From: Tom Rauch <firstname.lastname@example.org>
Sent: Jan 5, 2005 11:40 AM
Subject: Re: [TowerTalk] Help need Vacuum Relay Isolation 1.8 to 30 MHz
If the wiring is what it sounds like to me, you are really
setting yourself up to eventually seriously damage your
> Two relays mounted next to each other in an aluminum box.
> between the relays. Both normally open. Connect one
terminal from each relay
> together and to an SO-239 connector feeding outside the
box to a 50 ohm
> dummy load.
What happens if one relay sticks and does move, has an open
coil and doesn't move, or a wire breaks? What happens if you
get a short someplace? What happens if a contact welds?
Look carefully at your contact wiring. If there is ever a
possibility a relay or electrical failure can cause two
radios to connect to one antenna port, I'd strongly suggest
you buy different relays!!
In the spacecraft power switching world there's all sorts of schemes to come up
with fault tolerant relay arrangements.
There are ways to use SPST relays for this that are single failure tolerant,
but it takes a lot of relays. One scheme is to use 4 relays, contacts wired
series parallel. (Draw a schematic and you'll see how it can work). If your
failure tolerance is just to insure that it always will open (the failed coil
or drive voltage scenario), then two relays in series works.
If you have SPDT relays, you can hook them back to back (like a 3-way light
switch) to create a redundant system. I think you can come up with a
configuration using 4 SPDT relays to do the transfer switch thing with no
"short two radio together" faults.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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