Roger,
Tnx for the MOV info. I'm also considering the addition of a second vacuum
relay inside the M2 prop pitch controller. That would actually be the
easiest solution and not be subject to an accidental chassis to circuit
ground path during a strike event if I were to do as earlier discussed
(i.e., isolate circuit from chassis ground). So, with two vacuum relays:
(1) the MOSFET is protected by the existing vacuum relay; and (2) the second
vacuum relay would apply +48V to the winding only during the time that the
rotator turn is engaged. In fact, the two vacuum relay coils can be
paralleled from the same source.
Tnx to all who provided ideas.
Paul, W9AC
On May 15, 2015, at 9:11 PM, Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
wrote:
Having worked with literally thousands of MOV,s from little ones up to
50,000 Joul ratings, They will typically fail shorted, BUT I have seen
many fail open. It depends on the power contained in the "final spike"
that causes them to fail. From power line spikes and nearby lightening
strikes the ODDS ARE it/they will fail shorted. If enough power/energy is
contained in that final spike at failure time, it becomes likely they will
fail open and that anything on that line will fail as well.
Based on experience, you can expect the MOV to fail shorted and it will
PROBABLY protect any electronic device connected to it. That being the
case, the prop pitch motor will PROBABLY need some sort of "fail safe" to
prevent over rotation.
That's a lot of probables, but it's unfortunately a fact of life, leaving
you playing the odds which favor the MOV failing shorted, but with no
guarantee it will do so. If the MOV works as we all hope, then you will
need some sort of "fail safe"
Since the big ground system was installed my tower has been hit at least 7
times that can be verified with nothing disconnected and NO MOVs in the
rotator lines. No damage was seen.
I can not say the same for my computers and network. A nearby strike did
nothing to either station, but it sure cost me a lot in network and
computer damage. The CAT-5 lines go through protection at the UPS for
each computer. The connectors were melted and the wire contacts welded
together at one UPS. The router, 8 port switch and most NICs were shot!
So, lightening and MOVs are a crap shoot. They will protect you from the
smaller stuff, but cause other problems. They are unlikely to protect you
from the larger stuff.
In the end, I'd use them, plan on the MOVs failed as a short, and hope for
the best There is no guarantee, but you have tilted the odds in your
favor..
73
Roger (K8RI)
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