[TowerTalk] from towers to shack

[Roger (K8RI) on TT]

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.
>
>
> 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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