MOVs (was: Transformer update)

[Brad Thompson]

At 07:53 PM 05/04/2002 -0600, rayfri wrote:
><snip>
>      I had no experience or knowlege of MOV's despite 30 years as a
>ham.  The device looks just like a disc capacitor and that's what I
>thought when I first saw it so I didnt pay any more attention to it...
>only to the black, burned spot on the primary side of the transformer.
>Well, I found a small hole in the MOV, cut out the MOV, put in a new
>fuse and turned on the power supply... NO SHORT!!!   Now to connect up
>the regulator circuit to the bridge rectifier again and see what
>happens, but checks on individual components showed no obviously bad
>ones.....    Keeping my fingers crossed...
>Ray  wa7itz

Hello--

"MOV" is an acronym (and probably someone's trademark) for the "metal oxide
varistor".
  IIRC, it's essentially a frozen stew of metallic (zinc?) oxide granules,
and below its
conduction voltage an MOV essentially looks capacitive. When a voltage
spike hits, the
intergranule barriers break down and the clamp the spike's amplitude to a
voltage
that's determined by the MOV's formulation-- and whatever impedance is in
series
with the incoming and return lines feeding the MOV.

Think of an MOV as a bidirectional Zener diode with a more gradual
conduction characteristic
than a Zener presents, and you're get the general idea.

Over time, the granular barriers get "used up" by repetitive spikes and the
MOV begins to look like
a blob of conductive material and will eventually short circuit. I've seen
MOVs with tiny beads or
"tears" of solidified metal extruded through the device's epoxy coating-- a
sure sign that
total failure is at hand.

That said, MOVs are relatively durable transient suppressors when compared
to their selenium
forbears. You must include a fuse or other protective device in series with
the MOV-protected
circuit, and sometimes homebrevwers forget to do this.

A few years back, GE (now Harris) and Panasonic both manufactured MOVs and
probably still do
73--
Brad  AA1IP

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