Towertalk
[Top] [All Lists]

Re: [TowerTalk] from towers to shack

To: towertalk@contesting.com
Subject: Re: [TowerTalk] from towers to shack
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Tue, 10 Dec 2013 00:46:22 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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)

_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>