Sorry, I was assumiing that the coil current was DC. However, a sudden
disconnection of the coil would still be likely to introduce a voltage
spike so some sort of supression would be advised.
 If this is the problem then I would have though the MOV you tried
would have showed some inprovement.

If using zeners, connect them back to back and use a voltage rating
2-3 times the applied rms.
However, whatever is doing the relay switching needs to be able to
withstand whatever voltage is likely to be generated.

The term "flywheel" diode comes from it's operation. With coil current
established, if the coil is disconnected the current in the coil
(inductor) will not change quickly and the voltage at the terminals
will increase to attempt to maintain the coil current. The "Flywheel
diode" is placed to permit the current to pass through the coil and
decay gradually due to the coil resistance. The current then rotates
though the diode and coil, in a similar manner to a flywheel winding
down. That's about as well as I can describe it anyway.
By replacing the standard diode with a zener or MOV the voltage builds
across the zener etc., the current is the same (due to the inductance)
and so the power level goes up. Since there is only a limited amount
of energy  in the system,  it dissipates more quickly.


RP

On 12/04/07, Carl Denk <cdenk@alltel.net> wrote:
> Not familiar with term "flywheel diode", tell me more. :)  Since the
> issue seems to be with decaying voltage wanting to be quicker, then the
> zener diodes or MOV's might help. Since this is AC current, I'm assuming
> MOV's might help, and am I correct in assuming that adding a MOV  at the
> coil might be a fix. Tried a V150LA10A MOV across the coil terminals
> without the capacitor, no good. Don't kow how to use zeners with AC
> current.
>
> Richard Prosser wrote:
> > Just a quick comment that may be relevent.
> > The relay will NOT take the same amount of time to connect and disconnect.
> > In order to connect the current in the coil has to increase to a level
> > sufficient to generate enough magnetic flux to start the armature
> > moving. To disconnect, the
> > coil current has to decay to a low level. Normally, the hold current
> > is a lot less that the pull-in current so there is considerable
> > hysteresis involved. If a flywheel diode is used the current can take
> > a noticable time to decay before the actual disconnection occurs. This
> > can be sped up using zener diodes (or MOVs etc.) in place of the
> > flywheel diode if required but the switching device must be OK with
> > the higher voltage generated.
> >
> >
> > RP
> >
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