making a mag field

Hi Carl, That is a nice book!

Re Helmholtz coils. I think it would need to be pretty small.

There are some coils in this pic,

For the permanent magnet approach one thing to be aware of is that you need to hold the magnets to the steel frame... The two pole pieces are attracted to each other. A donut shaped piece of magnetic material would be ideal.

For less field one could also add some bigger pieces of iron to the field side of the magnetic material.. spreading out the field... again a donut shaped piece of iron might be nice.

RE: field sensors, we use these hall sensors,

George H.

Add pieces of iron to the pole face as a field 'spreader'. That should get the field down. I'd order the gizmo from pasco. $200 is not that much if it will get close to what you want. You could easily blow ~$200 playing a round with the vise idea.... (and you've got to hold the magents to the vis e.. or 'look out'!)

Hmm, fringing fields outside of a super conducting magnet?

George H.

Yeah, I'd have to epoxy or weld the magnets to the jaws of the vise, otherwise they'd fall off?

get the field down. I'd order the gizmo from pasco. $200 is not that much if it will get close to what you want. You could easily blow ~$200 playin g around with the vise idea.... (and you've got to hold the magents to the vise.. or 'look out'!)

rwise

Well no, They'll stick to the iron. But it's my impression that if the po le pieces get too close together they'll be pulled off of the iron...maybe this is not correct? (I'm a little afraid to try the experiment.) But per haps you'll give it a whirl and let us know.

George H.

I wasn't planning to close the jaws on the magnets, but I might try that. I'd expect that, if two magnets touched, the attraction between magnets would be more than magnet-to-steel, so if one then opened the jaws one side of the stuck pair would pull off one jaw. Maybe.

It'll take a couple of weeks for my field meter to show up.

Do you need to detect this threshold or just not have any negative performance effects?

Go for broke and test to 1T with permanent magnets and assess if you need to lower the level.

Most failures will occur from changing magnetic fields not static. Unless you are testing Rats or Reed Relays.

Big magnets can take a nasty nip out of a finger or, as I've had happen, shatter when they slam themselves together.

The customer spec says that the board has to work in fields up to 600 gauss. I don't see how that's going to happen in a VME crate. Our first plan is to try to talk them out of it.

I've seen it happen around big physics experiments. Specifically, I measured about 100mT around some big LEP experiments, and that was just the leakage. Very hard to shield.

Ideally, you want your electronics away from the field, but in an underground area, you can go only so far in any direction.

Jeroen Belleman

This is similar to EMP protection right? If you can't get distance, wouldn't welded iron plate boxes be good? Wouldn't wires going in and out of the box become generators?

sured about 100mT around some big LEP experiments, and that was just the le akage. Very hard to shield.

ground area, you can go only so far in any direction.

Wrong. This thread is about static magnetic fields, not electromagnetic pul ses.

Wouldn't wires going in and out of the box become generators?

Not with a static magnetic field.

John Larkin has said that his board has to work inside a VME crate, so it's difficult to imagine where the welded iron plates might go, and how they m ight shield the relays from the external flux.

If the flux was strictly through the board. you could put a Hall effect fie ld sensor or two fairly close to the relay and have a coil in the plane of the board to generate a counter-acting flux, so that the Hall sensors see e ssentially zero net flux at the relay, but setting up two extra orthogonal coils and sets of sensors to null the field in all three dimensions would g et messy.

Solid state relays might be an easier solution.

The only problem would be the relays, which would be used for BIST, self-test of the input channels.

My ebay field meter has been shipped, so I can test some relays after the New Year break.

One of my customers has a conference room with a chain of paper clips with one end stuck to one wall, sticking out a foot or so at about a

Plain old soft black iron is fairly good at shielding from magnetic fluz lines isn't it? Iron boxes are fairly good for such shielding, right? Is there any flux advantage to placing an iron box inside of another iron box with an air gap or wooden shims gapping the inner box from the outer box? Slowman, how "static" is the super strong magnetic field going to be when the machine is powered on and powered off? How is that situation so extremely unlike EMP protection when an extremely powerful magnetic field is powered on and off? Do you really think that wires going in and out of such a box would not generate current because of the super powerful magnetic field powering off and on?

JL > The only problem would be the relays, which would be JL > used for BIST, self-test of the input channels. Mr. Larkin: Can't that be done with mechanically actuated switches? Like with a plastic or wooden shaft actuating a series of switches instead of a relay? This super powerful magnetic field will be powered on and off with the machine, right? The leading edge and trailing edge will act a bit like EMP, right? Inducing current in every conductor?

Seems really iffy to have a 600 g field in large equipment like that. Is this for an MRI suite? They generally keep all electronics possible out of the magnet room (due to FRI), and have vast mu-metal shields to try to contain the field.

Jon

And welding any magnet will kill it...

Not a problem according to an recent article I read, they seem to think high static meg fields may have some health benefits!

Jamie

The only major health effect I've heard of of is capillary ischemia (restricted blood flow). Apparently red blood cells have a net magnetic moment, so in a sufficiently strong magnetic field, they want to keep a particular orientation. They're too large to go sideways through the smallest capillaries, so they don't.

Cheers

Phil Hobbs