On Sat, 2 Feb 2019 14:44:25 +0000, alan_m
wrote:

On 02/02/2019 13:16, Archibald Tarquin Blenkinsopp wrote:


The MOV's are VDR's, and as long as they are not called on to
dissipate more than their rating, they will work forever.

But their rating is often for as little as 10 to 100 spikes.


They are simply a Voltage dependent resistor. They have an infinite
lifespan if not overloaded.

That's the key "if not overloaded". Define an overload.


VDR's or MOV's do not simply fill up with spikes and fail in their
twilight years.

MOVs have a capacity to deal with a specified number of spikes (of a
certain energy) before they fail. The problem is often the mechanical
construction of the circuit. When a MOV fails it often blows apart. If
it is mechanically constrained by other components and is still held
physically together and can start sparking.


They are a Voltage dependent resistor and whereas your 10K is always
10K whether it has one or ten Volts across it, a VDR is what it say's,
"Voltage dependent".


But the characteristic is the wrong way around for infinite life.

High value resistance at lower voltages = low power dissipation
Low value resistance at higher volatges = high power dissipation -= bang!

In a domestic situation, the life would be infinite. Pulse levels of
hundreds of Amps have to flow before a cumulative effect takes place.
By the time cable resistance and plug contact resistance is taken into
account, there just wouldn't be hundreds of Amps available.

I have never seen one obliterated in a domestic environment. They do
get zapped on factories where the wiring is more substantial and noise
levels are high, even so I have yet to see a periodic maintainance
ticket for MOV replacement anywhere.


AB