>Note abt MOVs:
>
>One thing that unfortunately does not appear in the mfr's data sheets, or in
>any parametric/dynamic curves, is the definition of how the MOV crystals are
>arranged and how many joules it takes to fuse each grain.
>
>Traditional MOVs (all that I've seen, anyway) are doped grains of metal
>oxides, primarily zinc oxide, that are placed in series, or in
>series-parallel combinations, to achieve the BV and J rating desired. A
>large surge can fuse one or two grains and not the rest. That results in a
>lower BV. This isn't an open nor a short, it's just a modified MOV which
>has a lower BV than it originally had. Unless you're in the habit of
>removing them to measure this characteristic after each surge, it's
>impossible to know what's happened within the MOV.
>
>As the BV is reduced and gets close to the actual operating voltage, leakage
>current within the MOV causes heating (of course), and as the MOV grains get
>hotter, their BV drifts more until eventually -- bingo! With luck, what you
>normally have is simply a blown fuse and an MOV you need to replace.
? With luck and good engineering, the MOV does not destruct when the
wolf knocks at the door.
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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