John Popelish wrote:
John Larkin wrote:
On Sat, 12 Aug 2006 18:28:14 -0400, John Popelish <jpopelish@xxxxxxxx>
John Larkin wrote:
So any MOV can absorb a megajoule?
100 of the largest of these in parallel might.
(10 kj each)
Yeah, but the big one clamps at about 7 kilovolts!
Some of the curves sort of imply a lifetime of 1 to 1e6 shots,
depending on the energy per shot, with a fuzzy disclaimer.
Oh, you have a clamping voltage requirement, also? ;-)
I would expect any MOV that absorbed its rated joules in a single pulse
to have fulfilled its purpose and be ready for replacement. Derate that
peak energy to half and it is an entirely different situation. Derate
it to 1/10th, and it can be expected to last a long time, as long as it
cools off between pulses. I don't think integrated energy is a very
good way to calculate life. It wears out by hitting peak temperature.
You want to replace the MOV anytime your
air conditioner switches? Its no 1shot device,
it acts like a zener diode, one voltage for any
current you apply.
It reduces any line transient to its rated voltage,
whatever the current.
Lightning by the way causes most damage by
secondary induction, up to 1000+ volts/meter
(or yard), and this happens much more than
direct hits, those are almost always fatal.
Those induction spikes can be found in a wide
range of voltage and current, for which the MOV
can be a nice protection, dont forget that a Megawatt
pulse of one microsecond duration is only 1 joule,
so that a 40 joule MOV can dissipate 40 MW for 1 usec,
and thats much more than your electronics can stand.
The rated joules is just the amount of energy/heat,
that will cause a tolerable temperature jump,and some damage,
because the MOV is overheated in some spots,with local failure.
When to much local damage is done, the MOv voltage will
go below the mains voltage,and it will burn out/blow the