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.
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.
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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