"The key point here is, when a mechanical
   relay switches, the NO and NC contacts are
   MOMENTATILY SHORTED TOGETHER."

I guess this would be *true* for a relay used
in an open-to-the-air environment where a PLASMA
forms (from the 'sparcing' in the air which then
is conductive) between the contacts -

 - as opposed to the use of a vacuum relay which should
not be subject to such a phenomonon?

RF Jim

----- Original Message -----
From: <llile@SALTONUSA.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, November 04, 2002 4:52 PM
Subject: Re: [EE]: Speeding up a relay - zero crossing detection


> >Exactly, and the easiest way to get the fastest
> coil field collapse is to just use no catch diode
> and have a switching transistor rated for the
> 250v spike that the coil produces.
>
> I found this to be true in the lab, Roman.  I could get almost the same
> speed with nothing in parallel with the relay, and also with a 6.2K in
> parallel with the relay but with signifigantly less pulse, about 100Volts.
>
>
> But, sadly, there is no way to turn off this relay fast enough due to
> excessive bouncing.  I changed the relay topology around signifigantly, so
> that the relay shorts across the power diode to achieve full wave or half
> wave power.  voila!  Very little noise on switching.  Didn't have to
> resort to even more extreme measures like parallel triacs.
>
> The key point here is, when a mechanical relay switches, the NO and NC
> contacts are MOMENTATILY SHORTED TOGETHER.  This causes a great amount of
> noise if they are at different voltages and also both in low impedance
> power circuits.  I am talking like
> playing-polo-on-the-antenna-of-a-commercial-radio-station kind of noise,
> that I could not ever filter out of my power supply with any sane
> measures.  When I changed the topology so that the NO and NC contacts
> weren't at different voltages (one of them isn't connected to anything)
> suddelny I was able to see but a tiny blip on the scope when the relay
> switches and no bounce.
>
> If you don't believe the momentarily shorted together bit, consult the
> details of a relay technical catalog.  One contact draws an arc which
> continues to conduct as it approaches the other contact.  If a high
> current can flow at that time, BOOM! it does for one half cycle until the
> arc is extinguished.  OTOH, once the NO contaact finally closes, it, too
> can draw an arc and remain closed.  If both contacts are involved in an
> arc, look out FCC here we come transmitting noise! This is especially true
> at higher voltages and when there is a big inductance anywhere nearby.
>
> -- Lawrence Lile
>

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