As I understand it, they degrade when they have to do something and if they have to do something often they will fail and if possible by burning. For a protection device that is pretty lame, so what is their real application?
Thanks, Jenalee K.
Protection devices are designed to protect.
how often is the seatbelt to work ? car bumpers are also doing endless cycles ? car airbags do how many cycles ?
Rene
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In message , dated Mon, 11 Sep 2006, Rene Tschaggelar writes
Once, now. Supposed to be changed after one use.
Not only cycles, but pedestrians and lamp-posts as well.
Definitely one only.
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Its a type of non-linear resistor whose resistance falls considerably when its rated protection voltage is exceeded, in most normal circumstances a transient on the protected line will be absorbed by the varistor and dissipated as heat - however if the powerline outside your house is struck by lightning the power surge is likely to be bigger than the varistor can dissipate, varistors tend to shatter when this happens and then your equipment is "toast"!!!
Lucky it doesn't like to shoot flames up your arse!!!
One in my Panasonic microwave likes to shoot flames out the top,
Having a sufficiently big VDR is pure luck. An over voltage spike is something that shouldn't happen. and thus is rather difficult to anticipate. Yes, usually a fuse is supposed to be in series with the VDR, but we know how the I^2*t of a fuse is. Even a 20mm VDR is long gone before the fuse breaks.
Rene
Because so many purchase grossly undersized protectors and saw them burn, then this myth of varistors burning in normal operation is widespread. Get datasheets for MOVs. No numbers and no specs for an MOV operating in the 'burning' region because that is unacceptable operation.
MOVs degrade. That means its voltage (Vb) changes after numerous operations. An MOV has degraded excessively when its Vb changes 10%. Why can that Vb be measured? Vaporization of MOVs is an unacceptable failure condition meaning the MOV is still functional to be measured. Quoted from one manufacturer datasheet:
MOV life expectancy is defined by joules. Joules is not energy of an entire transient absorbed. MOVs do not protect by absorbing transients. If they did, then MOVs would be series mode devices. MOVs are shunt mode devices. They operate by becoming more 'wire like' during a transient. Energy absorbed by the MOV is due to not being conductive enough. Energy dissipated inside an MOV decreases exponentially as MOV joules increases. From
How to prolong MOV life expectancy? As joules increase, life expectancy increases exponentially. As joules increase, an MOV becomes significantly more conductive during each transient. As joules increase, an MOV degrades less during each transient.
If selling ineffective protectors, then install too few joules. A transient too small to overwhelm protection already inside an adjacent computer, may immediately vaporize MOVs inside that ineffective protector. Then the naive will assume, "The protector sacrificed itself to save my computer", and buy more ineffective protectors. In reality, the computer saved itself. And because it was grossly undersized, that 120 volt protector permitted voltages to rise above
600 volts (voltage when the MOV vaporized must rise excessively during vaporization as made obvious by charts in manufacturer datasheet). So what kind of protection is that? Ineffective. But it sells more grossly overpriced protectors. Extremely profitable to sell protectors with too few joules.MOVs only degrade when properly installed and properly sized. Surges should occur without the human ever knowing the surge existed. That is what effective protectors do when sufficiently sized. Earth a surge and remain fully functional. If MOVs vaporize, then the protector was defective and ineffective - too few joules.
MOVs that vaporize are a human hazard. Some pictures demonstrate plug-in protectors:
Come on over - we'll test it on you.
Nah - practice on your own for a while.
Protection devices.
Tim
In this day and age, for a new design, you don't use a varistor. You use a Transzorb. I don't work for the Transzorb folks, I'm just a happy customer. I once worked at a place where they had a bunch of different HV power supplies, with transzorbs all over the place, and they were a constant headache. When whoever makes Transzorbs announced them in the engineering mags, we ECO'd out all of the MOVs (Metal Oxide Varistor[s]) and put in Transzorbs.
Apparently, the savings in service calls more than paid for the change.
Hope This Helps! Rich
How long did it take you to learn to parallel-park? ;-)
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