MOVs (was: Transformer update)

[rayfri]

 IT LIVES!!!   power supply back to full operation..... 13.6 volts output.
I just cut out the MOV, put in a fuse and reconnected the voltage regulator
board and it came alive.  Reason I'm so excited is because this power supply
had
been thrown on the junk heap because of the belief the transformer was gone.
Thanks again to all who offered comments, suggestions and solutions.
Just gotta call Astron monday and see if I can buy a couple MOV's ... one to
replace this one, one to have as a spare....
Ray  wa7itz

Brad Thompson wrote:

> 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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