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While they do not wear out if they are not taking hits. When they do = take hits they suffer some damage and the effect is cumulative. The damage = for any hit is roughly proportional to the energy dissipated. Enough damage and it fails (normally to shorted).
You _can_ get reasonably close with TANH...
And it's stable in a simulator, as opposed to the log-based models which throw up at I=0. ...Jim Thompson
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Remember: Once you go over the hill, you pick up speed
In real life, the reducing impedance will not approach the shorted condition, unless the normally-applied signal is energy-limited. Over-temp and decomposition to explode or to fuse open will result from the higher power application.
Modelling, in either case, would need to evaluate joule-second effects that are not linear - even part style specific - as an accumulated condition. A pen and paper would probably be faster, for a specific design effort. This, along with evaluation of other parts being stressed (ie series limiters) would only give an approximation of useful life - for maintenance purposes.
Some thermally-limited parts are available, with facility for failure indication.
RL
Well, the one I have in mind is catching spikes from across a 210A MOSFET bank made from 6 x 80A MOSFETs, but the DC rating of the MOV is same as DC max voltage on the MOSFETs, so it's possibly not seeing much apart from the sharpest of pins as the MOSFETs will take the avalanche?
Any way, I replaced the MOSFETs and the MOVs just in case. No idea what caused the blowout, metal swarf or filings possible since it's a welder controller.
Yeah, thought as much. Couple times I've met MOVs where I'd put TVS diodes plus snubbers. In this case the inch by inch busbar holding the FETs is not much of an impedance, so perhaps it's the cables, I've not seen the unit, only one PCB for repair.
Thanks, Grant.
Anyone who uses an MOV after the invention of the Transzorb is an idiot.
Hope This Helps! Rich
Both have their places. In terms of energy handling, the hierarchy is still free air, gas/vacuum controlled spark gap, chemically formulated mov and doped silicon. These usually have to be coordinated in a shared environment, with suitably-rated series current and volt-second limiters, to survive the lower-impedance threats. Devices designed for this application will be specified for x number of events at a specific current peak amplitude and wave-shape/duration.
We are NOT talking static electricity, when considering lifetime or degradation due to energy absorption of a(n) MOV.
RL
OK, I must admit that I don't know the physics of the issue from a hole in the ground, I'm "only" a tech after all. ;-)
But I do remember that at my first job out of the USAF (ca. 1976), I was an "engineering tech," and the company made lots of different high-voltage power supplies to drive electron guns, ion guns, X- ray emitters, cylindrical mirror analyzers, and etc; they had MOVs all over the place because nasty arcs were pretty common.
That was also the easiest job interview I've had before or since; the HR guy said, "Bring in your 214," and I was working practically the next day. :-)
Then one day there was an announcement of the Transzorb® in some trade rag; they tried some, and everybody was so amazed at their performance that management issued an order to replace _all_ the MOVs with Transzorbs, even those in the field. (I was one of the guys who wrote the ECOs.)
But that might have had to do with the fact that the nasty arcs were so common that it was cheaper to send out field engineers to replace the MOVs with Transzorbs, as the service call rate plummeted.
I guess I have no quarrel with MOVs in "surge suppressor" power strips; they're cheap, and not too likely to take a direct lightning hit. ;-D
Cheers! Rich
An interesting story. The Transorb trademark was first introduced popularly by General Semiconductor (in 1969), who paid for most of the advertising you refer to. Microsemi called them TAZ or LCE, but never registered the trademarks. They were available previously under the more general label of 'Transient Voltage Suppressor' and are basically beefed up zeners. For a long time, it was not easy to find suitable second-sources, which made them a hard sell and fairly pricey. Only military requirements forced some kind of standardization through Jedec, but never quite covering the range.
Using just one type of part in all situations, would likely lead to some kind of grief. It may have looked automatic, from your end, but it's more likely that a specific circuit was targeted to address the issue mentioned, at that time.
Actually, that is one (...the main) application where, due to frequent misapplication, safety standards have had to be revised. Parts are now being evaluated to a revised UL1449 (3ed) - hence the ~rushed inroduction of thermal limiting into their construction.
RL
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