[ARC5] ARC5 CW Question

[mac via ARC5]

> On 6/22/2014 7:50 PM, Dennis Monticelli wrote (in part):
>> When the current in the relay is suddenly interrupted it has to go  
>> somewhere; it cannot simply be "blocked."

and On Jun 23, 2014, at 6:27 AM, Mike Hanz wrote (in part)::

>
> This is not quite correct - let me try to explain why.  The collapse  
> of the magnetic field in an ideal inductor could theoretically  
> produce a voltage spike of infinite proportions in zero time.  But  
> we live in the real world, and the relay coil has distributed  
> resistance and capacitance.  A typical small 28 volt relay coil will  
> have perhaps a 200 ohm DC resistance in the winding wire, for  
> example.  Absent any other external current paths, the leading edge  
> rise time of the voltage spike will be lengthened by inter-winding  
> capacitance, and the coil will self discharge the field energy into  
> that 200 ohm resistance on the trailing edge of the spike.  That's  
> what creates the pulse of voltage across the coil terminals.  It's a  
> voltage pulse - not a current pulse - at those terminals.  It can  
> only become a pulse of current if you put an external load of some  
> sort on it.
>

Yes, the energy does have to go somewhere and Mike explains just where  
and how.  A typical relay coil comes with a measure of built-in self  
snubbing capability.

The voltage "kick" on a fast current break can still be very high,  
though, and in circuits such as those found in Command sets, relays  
are typically being energized and de-energized through the contacts on  
other relays or mechanical switches and, in the absence of any other  
snubbing arrangement, ARCING across these contacts on break is  
probably where most of the energy is actually dissipated (snubbed).

Dennis D.  W7QHO
Glendale, CA