[TowerTalk] from towers to shack

Tue Dec 10 00:46:22 EST 2013

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 at contesting.com
> Subject: Re: [TowerTalk] from towers to shack
>
> On 12/9/2013 10:35 PM, john at 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 at contesting.com
>> Subject:Re: [TowerTalk] from towers to shack
>> From:Jim Lux <jimlux at earthlink.net>
>> Date:Mon, 09 Dec 2013 06:58:08 -0800
>> List-post:<towertalk at contesting.com">mailto:towertalk at contesting.com>
>>
>> On 12/9/13 6:23 AM, john at 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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>>
>>
>>
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