Magnets on the work bench

Perhaps he means "ferritic". ;-)

The 400-series (ferritic) stainlesses are quite ferromagnetic- you can pick up big chunks with an electromagnet.

If you feel like experimenting, it would be interesting to see if a BFM (big fat magnet) would kill an energized SMPS wall wart.

Did you look at the energy density of the field? Unbelievable.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.blogtalkradio.com/onetribe - Occult Talk Show

On a sunny day (Thu, 11 Feb 2010 15:12:30 +0000) it happened Dirk Bruere at NeoPax wrote in :

There is a site where you can have a star named after you.

I bet its not endorsed by the IAU

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.blogtalkradio.com/onetribe - Occult Talk Show

Hmm, depending on orientation, it might enhance efficiency.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

The type 50M shows a particularly high remenance of 1.40-1.46T, from a magnetization of 860-995kA/m (which means, for a 1 cm thick magnet in a steel fixture, you need to pulse about 10kAt around it -- 100A * 100 turns let's say!).

Assuming the B-H curve is exactly that square, the "energy product" is:

1.4527M T*A/m = Wb*A/m^3 = V*s*A/m^3 = J/m^3.

(Webers are volt-seconds, the amount of flux applied. Volts get into it because integrating EMF over time gives flux, hence, V*s. Flux actually has nothing to do with amps, which sounds funny for magnetism which is all about amps, but it's like how current and voltage are dependent on resistance, there's a connection, it's just not a direct connection.)

But then you'd have to magnetize it yourself!

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

300 series stainlesses (304 and 316 are the most common) are slightly magnetic, more so when work hardened. A flat panel of 316 might not be particularly magnetic, but a sharp bend in the same sheet may be sticky enough to hold a light magnet.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On a sunny day (Thu, 11 Feb 2010 15:46:40 +0000) it happened Dirk Bruere at NeoPax wrote in :

And neither by the Republican party... But who cares, those guys cannot even make up their mind if something is a planet or not. Not the mention the rest of the crap theories they support.

'A star named Panteltje' sounds nice, could be the title of a book. maybe I should fork out for it :-)

On a sunny day (Thu, 11 Feb 2010 09:50:45 -0600) it happened "Tim Williams" wrote in :

Core saturation would _enhance_ efficiency?

I've known people stick floppy discs to filing cabinets with magnets. No harm done apparently

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.blogtalkradio.com/onetribe - Occult Talk Show

On a sunny day (Thu, 11 Feb 2010 21:20:24 +0800) it happened "Royston Vasey" wrote in :

I just tried a strip of SMD zeners, those are attracted indeed!

Some optical media is only specified to last 10 years (though that doesn't mean it will crap out *at* 10).

I've noticed that many of my commercial LVD's have developed laser rot -- though they are now approaching 30 years old.

several partitions on the 1TB.

If you run The Bleeding Edge and had been doing it for almost

20 years (I think 1992 or 1993, trust me, you'd lose. :> I can remember installs from scores of 5" floppies :>

I worked for a hand-tool company one summer. Did wonders for my tool kit! ;-)

several partitions on the 1TB.

Who said anything about power loss? If the OS contains a bug in the driver -- or, in my case, the disk had some funky characteristics in the SCSI interface -- then the VTOC can get trashed in one write.

I never investigated the actual cause. I watched the system trash my disk (4G at the time -- but 4G disks were $1K then). Assuming the drive had failed, I replaced it with my warm spare. And watched that drive get trashed just as quickly.

I then rolled back the OS update and restored the image from an offline backup learning some very important lessons:

- other things besides "disc failure" can take away your data

- have multiple backups

- make sure at least one of those backups is offline

- don't rely on DASD media for backups Of course, there are other thihngs to be learned from this but those were the issues I learned THAT INSTANT! :>

I know that the drive in question was ultimately added to the "fixup table" because of this "problem". But, never sorted out what the actual cause was (I want my tools to *work*, I don't want to spend time *making* them work... updates, etc.)

The magnets on most speakers are sufficiently far from the edges of most cases. Field drops as inverse square of distance so there's little threat from a speaker (to *media* -- you might note that unshielded speakers near a CRT will cause problems, especially for a very high resolution CRT!)

"Refrigerator magnets" are also no threat (to media -- usually) since most refrigerator magnets can barely hold a piece of paper on the refrigerator (nowadays).

Now, where do you draw the line? Do you inspect each magnet that comes into the house and characterize the strength of its field? What criteria do you use to determine "this one is no threat; this one IS a threat"?

When I use my demagnetizer, I move to the far side of the house. It's field is strong enough that it can support a 7" reel of tape. Unfortunately, the only control is on/off so there is no way to slowly ramp the field down in intesity. Apparently the sudden interruption of current can cause a large spike/"magnetic event".

You need to shred the platters if you want the data to be unrecoverable. I'm *sure* there are semi-"photographic" techniques used to "image" data from damaged discs -- if the motivation is high enough. We used to do it with *tape* decades ago...

I discovered that it's a bad idea to put a Nd magnet in the same pocket as a credit card.

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.theconsensus.org/ - A UK political party
http://www.blogtalkradio.com/onetribe - Occult Talk Show

On a sunny day (Thu, 11 Feb 2010 11:17:52 -0700) it happened D Yuniskis wrote in :

I trust my optical disks a lot more then the old magnetic media. I wrote my own CP/M clone, and it used 5 1/4 inch floppies, some of those can no longer be read back (fungus?) also after taking those out of the envelope and cleaning. Same with some 3 1/2 inch floppies. VHS tapes and other tapes develop dropouts. My first Imation DVD+RWs had 'certified for 50 years' written on the disks. I have had dark spots in some TDK disk, probably fungus too, causing read errors. These days (well the last 7 years or so) I only use Verbatim disks. Exactly zero write errors, and no failures. Not one. Then a lot is now on FLASH. FLASH (USB sticks, but especially SDcards) have a simple interface, that will always be easy to reproduce, unlike mechanical rotating stuff. So all you have to do to keep the FLASH alive is every so many years read it in, and write it back. Add a MD5 checksum to see if it is still OK. So the essential stuff is backup-ed at least 3 x here, hard disk, optical, and FLASH. FLASH gets cheaper all the time.

Ya- if it's a flyback with a simple core (I'm thinking rod core, but that's never used, for RFI reasons), the correct polarity will prevent it from saturating in the forward direction. This is actually used: I find a lot of magnetized rod or bobbin type inductors in monitors. They are used to boost the horizontal deflection supply (to allow for different sweep rates on the flyback transformer). Hook it up one way, you get almost twice the saturation magnetization -- hook it up the other and it just goes 'phut'. :-)

Average supplies have a transformer with a closed loop, usually C or E. In this case, one side would get closer to saturation, while the other would get further away. There would still be a small advantage, because the magnetic field under the windings is stronger than in the limb(s), due to fringing. An external bias opposing this would force saturation to occur more evenly (= sharper saturation?), maybe gaining an amp-turn or two (out of ~200At total). Still, 1-2At is slightly more energy.

Now, an average forward converter, that's easy. Even without opening it up, the steel case will saturate quickly, meanwhile the [nearly] ungapped power transformer will saturate quite easily, so it won't really take much external field to screw it up. Most forward converters have current protection, so it would just shut down, unless it's a *really* cheap one, in which case it will pop the transistors and blow the fuse (if you're lucky, the transistors will erupt in flames first :) ).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Agreed. My point is, even *those* may not be "safe".

Yup. You also have to periodically wind/rewind tape to "move" the magnetic domains on one "layer" of tape from the layers immediately above/below to minimize print-through.

Expecting to keep a tape (or any other backup medium) "indefinitely" is a recipe for disappointment.

Yeah, and Bernie Madoff had "a sure thing"! :>

errors.

in, and write it back.

You are better off using a correcting code than a simple checksum. E.g., mount the devices in a raid array and write the data that way (i.e., three devices holding the data of two).

FLASH.

Getting many TB of flash is expensive. :> And, I'm not sure the verdict is yet in on long term reliability of this medium -- especially in the "removable consumer grade" packages. :-/