So I was repairing a power supply which had a blown fuse and MOV. If MOV gets old and eventually explode, oozing out a metallic stuff, why don't we make em like fuse, easily changeable by just popping a hatch?
A MOV isn't like a fuse that breaks a circuit, it effectively shorts a circuit to clamp transients. Putting it in a socket adds additional contact resistance that could degrade over time which would reduce the performance of the device. Also making it user replaceable like a fuse is inviting disaster, the wrong voltage value WILL be fitted meaning it either doesn't work or it goes bang as soon as it's plugged in.
If MOV gets old and eventually explode
MOVs do not explode when old.
If MOV gets old and eventually explode
MOVs do not explode when old.
Depends on what you define as getting "old".
And as for not exploding ....
The MOV is what is inside the yellow box.
Looking at this photo it seems to me that X2 cap made a good job by preventing fire and soot at it's side.
If it was two X2 caps at bith MOV sides - it could be better.
And it looks unsafe to place a MOV near a fuse.
If MOV gets old and eventually explode
MOVs do not explode when old.
MOVs sometimes explode when they become old but it does not happen out of nowhere. They gradually decrease clamping voltage
when exposed to transients until it becomes too low they overheat/explode.
They gradually decrease clamping voltage when exposed to transients until it becomes too low they overheat/explode.
Yep. That was the fault at the heart of this. I put in a new MOV and the circuit survived for just under 2 minutes before a repeat performance. Quite pyrotechnic it was.
A MOV on the AC mains side always has the possibility of a RUD (random unexplained disassembly) due to the high prospective current supply from the mains. That is why MOV's are typically covered by a heatshrink sleeve. It's a double edged sword, you either protect sensitive parts in the device, or you suffer the consequence of cleaning up the mess.
If MOV's fail then that is a good heads up to look at your AC distribution and earthing system. If you have expensive or unreplaceable gear, then don't just rely on MOV's for protection, spend some time and effort on global alleviation techniques for your AC distribution.
They gradually decrease clamping voltage when exposed to transients until it becomes too low they overheat/explode.
Yep. That was the fault at the heart of this. I put in a new MOV and the circuit survived for just under 2 minutes before a repeat performance. Quite pyrotechnic it was.
Nah, you either fitted the wrong voltage rating MOV or there was something very seriously wrong with your supply. Brand new MOVs don't explode catastrophically after 2 minutes!
Brand new MOVs don't explode catastrophically after 2 minutes!
That is why I always check "Best before" date.
Brand new MOVs don't explode catastrophically after 2 minutes!
That is why I always check "Best before" date.
There is no best before date for MOV. They don't degrade by sitting on the shelf. The same for most of other parts except that lead solderability degradation is a thing.
Brand new MOVs don't explode catastrophically after 2 minutes!
That is why I always check "Best before" date.
There is no best before date for MOV. They don't degrade by sitting on the shelf. The same of most of other parts except that lead solderability degradation is a thing.
They
do degrade each time they suppress an overvoltage, though.
Yeah, MOVs usually come with rating curves -- big boy lightning surges are down in the 100s, or 10s or 1s of cycles lifetime range. There is a limit, where lifetime goes towards infinity; it's just completely useless for surge protection purposes. If you needed so little (a few J) protection, you'd put in a TVS instead.
Or, if you have an application where the pulse width is much longer than direct/induced lightning, in which case there is some economic argument in favor of them (MOVs are cheaper than TVSs, for the same energy capacity for long pulses). That's like 10ms+ pulses.
(Interestingly enough, they do make TVSs for lightning suppression; they do not have a wear mechanism, they just fail shorted (or, if your fault current or fusing isn't very good, they'll probably still blow the fuck up...
) when absorbing too much energy. Downside: they're a bit bigger than MOVs, and
hella more expensive!
)
Tim
They do degrade each time they suppress an overvoltage, though.
You just repeated what I said in the post #5.
MOV's do not degrade when suppressing an overvoltage below a rated level of absorbed energy - read the datasheet. Typically a MOV has to be on a mains supply feed to get to transient levels that could cause degradation - not all applications are mains fed supplies, and not all transients on mains fed supplies would cause degradation.