K5GW wrote:
>Hi Paul, please pass along the information about where to purchase these gas
>discharge devices and to get more info about them.
>
Epcos (Siemens) and Semitron make them, among others. For example, a
search for GAS DISCHARGE TUBE at www.farnell.com will produce 37
different kinds.
Unfortunately the technical data via the Farnell page is a bit sketchy.
There is more information at www.epcos.com under Product Search... Surge
Arresters.
The ARRL Handbook also mentions them in the section on HB lightning
protection. There are sure to be many US suppliers.
>Perhaps this is what we need for our mid size tetrodes used at 432 and
>1296MHz for screen grid to anode flashover protection.
The advantage of these devices is that they will handle huge currents
with a very low clamping voltage... but only IF they strike an arc.
It needs a very high overload voltage to be sure that the tube will
strike very quickly.
VDRs are better if you want them to operate quickly at a relatively
small overload voltage, and they can handle most amateur-sized
flashovers. However, they operate more like a zener, so a higher current
will cause the voltage to rise. They don't clamp the voltage to a low
value, like a gas tube or a thyristor would.
A thyristor is really the ultimate answer for screen protection. With a
zener to trigger it, a thyristor can be made to fire very quickly, at
only a few percent higher than the normal screen voltage. Of course the
thyristor then switches into a low-voltage, high-current condition which
protects the screen very effectively.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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