Re: Toroid cores for GDTs

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Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> Since powdered iron is used in high power RF baluns at carrier freqs 
> up to several tens of MHz at still high Q I wouldn't be surprised if 
> these would work fine as a GDT, too. :-)

Powdered iron cores don't have terribly high permeability. The 
carbonyl ones for RF use up in the MHz only have a permeability of 
about 10. The hydrogen-reduced ones for low frequency power use (ie 
what you find in the filter choke inside a PC power supply) have 
somewhat more permeability but it's still nowhere near the 2000-5000 
of a Mn-Zn ferrite designed for low-frequency power. So you will end 
up drawing a lot of magnetizing current with these.

I think they look somewhat different from a casual inspection. Mn-Zn 
ferrite (the good stuff for GDTs) is a kind of matt grey. Ni-Zn (the 
stuff EMI suppressors are made out of, not so good, though they still 
seem to work) is a lighter shiny grey and very smooth like marble. 
Iron powder toroids always seem to be painted some colour or other, 
if not two different colours. I have seen them moulded in plastic too.

Steve Conner
http://www.scopeboy.com/