Answer at bottom.
Wolfert, William R. wrote:
> Gentlemen
>
>
>
> I'm putting together a switchbox for a phased array. I had many spdt relays
> on hand so I'm using them. 2 relays per antenna will work fine. The array is
> an 8 circle
>
> <snip>
> The feed coax is grounded when not in use. Adding a third relay per element
> would allow me to ground the system when not in use, but I'd rather not have
> 3 relays per element. Are diodes across the relay coils necessary? And if so,
> how can I detect a failed diode (a short, correct?) before turning on the
> power? The entire system (house, towers, shack, etc) is properly grounded by
> SPG. Your input is appreciated.
>
>
This thread has morphed into several distinct lines (hot switched and
cold switched) with answers to one line easily confused with the other
and none addressing the subject "relay lightning protection (that I found)
1. As the relays are switching antennas they _should_not be hot switching.
2. Will the relays have diodes across them?
Not unless you put them there.
3. Are the diodes necessary? I can answer that with a definite ...
maybe. The reason is two fold. There are better methods of spike
suppression when power is taken off the relay coil than a simple diode
and are discussed in the thread with pointers to manufacturers data.
OTOH with cold switching and no time requirement a simple diode would
probably be sufficient.
Certainly some form of spike suppression is recommended as the
back EMF from a relay can be up in the 700 plus volt range. The LC of
control leads will suppress this spike to some extent depending on
length and would *probably* be good enough when keyed by mechanical
relay contacts in conjunction with the leads being bypassed at the
source. (neglecting the super tiny relays in some of today's transceivers)
4. How do you detect a blown diode BEFORE turning on the power? Other
than a resistance test for open or short while remembering the
resistance of the coil is across the diode I can't think of a simple test.
73
Roger (K8RI)
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