit will work as-is.

Yes, that should protect the transistor, but I still think that's the wrong
place to deal with the relay load dump.  The relay dump current now has to
go from the relay, thru the bottom lead, thru the TVS, thru the power
supply, and then back to the relay.  The voltages at the transistor should
be safe, but there will still be large voltage spikes elsewhere.

There should be something at least to deal with the high frequencies right
at the relay.  A cap accross the relay will limit dV/dt so that other
components can handle the spikes better and to reduce RF emissions.  Then a
reverse diode to catch the current.  If that causes the relay to turn off
too slowly, put a resistor in series with the diode, but make sure the
transistor is rated for the supply voltage plus the relay current times the
transistor.


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