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.
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)
John KK9A
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] from towers to shack
From: "Jorge Diez - CX6VM" <cx6vm.jorge@gmail.com>
Date: Mon, 9 Dec 2013 11:29:59 -0200
Thanks to all for the answers, interesting feedback!, thanks!
<snip>
Lighting is something to improve. A project for next year, very expensive
since I have more than 18 cables (coaxial and control lines) going to the
shack. It´s obvious that will need a Polyphaser sponsorship at CW5W :-)
Thanks!
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