Gary Schafer wrote:
> Polyphaser has used gas tubes in their coax protection devices from day one.
>
> The reason for the series capacitor is to allow the gas tube to fire before
> enough energy is passed into the circuit following the protector. If there
> was no capacitor and the radio or cavity following the protector had a coil
> in its circuit that provided a DC ground then the gas tube would not fire
> until a large amount of energy passed into the coil and into the radio etc.
>
> 73
> Gary K4FMX
>
>> -----Original Message-----
>> From: towertalk-bounces@contesting.com [mailto:towertalk-
>> bounces@contesting.com] On Behalf Of Steve Davis -Davis RF Co.
>> Sent: Friday, September 11, 2009 12:12 PM
>> To: towertalk@contesting.com
>> Subject: Re: [TowerTalk] LightningProtectionDevicesPolyP,Response DAVIS RF
>> Co.
>>
>> Hi Steve,
>>
>> Polyphaser's various types of lightning/surge in-coax line devices
>> , in part, incorporate a filter circuit .
>> These circuits are freqcy sensitive thus using an outside spec freqcy
>> application will introduce problematic attenuation to receive signals.
>> A ref. was made to "gas tube", I believe in same context with PolyP.
>> It
>> is only recently that Poly P introduced a gas tube type, which is a slower
>> response time, albeit high current ability ( 50 KA). They stayed away
>> from
>> these for quite some time due to the superiority of their diode and
>> MOSFET,
>> patented devices. But, IMHO they went to gas tubeause there was a proven
>> market for it,
>> less expensive (but what is $ 30 or so more to ultimately protect your
>> cable, your equipment, your wife , kids and the dog (my friend lost his
>> house and dog, now uses PolyPhaser).
>> We discount PolyPhaser. Any questions feel free anytime to contact me.
>>
The actual gas tube (or vacuum gap, such as the Victoreen or Philips
ones used commonly) fires pretty fast (nanoseconds, certainly less than
microseconds), although it's rise time and voltage dependent (more
overvoltage=faster breakdown). The primary limit is the series L of the
leads interacting with the series C across the gap, but that's
nanohenries and picofarads. There's a fairly small time for the ions to
propagate across the (very short) gap, as well.
Looking up gas tube devices, I found the first one in Digikey, a Lumex
GT-BG75L, which claims it breaks down at 75V with 100V/s and 450V with
100V/us rise time (= 4.5 usec).
It also says that with 1kV/us the response time is 10 ns, and with
5kV/us it breaks down in 1 ns. 1kV/ms is a pretty sluggish 10 us.
Zeners and fets are fast, but probably not a whole lot faster,
especially for substantial overvoltage. They might be faster for small
overvoltages.
I suppose a lot really depends on the kind of transient you're likely to
see, and what kind of clamping behavior you want.
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