Ferrite desaturation in slow motion

On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:
> George Herold wrote:
> 
> > One purpose of the over wrapped coil was to cancel the
> > Earth's B-field... so that gives you some estimate of the
> > fields involved.
> 
> Yes, fluxgates can be sensitive and accurate, but they are pretty slow.
> The BW of those I know of is <1MHz. Here I have quite the opposite 
> problem: accuracy can be low, and no linearity is required (a magnetic 
> window comparator is what I need), but the time scale of H change is on 
> the order of 10ns. I am trying to figure out what the B change would 
> then be.
> 
> 	Best regards, Piotr

Here is a magnetic field sensor with a bandwidth of 200 MHz:
https://tinyurl.com/y364ubhy

Flyguy wrote:

> Here is a magnetic field sensor with a bandwidth of 200 MHz:
> https://tinyurl.com/y364ubhy

Access denied.

	Best regards, Piotr

On Sunday, September 27, 2020 at 6:07:42 PM UTC-4, George Herold wrote:
> On Friday, September 25, 2020 at 5:40:42 PM UTC-4, Piotr Wyderski wrote: 
> > Hi everyone, 
> > 
> > The following appears to be more physics than electronics, but is very 
> > relevant to the latter and many of you have already amazed me with your 
> > knowledge. So here is the question. 
> > 
> > A ferrite toroid is saturated by some current defined by the geometry of 
> > the core/winding and some material constants. The exact values of I and 
> > B(I) are not important, assume they are sufficiently high. 
> > 
> > Now, as the current is decreased, B(I) eventually decreases to some B_r. 
> > This is a relatively accurate collective description of the underlying 
> > phenomena. But what are these phenomena? What causes the domains to lose 
> > their alignment? Thermal excitations? What is the time scale? What 
> > actually happens in the ferrite when observed at nanosecond resolution? 
> > I know what the situation is going to look like after a microsecond, but 
> > what is the dynamics of the change? 
> > 
> > Could you please suggest me some good reading on the transient phenomena 
> > in ferrite ceramic materials? I would like to understand that far better 
> > and beyond what the typical magnetics design books have to offer. 
> > 
> > Best regards, Piotr
> I know almost nothing of magnetics. But we made this ~flux gate 
> magnetometer out of an inductor* (kinda over driven) and observing 
> it come in and out of saturation was very interesting to me. 
> 
> My only thoughts, 
> George h. 
> 
> *in some external B-field (over-wrapped coil)
Hmmm, forerunner to a flux capacitor in a DeLorean? lol

three_jeeps wrote:

> Hmmm, forerunner to a flux capacitor in a DeLorean? lol

Actually, a very old and ingenious concept, predating the WW2 days:

http://web.mit.edu/6.101/www/s2016/projects/farita_Project_Final_Report.pdf

It is amazing that something that crude can offer this level of performance.

	Best regards, Piotr

On Sunday, September 27, 2020 at 11:28:00 PM UTC-7, Piotr Wyderski wrote:
> George Herold wrote: 
> 
> > One purpose of the over wrapped coil was to cancel the 
> > Earth's B-field... so that gives you some estimate of the 
> > fields involved.
> Yes, fluxgates can be sensitive and accurate, but they are pretty slow. 
> The BW of those I know of is <1MHz. Here I have quite the opposite 
> problem: accuracy can be low, and no linearity is required (a magnetic 
> window comparator is what I need), but the time scale of H change is on 
> the order of 10ns. I am trying to figure out what the B change would 
> then be. 
> 
> Best regards, Piotr

Why would you want a fluxgate (very accurate) measurement?   For 1 MHz and above,
you can detect changing fields quite well with a coil.

whit3rd wrote:

> Why would you want a fluxgate (very accurate) measurement?

Would I?

> For 1 MHz and above, you can detect changing fields quite well with a coil.

I know of no CT with 100MHz+ bandwidth. Only the Rogowski coil comes 
close, but even then it is far from trivial, as the output voltage is 
tiny and the integrator brings its own problems.

I want to detect if current is below some arbitrary threshold (the exact 
value is not very important, an amp or two), but I want to detect that 
event fast and in a lossless manner.

	Best regards, Piotr

On Wednesday, September 30, 2020 at 10:42:05 AM UTC-7, Piotr Wyderski wrote:
> whit3rd wrote: 
> 
> > Why would you want a fluxgate (very accurate) measurement?
> Would I?
> > For 1 MHz and above, you can detect changing fields quite well with a coil.
> I know of no CT with 100MHz+ bandwidth. Only the Rogowski coil comes 
>close, but even then it is far from trivial, as the output voltage is 
>tiny and the integrator brings its own problems. 

Current transformer offerings aren't intended for that, of course, but a Rogowski coil IS
completely acceptable; any radio that tunes UHF has enough bandwidth, and enough
sensitivity, with the coil connected as an antenna.   The 'output voltage is tiny'  isn't an
issue if the signal/noise ratio is good and the impedance is low.

On Monday, September 28, 2020 at 2:28:00 AM UTC-4, Piotr Wyderski wrote:
> George Herold wrote:
> 
> > One purpose of the over wrapped coil was to cancel the
> > Earth's B-field... so that gives you some estimate of the
> > fields involved.
> 
> Yes, fluxgates can be sensitive and accurate, but they are pretty slow.
> The BW of those I know of is <1MHz. Here I have quite the opposite 
> problem: accuracy can be low, and no linearity is required (a magnetic 
> window comparator is what I need), but the time scale of H change is on 
> the order of 10ns. I am trying to figure out what the B change would 
> then be.
> 
> 	Best regards, Piotr

OK, HF magnetics is beyond my experience... (except for winding 
transformers/coils on some ferrite)  
One thing I found interesting with flux gates is how they go 
in and out of saturation... that's a large field effect as 
well as being ~DC.

George H.   

On 1/10/20 3:41 am, Piotr Wyderski wrote:
> whit3rd wrote:
>> Why would you want a fluxgate (very accurate) measurement?
> Would I?
>> For 1 MHz and above, you can detect changing fields quite well with a 
>> coil.
> 
> I know of no CT with 100MHz+ bandwidth. Only the Rogowski coil comes 
> close, but even then it is far from trivial, as the output voltage is 
> tiny and the integrator brings its own problems.

Directional coupler built from a bit of coax?

CH

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:rl2g15$1usdb$1@portraits.wsisiz.edu.pl...
> I want to detect if current is below some arbitrary threshold (the exact 
> value is not very important, an amp or two), but I want to detect that 
> event fast and in a lossless manner.

"Lossless" is a physical impossibility, the real question is how much loss 
can you tolerate, under what conditions?

Notice it's not cheating to use, for example, a CT with a diode shunting the 
burden resistor.  Fine resolution at low currents, modest dissipation at 
high currents (if high average current, or pulse load, is a requirement). 
The burden resistor can be a low value, giving a short time constant with 
the diode capacitance, or transformer strays.  (The transformer does need to 
be good enough for the bandwidth, typically with winding length much shorter 
than the wavelength in question, which significantly limits the number of 
turns.)

Tim

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Electrical Engineering Consultation and Design
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