That OK, the tread is getting a little long. The idea was (is) AC power 
drops,  relay switches hopefully before the PLC drops out. Any thoughts 
about adding MOV's in parallel across the coil, or a smaller relay with 
less mass to the moveable contact and less induction allowing the 
contact to move more quickly. The current relay is 10 amp contacts,  I 
only need 3 amps. (and I'm brainstorming) or a rectifier bridge, DC 
coil, and voltage dropping resistor.

:)

Richard Prosser wrote:
> 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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