Protection for "remotely located relays" (out at the antenna)
In the case where the relays are remotely located (at the end of a long
control cable) I use absolutely nothing. The cables were about 180-200'
long. I've since changed to two set ups using about 120' and 50'. Time
will tell how they stand up in this environment. With all the
lightning strikes when the leads were longer I had no problems.
Now as we are looking at one installation and anecdotal evidence. I've
had no lightning strikes in the last year and a half, or since I changed
the cables to I can truthfully say the evidence indicates those long
leads were the *apparent* reason my tower took so many hits... <:-))
But, back to the subject, I figure the capacitance and inductance
between the remote relays is such that all that cable makes a good spike
suppressor and have never bothered. OTOH where the control (relay or
SS) is close to the relay coil , I'd go with a small network or MOV.
But with all the direct hits to the tower I've never lost a remote
relay, but that's one installation so the sample is too small to be of
any real significance.
73
Roger (K8RI)
K4SAV wrote:
> Across relay coils I recommend using a zener rated at at least 3 watts
> and preferable at more than that. Look for one with a high current
> surge rating. The reason being that lightning can sometimes induce
> large voltages into long control lines and associated circuitry. There
> is also the problem of voltage drop in the ground due to lightning
> strikes. I have lost many small relays in receiving antennas due to
> burned out coils from lightning when not using zeners for protection. I
> also use these zeners across other electronics located at the antenna
> and in control boxes inside the station that are connected to long
> outside lines.
>
> If there is enough voltage and current induced, these zeners may fail
> and they will fail shorted (unless there is enough energy to remove them
> from the board). (Note: Small diodes usually fail shorted but with
> sufficient current the junctions may be destroyed and they then open.)
> So in some cases you may have to replace zeners after a strike, but I
> find that preferable to replacing relays and a bunch of electronics.
>
> The last lightning strike I had on the tower resulted in 2 shorted
> zeners but no other damage. That was due to a ground voltage surge
> between the tower (where the lightning hit) and the receiving antenna
> 400 ft away. (The actual cable length for this path was 600 ft.)
>
> True a zener, diode, or a cap across a relay coil will significantly
> impact relay speed, but this isn't usually a problem in a manually
> switched antenna. It also does not contribute to early relay failure as
> might be possible in a system where they are switching significant current.
>
> Don't put these zeners across an RF line. They will generate intermod.
> Use a gas tube for that.
>
> Jerry, K4SAV
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