On 1/18/2021 11:53 AM, [email protected] wrote:
MOV can be used even in AC power applications (50/60Hz) , but only if the
potential thermal run away effect ( drawing more leakage current after
energy absorption/exposure or by aging ) is controlled by a series connected
high impedance (no-fire mode) spark gap. What is however critical under
non-fire RF conditions (MOV -ZnO has big nF capacitance and is a
semiconductor) is the nonlinear diode effect. This can lead to RF
rectification (harmonics, emission issue) before the firing level is
reached.
There's FAR more to it that these issues. The problems include 1) where
they dump the current when they short out a strike; 2) accumulation of
discharge current over time degrades their performance, causing them to
fail shorted or open; and 3) they can start a fire! When an MOV shorts
the strike to the equipment ground ("green wire") it raises the chassis
potential of connected equipment; when that equipment is connected to
other equipment by "low voltage" wiring (audio or video cables, Ethernet
or other computer cables) to equipment plugged into different outlets
(or even a different MOV device), the potential difference between the
two pieces of gear is likely to fry I/O circuitry.
Many years ago, we in the world of pro audio learned this, with
equipment for large audio (that sometimes included video systems) spread
out over wide areas. As long ago as the '90s, very well-educated EEs
recounted stories of computers in their design offices networked by
wired Ethernet being fried by lightning induced on the Ethernet cables
and computers with MOV surge protectors. No intentional antennas
involved (not hams).
All of this is a great reason for using WiFi rather than wired Ethernet
unless the added latency of WiFi cannot be tolerated (remote operation,
for example). I dumped the use of wired Ethernet a couple of years
before moving from Chicago in 2006. Decent WiFi hardware provides plenty
of bandwidth for my uses, including streaming audio and hi-res streamed
compressed video.
73, Jim K9YC
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