piclist 2002\10\29\174049a >
Thread: Speeding up a relay - zero crossing detection
www.piclist.com/techref/io/motors.htm?key=relay
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face BY : Russell McMahon email (remove spam text)



> >A resistor in parallel with a zener may give you a better result.
>
> That is essentially the same as the suggestion I made.  However, my
> original reasoning still stands: you want the magnetic field to decay as
> fast as possible which means that you want ZERO current to flow.  That
> means allowing the voltage to rise as high as possible before allowing
> current flow - the zener does this.
>
> I *think*.

Not quite. A capacitor cannot "allow" an instantaneous change in voltage. An
inductor cannot "allow" an instantaneous change in current. At turnoff the
coil current will continue and the voltage will rise to whatever level is
required to support this. With no clamp components the current flows into
the stray capacitance !

You are correct about it being better for voltage to rise as high as
possible (subject to capacitive storage second effects). Higher voltage x
initial current = higher energy dissipation.



       RM

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Subject (change) Speeding up a relay - zero crossing detection

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