On 12/10/2013 12:00 AM, GARY HUBER wrote:
Here's another MOV solution;
http://www.arraysolutions.com/images/SurgeSupressorManual_1.pdf .....
simply (8 or 16) 65 volt MOVs in a NEMA box. I mounted the 16 terminal
version at the ground window and cable entrance of my shack. The
ground window is bonded to the tower ground system on the outside and
is also bonded to the supplemental and primary system electrical
ground systems in accordance with the National Electrical Code. For
coaxial cables, I use bulkhead connectors to penetrate the ground
window (aluminum plate , Plexiglas / plastic , aluminum plate
sandwich) with ICE co-axial protectors;
http://www.arraysolutions.com/ManualsandAppNotes/AS-3xxManual.pdf
73 ES DX,
Gary -- AB9M
-----Original Message----- From: Roger (K8RI) on TT
Sent: Monday, December 09, 2013 10:08 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] from towers to shack
On 12/9/2013 10:35 PM, john@kk9a.com wrote:
I wonder why transorbs and gas discharge tubes are not commonly used to
protect control lines in ham radio installations? The $135 Polyphaser
IS-RCT just has eight inexpensive MOVs inside. I used GDTs on one
rotator
where I got tired of replacing leaky MOVs and it has worked fine for
over a
year and though one direct lightning strike.
Gas discharge tubes have too high a breakover voltage to protect most
solid state devices.
They are fine for most rotator controls.
73
Roger (K8RI)
John KK9A
To:towertalk@contesting.com
Subject:Re: [TowerTalk] from towers to shack
From:Jim Lux <jimlux@earthlink.net>
Date:Mon, 09 Dec 2013 06:58:08 -0800
List-post:<towertalk@contesting.com">mailto:towertalk@contesting.com>
On 12/9/13 6:23 AM, john@kk9a.com wrote:
Polyphaser rotator protectors are very expensive. I would not wait too
long to protect your system. You can purchase MOVs for very little cost.
I would suggest using TranZorb type devices (basically back to back
zeners)
as clamps rather than MOVs, or even better, some sort of 4 layer
device like
a Diac.
http://www.vishay.com/diodes/protection-tvs-esd/trans-zorb/
Mouser has them in a variety of voltages and ratings.. about $1-2 each
MOVs die a little each time they take a hit, with the leakage current
increasing. Their only big advantage is that in large quantities they
are
very cheap and a loose tolerance part to manufacture, so consumer plug
strips which need to sell for a few bucks can afford them. They're
cheap to
make physically large, too, which helps with the energy dissipation.
MOVs have two ratings, Voltage and Jouls. Get the with a high enough
voltage rating that they will protect the device, but have plenty of
headroom so it will take a long time to get to that point. Get enough
capacity in jouls so the won't be damage except for a direct strike,.
I've not found MOVs to be satisfactory for protecting SS devices. The
overhead has to be small to keep the voltage down. MOVs are much more
like a voltage variable resistor. The "start" conducting at some
voltage and as the voltage goes up, they conduct more So the voltage
continues to rise with the current to the point of failure. That's why
the higher capacity MOVs (in Jouls) will stand up longer) Still as in
all MOVs the conduction starts lower and lower after each event. Bear
in mind that it's likely the MOV will eventually become a short and
allow for this in the circuit. Zeners of substantial capacity with a
series resistor can be used in conjunction with MOVs for a relatively
low cost. I have seen MOVs fail open from extreme events. I've been
within 6 feet of a 50,000 Joul MOV that simply ceased to exist and the
only thing left was a pair of #6 wires sticking straight out. My ears
rang for over an hour. The MOV was across a pair of back to back 480V
1200 Amp SCRs (they failed too)
BTW the earlier MOVs are rated in break down voltage, not RMS.
OTOH with all the direct strikes my system has taken, I have had no
damage to any equipment including rotators and their controls with no
protective devices. OTOH the next strike could take out the whole station.
73
Roger (K8RI)
A clamp works the same every time. The disadvantage of a Zener type
clamp is
that the voltage drop is constant, so if there's significant current,
the
power dissipation can be high. For a short duration transient this
may not
be an issue, or one can put series impedance in the system to limit the
transient current. An LC low pass filter probably wouldn't be a bad
idea on
a rotor control line, since it's not like you're trying to pass RF.
4 layer negative resistance devices (Diacs), or even neon bulbs (NE2) or
even better, Vacuum Spark gaps, are nice because once they breakdown,
the
voltage drops dramatically, reducing the power dissipation.
Here's a typical gas tube type TVS
http://www.sankosha-usa.com/y08js.asp
Mouser has tons of these from various mfrs, and they run about a 50
cents or
buck each. Look for "Gas Discharge Tubes" The lowest voltage is
typically
around 70V, so they're good for protecting 24VAC circuits (about
twice the
peak V), but not so good for circuits that directly touch ICs, where you
need the 5-10V zener type clamps.
The challenge with a spark gap (in air) is that the minimum breakdown
voltage is around 300V. That's fine for a coax line running power,
but not
so great for a 12 or 24VDC line. So what I would do is combine the
spark gap
(which will clamp to 300-400 V) with some other device to take it the
rest
of the way (with some current limiting component between spark gap
and other
device) (Gas Discharge Tubes use some gas like Argon at low pressure
with a
lower minimum sparking voltage)
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