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An ohmmeter will only tell you if a MOV is bad by indicating conductivity, typically a short. A good MOV will always measure "open" with an ohmmeter, but this is not a definitive test. It is not easy to tell if the MOV is good. It requires a special test setup to apply an energy-limited pulse of proper amplitude to measure how the MOV turns on and dissipates the energy. The energy in the pulse should be much less than the rated energy of the MOV, lest you damage the device by testing it.
Most MOVs fail by shorting and will physically display a discoloration on the outside when this occurs. When in doubt, replace the MOV with a new one.
Most MOVs fail by shorting and will physically display a discoloration on the outside when this occurs. When in doubt, replace the MOV with a new one.
or physically explode
Tha fios agaibh
- Aug 11, 2014
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Good advice, I will add that they should also be replaced due to age.
I would venture a guess that the majority of movs in equipment 15 years or older for suppression of line voltage, offer little to no protection.
I would venture a guess that the majority of movs in equipment 15 years or older for suppression of line voltage, offer little to no protection.
a lot of things will do this in the right circumstances (transistors, diodes, ...).
recently (last night) basically had a board blow-up, where transistors and similar lit up like Christmas lights (turning bright red) and started spewing smoke and fire, the event also destroying several 30A MOSFETs and similar as well... (not visibly, but the legs are shorted).
nothing exploded per-se (this time), but some of the transistors did shatter afterwards (after cooling). but, yeah, that board is pretty much trashed...
a MOV or TVS diode could probably have prevented this (the board was driving an inductive load, and lost contact with the power supply and blew up).
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