piclist 1997\08\04\235753a >
Thread: Real Time Clock
www.piclist.com/techref/timers.htm?key=real+time
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face BY : John Payson email (remove spam text)



>         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.

Note that the "zero point" in this context is not the zero-voltage point,
but the zero-current point.  If you open at the zero-voltage point a relay
that's driving a large capacitor, it will have to interrupt substantial
reverse current.  If it's driving an inductor, you will have to deal with
substantial foward current that will produce large voltage spikes if
undiverted.
<199708050356.WAA26993@Kitten.mcs.com> 7BIT

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