I though that MOVs degrade with each protection event and that their
breakdown voltage decreases. Is a 45v MOV appropriate for a 33v system?
John KK9A
To: "'Rick Stealey'" <rstealey@hotmail.com>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] STEPPIR QUESTION
From: "Dick Green WC1M"
Date: Wed, 15 Jul 2009 14:48:00 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
When my lightning disaster occurred, I was using K5FD suppressor boards,
which have two MOVs per line plus a 5A inline fuse. Each wire in the control
cable was connected to a suppressor board at both ends of the cable run,
tower and shack, for a total of four MOVs per wire. The ground system at
tower and shack is extensive and well-bonded, with large conductors, and
it's bonded to all other grounds such as AC, telco, etc.
Every 5A fuse blew, indicating that considerable current flowed through each
wire to/from the controller. None of the MOVs was shorted, but it wasn't
possible to tell if they were destroyed in an open condition. As mentioned
earlier, the MOVs were 82V units, which are standard in K5FD and Polyphaser
suppressors.
In trying to prevent a repeat of this incident, I had K5FD replace the MOVs
with 45V units, which is the value recommended to me by K7IR. The problem
with selecting a value for the MOVs is that there's a voltage spike above
the nominal 33V operating level for the driver chips (assuming one is using
SteppIR's 33V power supply.) Further complicating selection is that MOVs
have about a 10% tolerance, so they won't necessarily clamp at exactly the
rated voltage. I've forgotten the research I did on this, but I think the
tolerance is only on the high side. In other words, a 45V MOV won't clamp
below that level, but may not clamp until the voltage is near 50V. My
recollection is that the driver chips have very little headroom for
overvoltage. At any rate, the exact voltage characteristics may vary from
one installation to another (or possibly from one wire to another!)
The problem you suggest, that an MOV shorted by a lightning strike would
result in the chip being destroyed the next time it operates is
theoretically possible, but my sense is that MOVs don't fail in the shorted
condition very often. In other words, I think the odds of that particular
failure scenario are somewhat low. But certainly not zero.
Perhaps a more likely scenario is related to the SteppIR drift compensation
cycle I mentioned in my previous email. What happens if a drift compensation
cycle coincides with the MOV shunting a surge to ground? Won't the driver
voltage also be shunted to ground, resulting in a short across the wire? If
so, the chip would likely be destroyed. If that scenario is possible, then
even if a MOV value can be that would fit within the normal operating and
overvoltage ratings of the chip, it won't do any good if the controller's
drift compensation voltage happens to get shunted to ground during a strike.
As for protecting the rest of the controller, my experience was that the
primary damage was to the driver chips and RS-232 interface. I tried
replacing just the driver chips on one of the boards, but it didn't work. So
it's likely the surge destroyed more components than just the driver chips.
It may not be possible to protect other components unless the power supplies
are isolated. The RS-232 interface is mounted on the driver board, and
probably shares that board's power supply. I think it might be possible to
protect the RS-232 interface by obtaining its power from the CPU board and
isolating it as much as possible from the driver board, but that would
probably take a fair amount of redesign to the boards. My CPU boards were
not destroyed by the strike, and appeared to work correctly for several
months, but subtle, undetected damage to the CPUs began to emerge and
eventually the chips had to be replaced. Hard to imagine how the CPUs can be
fully protected.
SteppIR's new controller optionally addresses some of these problems, but I
still have questions about the detection and cycle time of the relay-based
disconnect circuit, as well as the isolation between the CAT5e cable and the
components at the controller and driver ends of the cable. SteppIR doesn't
100% guarantee that the disconnect will protect the driver board, and
locating it remotely would complicate or eliminate manual disconnect as an
option. Even if optoisolators are used for the CAT5e cable at both ends, I
would think the optoisolaters would be destroyed in a surge (hopefully,
they're socketed.) All that said, if there's reasonable isolation between
the CAT5e cable and the rest of the controller circuitry, the surge won't be
able to propagate through the RS-232 interface, which is what caused the
bulk of the damage in my case.
73, Dick WC1M
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