Re: Lightning Arrestors (was Geek Pig)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Tesla list wrote:
> 
> Original poster: "Richard Wayne Wall by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <rwall-at-ix-dot-netcom-dot-com>
> 
> Hi Bert,
> 
> Excellent pictures of the lightning arrestor.
> 
> I have a question about the silicon carbide "MOV".  What is the
> architecture and composition of the MOV.  Is it classified as a true MOV?
> Years ago I was told that the MOVs in question were innumerable random PN
> junctions and both the forward and reverse voltage drops across the MOV
> were due to the cumulative PN voltage drops.  Does anyone know how they're
> constructed and how they function?
> 
> RWW


Rick,

That's a very interesting question! In older silicon carbide type valve
arrestors, the silicon carbide is not exactly a "MOV" compound, but it
has similar VI characteristics and can handle high instantaneous power
levels. You question reallyu forced me to do a bit of digging in the
dusty bowels of my technical library. One answer come from Edward Beck,
in "Lightning Protection for Electric Systems", 1st ed., McGraw-Hill,
1954, 313pp. The following is extracted from this excellent text: The
particular material is typically a mixture of silicon carbide crystals
(typically 40 to 100 grit) and a suitable binder compressed at high
pressure, and then baked or fired. The unique properties are a result of
the electrical properties of the contacts between the small SiC crystals
- relatively good conducting grains are separated by highly insulating
grain boundaries. 

A couple of more modern explanations can be seen at:
http://www.omsproject-dot-com.jo/publications/training/tm11en.htm
http://www.mse.eng.ohio-state.edu/~ms_rosenthal/mse672/applications.htm

Summarizing from all the references above, it appears that the
fundamental properties stem from a combination of tunelling and contact
phenomena. While Silicon carbide and ZnO are both Voltage Dependent
Resistors VDR's), only ZnO is a true "MOV" (at least chemically
speaking). That distinction aside, the electrical behavior of both is
quite similar. A VDR behaves like a complex interconnected network of
very small resistors... increasing the applied voltage reduces the
contact resistance between the individual grains. The reason ultimately
appears to be related to intergrain tunneling - once breakdown voltage
is achieved, large current conduction results due to tunneling across
the many grain boundaries. The grains themselves are comparatively good
conductors, so once tunneling begins, device current climbs quite
rapidly. 

-- Bert --    
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com