Lawrence Lile writes:
>One other problem is with power relays (which I use).  If you trigger
>a relay at the same spot on the AC sine wave every time it's similar
>to switching DC.  One contact of the relay tends to plate out on the
>other, resulting in pitting and reduced life.  I've been warned to
>introduce a time delay so the relay switches at different polarities.
> Is this a fairy tale from an overzealous relay salesmqan or has
>anyone seen suych a problem?

        What the salesman said is true as far as it goes.  If you could
be sure that the relay opened and closed at the zero-point in the sine wave,
then there would be no pitting and no sparks.  It is hard to do that on
an electro-mechanical device like that.  You didn't say what kind of currents
were being switched, but the bigger the relay, the more inertia there is in
the iron that is being moved and the more uncertainty there is in the exact
microsecond that the contacts will either make or break.

        If I were building such a device, I would try the lazy man's
way, first.  I would look for one of the solid-state relays which was designed
with a zero-crossing detector in the closure circuit.  These usually have a
TRIAC as the switch, (relay contacts), so they will by nature open on the
next zero-crossing after you remove the DC control voltage from the LED
or what would have been the coil.  It would most likely be necessary to
install spike suppressors on the TRIAC side and a device called a snubber
to prevent high-voltage spikes on the mains from tripping the TRIAC, but
this circuit would be electrically clean and quiet as well as acoustically
quiet, also.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group