Chad Kurszewski WE9V wrote:
>
> At 03:20 PM 4/22/97 -0400, Vincent Fedele wrote:
> >You can not open a vacuum relay with voltage present at the contacts, as
> >an internal arc will ionize metal off of the contacts in the process of
> >opening same.
>
> Wouldn't an arc only be present if current was being drawn? My line of
> thinking was that the tube was in cut-off and no current would flow,
> therefore no problem. Obviously this wasn't the case. :)
>
> ---
> Chad Kurszewski, WE9V
Chad:
The application booklet for Jennings vacuum relays notes several
precautions to observe when using vacuum ralays to switch high DC
voltages.
<<The type intended for this application have coils that are especially
well isolated from the contacts.>>
You're correct about interrupting a DC current, which is an even more
challenging task, due to back EMF. Keep in mind, when switching from
amplifier deck-to-deck, small capacitances are being charged, but at
high voltages, therefore, some current may be flowing, even when the
tube(s) are cut off. A small arc will short [internally] to the case of
the incorrectly applied relay, and that's what starts the catastrophic
failure mechanism.
Vince
WA2PKE
>
> ---
> Chad Kurszewski, WE9V e-mail: Chad_Kurszewski@csg.mot.com
> The Official "Sultans of Shwing" Web Site: http://www.QTH.com/sos
>
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