[TowerTalk] What Killed the Computers? was: Re: Grounding ofAmateur Radio installations

K8RI on TowerTalk K8RI-on-TowerTalk at tm.net
Sat Dec 9 03:40:51 EST 2006

> None damages as far as you know...solid state devices don't always die
> immediately...some fail months or years later from small burnt spots in 
> the
> solid state material of the devices.  In other words the longevity of the
> device may be significantly reduced rather than experiencing an immediate
> poooof.
> I use UPS on my end points (those boxes have the corcom type chokes and 
> the
> 30 cent MOVs, etc. Unlike you Jim we live in a lightning alley where
> voltages vary a lot.  I've replaced quit a few 130v MOVs and I'm happy to 
> do

Unless they've changed the way they rate them, those are way too low a 
voltage .  Line voltage is 120 * 1.414 or basically 170V PP.  The MOVs 
should be at least a little above this voltage.   With DC I try to keep them 
as far above the operating voltage as possible, but below where the 
components will be damaged.

Back when working in industrial instrumentation I built a lot of circuits 
using MOVs from the little ones on DC circuits to 50,000 Joul models across 
a pair of water cooled SCRs about 3 inches across. I think the silicon slice 
was about 1 1/4 inches in diameter.  The MOVs were to protect the SCRs 
operating of 480 VAC from switching transients.  "As I recall those SCRs 
were rated at 1200 Amps and the MOVs were not always sucessful in protecting 
them.  It sure was spectacular when they'd let go and even more so from 
about 3 to 5 feet away. <:-))

I'm for the whole house protection with a substantial ground system for the 
antennas and station with everything tied into a single point ground or as 
near to one as I can get which is not near as I'd like.

> it.  While my neighbors are replacing coffee makers, TVs, refrigerators,
> telephone answering machines, etc. I just plug-in another MOV ;-)

When it comes to TVs and answering machines the MOVs can indeed be 30 cent, 
but to protect the innards of a refrigerator would take a bit more unless 
you have one with solid state controls and are only protecting the controls. 
Typically the compressor motor goes and to protect that would take a much 
larger MOV.

Metal Oxide Varistors  or MOVs are really nothing more than a lot of metal 
oxide diodes compressed into a disk.  Composition and size determine the 
knee of the break down curve with conduction increasing rapidly above that 
point.  _Any _Time an MOV conducts some damage is done to the device 
although depending on the energy in the spike the device may take hundreds 
of thousands of them before giving up. IOW they age with use and their 
breakdown knee slowly lowers.

They can fail open or shorted which makes checking them with a meter only 
partially sucessful. The ones in outlet strips are more prone to failing 
open but they too can go either way.

Where I worked we used to be able to purchase surplus equipment and 
components including control wiring, coax, and shielded wiring.  I have a 
small drawer full  (26) of 130 V 200A MOVs.  There's more writing on them 
but the one I have here has nothing on it.  "I think" that one is 130 VAC PP 
and 2000 Jouls, but the only thing I'm sure of is the 130 VAC PP. It about 
the size of a large .01 bypass cap and twice as thick.  I have another that 
is about 1" in diameter and about 5/8" thick. The only writing on that one 
is Power MOV Varistor.  The big ones were about 2" X 3" and maybe 1" thick 
with a rating to run on 480 VAC RMS and 50,000 Joul capacity.  I don't have 
any as they didn't part with those. Well they did, but that was after one 
failed and about all we could find would be the base plate and leads which 
went in the waste basket.
The reason I have the 26 130s?  They were purchased for a project that used 
120 VAC. proved to be inadequate and were replaced by ones with the proper 
voltage rating.   After sitting around for years they turned up on one of 
the surplus acutions.


Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
www.rogerhalstead.com (Use return address from home page)
> 73,
> dave
> wa3gin
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Lux
> Sent: Friday, December 08, 2006 4:43 PM
> To: Pete Smith; Jim Brown; Tower Talk List
> Subject: Re: [TowerTalk] What Killed the Computers? was: Re: Grounding of
> Amateur Radio installations
> At 12:07 PM 12/8/2006, Pete Smith wrote:
>>Hmmm ... interesting k
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