At 11:39 AM 4/18/2010, Janne Ruohom�ki wrote:
>Isn't this something what the diodes installed in parallel with the
>relay coil are for, in transistor controlled circuits ?

Hi Yanne -- yes, that's what the diode is 
for.  It's reverse biased against the coil drive 
voltage, so it doesn't short out the coil; but 
when the circuit opens the collapsing magnetic 
field in the coil drives its input terminal 
negative trying to keep the current flowing.  The 
diode is then forward biased and discharges most 
of the stored energy in the coil.

A capacitor would lower the voltage of the spike, 
but store the coil energy which would then pass 
back and forth, stored as an electrical field in 
the cap and as a magnetic field in the coil as it 
gradually used up the energy.  This is how the 
old Kettering (points-type) ignition system 
works, and why the points burn up rapidly if the 
capacitor fails open or is missing.

Yours,
David