Ferrite desaturation in slow motion

The usual picture of grain magnetization has small domains which grow and shrink. A true saturation would make the whole grain ONE domain, and coming out of saturation means generating new domain boundaries, not just moving preexisting ones; that's a spontaneous symmetry break, and it always means entropy, i.e. energy loss to heat. Saturation might depend on grain sizes.

Reply to
whit3rd
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"Lossless" = not introducing much additional losses beyond what already is being dissipated. That limits the methods available to magnetic field sensing and busbar voltage drop sensing. 100 amps now, 500A in the future, low voltage, loss budget is 200mW. I want to detect that there is no significant current flowing, I don't care how high the current is if it is beyond some threshold. Quite a similar problem to synchronous rectification.

The sensor needs to be relatively small, fast and reliable and cost under 30 dollars in low volume. Hence my musing about saturable ferrites, which have all the required properties, perhaps except for the speed: 10ns is my pain threshold, 5ns would be perfect.

No problems with that.

Best regards, Piotr

Reply to
Piotr Wyderski

Bus bar? So, this thing is big?

What characteristic impedance is the bus bar? Low SWR? If it's not known with confidence this is a meaningless measurement.

This is where directional couplers take over, with good reason.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

The busbar is 14cm long, 3x16mm copper slab. Dunno if that's big.

I can measure that, but I am not sure the wave properties are relevant at this scale.

I was referring to microwaves just because a saturable reactor sampled at 1GHz would provide me with 1ns resolution (or 500ps, in fact, if I use both halves of the sampling waveform). But the ferrite would need to be comparably fast. For example, switching a 14mm 3R1 toroid takes about a microsecond, which would be 1e3 times too slow. I have various core memory rings from USSR and DDR factories, but I don't expect them to go beyond 10MHz. Don't ask me how to pass 500 amps through a 1mm ID core, though... My googling shows that if ferrite needs to be fast, then that would solely mean the gyromagnetic effects known from the lithium-doped microwave ferrites. That is what Bill Sloman was referring to. Doable in principle, but the complexity would go through the roof at supersonic speed ? apparently, such a beautiful idea killed by such an ugly fact.

Best regards, Piotr

Reply to
Piotr Wyderski

Sadly, no. I was just talking about isolated transition metal atoms in chem ical compounds.

If you wanted to measure a magnetic field fast, a spectroscopic approach mi ght do it. You can get magnetic-field-dependent splitting of electronic ab sorbtion spectra.

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Turning that into a scheme for measuring current in 14cm long 1.6cm wide ,

3mm thick copper bus bar could be tricky. The Brazilians used a tungsten la mp as their light source, but the molecular beam deposition scheme to get t he light-absorbing sample would be more difficult to duplicate.

rsonic speed ? apparently, such a beautiful idea killed by such an ugly fact.

It is a beautiful idea . I wish that I'd had it.

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Bill Sloman, Sydney
Reply to
Bill Sloman

Ok... what else is around it? Currents don't just show up out of nowhere, definitely not at these time scales.

A figure for Zo is basically answering this quantitatively.

What's making the current (or the lack of it), anyway? Why is the time scale so short? Why is current the best way to measure it, why not voltage or whatever?

(You gave synchronous rectification as an example application, but the scales aren't commensurate, based on what I've heard. For example, a ~100A bus bar at mains frequency, could take dozens of microseconds to commutate, who cares. A 100A bus bar strung between IGBT modules would be very much that size, but still not the rate, 100ns being adequate. And neither would seem to require the precision asked for?)

Not quite, no, but you're in the cutoff region where, depending on system Z vs. Zo, either wave properties or their LF equivalents are mandatory to consider. Put another way, you cannot design such a system using statics.

Put still another way -- current transformers for instance don't depend on ferrites at all, they're just improved with them. Apparently being in the HF cutoff region, the core hardly matters at all, and it's all about winding geometry. You can still play with magnetics, sure, but you aren't going to be able to

Or like, what's the repeat rate? You said 200mW (which is a tiny, TINY fraction of anything you'll notice from that bus bar; who's asking, the CIA?..), but the de/magnetization of even a small amount of ferrite, at more than a modest repeat rate, will consume that easily. (The current transformer, given most likely geometries, will probably be an order of magnitude higher, or even more.)

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Design 
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Reply to
Tim Williams

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Reply to
Robert Baer

[...]

Current transformers for ~100A at ns timescales? That's the sort of device I make for particle accelerators. Big expensive things with coaxial geometries. There are two types of ferrite in them: Big rings with highish permeability to extend the frequency response down to about 100kHz, and absorbing tiles so that cavity resonances don't spoil the high frequencies too much. At high frequency, the rings might as well not be there.

I get sub-100ps risetimes, or equivalently, bandwidths of the order of 4GHz. Here's a picture of two of them in CERN's PS: .

Jeroen Belleman

Reply to
Jeroen Belleman

Your "coaxial CT" resulted in a nice finding:

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And their application is very similar to mine. Thanks a lot!

Best regards, Piotr

Reply to
Piotr Wyderski

So is mine, but I would like to change that. I have already performed some crude quasi-static tests with a single toroid crossed fields saturable reactor. These preliminary results are very encouraging; a 2:1 inductance change is easy to achieve. Now I am going to pump it with some GHz reconnaissance waveform and see what happens. The problem is that I have no microwave generator, so I will need to wait for a delivery from Mouser.

If it works, the result will physically look pretty much like a YIG oscillator: one wire through the core to create the toroidal field, one around the equator to create the poloidal component. Just not sure if my layered saturation shockwave theory is right, will see. At low frequencies, where the transient phenomena can be ignored, it works just fine.

Best regards, Piotr

Reply to
Piotr Wyderski

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