>>IOW, what is the optimum value for the glitch resistor all other
>>things (surge capacity) being equal? I think Ian recommends (as a rule
>>of thumb) something like 20 ohms per/KV on his website.
Not my thumb, but Eimac's. Based on the 40A limit that Phil summarizes
below, it would actually be 25 ohms per kV. However, many people tend to
use less than this (in consideration of the voltage drop in normal
operation, especially if there is significant ESR in the capacitor
stack) and it obviously protects the tubes OK.
>
>Eimac's bulletin says that they want the peak energy limited
>to 50 joules. They say that a 50 ohm resistor will limit current
>to 40 amps. If the value of the resistor is larger than necessary,
>it will delay the action of the primary breakers/fuses. All these
>things have to be considered in designing a HV glitch protection
>circuit.
As Phil says, it's actually based on the *energy* that might otherwise
be dumped into the tube. For oxide cathode tubes, the limit they quite
is just 4 joules - yes, four! For tubes up to 1500W, the 40A peak
prospective current (= Vmax/R) is Eimac's rule of thumb to achieve this.
The limits for tubes with thoriated-tungsten filaments are much higher,
but for smaller triodes such as the 3-500Z they quote a limit of 50J,
based on protecting the grid structure.
The energy stored in the capacitor bank is typically 100-200 joules (eg
30uF @ 3kV = 135J) so you have to do *something* to protect the tube...
not to mention the other components through which the surge current
flows (like your grid current meter).
>The Eimac bulletin is a valuable tool in designing and
>testing such a device.
It's high time we had a comprehensive "Flashovers FAQ" page!
As well as Eimac Bulletin #17, my file of photocopies contains similar
information from Siemens and Burle (RCA), and others going all the way
back to the 1930s and earlier. It also contains information about
several kinds of electronic trip circuits.
I will put this all together so that it's all publicly available in one
web page. Then other contributions can be added in (all sources
acknowledged, of course) too pool everyone's knowledge.
Can't say exactly when this will be done, because there's a lot of
material to scan/OCR/edit/write/format (which sounds a lot like my day
job... which is waiting, and it's Monday) but I *will* do it.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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