[TowerTalk] Remote switching design choice

[Jim Lux]

The advantage of the scheme where you put power to the relay common, and
switching the grounds to actuate one or more of the relays, is that the
switching can be done by open collector NPN drivers.  Low side switching is
very common, but, as John points out, it does leave the inactive terminals
"hot" relative to ground.

----- Original Message -----
From: "John Tait" <bravo at iol.ie>
To: <towertalk at contesting.com>; "Pete Smith" <n4zr at contesting.com>
Sent: Sunday, November 09, 2003 12:15 PM
Subject: Re: [TowerTalk] Remote switching design choice


>
> Subject: [TowerTalk] Remote switching design choice
>
>
> > Can someone explain to me why some remote antenna switch manufacturers
use
> > a scheme that supplies 12V to the relay box and switch the ground side
of
> > the relays to select antennas, while others supply a common connection
and
> > switch the 12V to the desired relay?  I can't see an advantage one way
or
> > the other,
>
> I prefer to supply the voltage to the relay, as I think it's safer. If you
> use the switch-to-ground system, it means that all the unused lines and
> relays are sitting at 12 or 24v potential. If any of those lines short to
> deck, then you've lost all control until it's fixed. With the other system
> all unused lines are at ground potential. and if one of them decks out,
> you've only lost one relay.. Also, all "floating" control lines have the
> propensity to pick up, and re-radiate interference.
>
> 73,
>  John EI7BA  http://www.iol.ie/~bravo/
>
>  >and it does have the effect of creating incompatibility among
> > makers' hardware.
>