[TowerTalk] Relay lightening protection

[K4SAV]

 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