> > I'm using a 100nF capacitor in parallel with the Mosfet.
>> It seems to eliminate the spike.

The coils 0.5Li^2 energy will transfer to the capacitor as 
0.5CV^2.
As long as C >= L.i^2/V^2 where i is the interrupted current 
and V is the maximum acceptable voltage then this will allow 
you to then dissipate the energy "at your leisure". You may 
(will) get a degree of ringing which may be unacceptable and 
the capacitor will "slug" (slow) the magnetic decay and thus 
increase release time if it is a solenoid or relay. The 
capacitor ESR will change things slightly but probably not 
enough to matter here.

A diode will dissipate energy via the coil resistance but 
greatly slow the field collapse.

A resistor in series with the diode will allow a V = I.R 
increase in voltage to some acceptable limit (here only 30V 
all included due to FDN377 rating) and reduce dissipation 
time.


        Russell







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