tempco of permeability for mu metal

[bcarso]

Anyone know? I waded through a bunch of Google results to no avail.

I got a reasonably decent ~80mH inductor with a 77 material cup core---low enough distortion with a little bias current running through---except that the tempco of µ is doing me in. It's changing about 0.5% per degree K, something that would be difficult to compensate. I see that the curve flattens at around 100ºC, but that would be an impractical operating point.

I wonder what Metglas is like (other than expensive)?

 

[edanderson]

no help as far as the tempco, but if you're curious about amorphous materials, contact VAC and get their engineering sample core kit:

http://www.vacuumschmelze.de/dynamic/en/home/products/coresampinductivecomponents/applications/cores.php

they have reps in the USA.

ed

 

[AnalogPackrat]

Can't find a reference for the tempco, but this link indicates Curie point at around 850F is where permeability goes to 1 (magnetically transparent). Does your data point that way?

 

[bcarso]

Thanks for those links. I do have some mu metal and could cut up some strips, maybe, and make a little bundle and wind some wire on it, and do the correction factor for a rod geometry.

The 850F Curie point is at least a second point on the curve! :grin: I suspect the transition to µ of 1 is fairly abrupt though, so a straight-line approximation will likely be pessimistic.

Got the little current transformers Ed, thanks. The one with inductance close to what I'm looking for has more-or-less the expected behavior with d.c. bias current---third harmonic going to a mix of second and third. Distortion just a bit high, and self-res frequency a mite low for the application. I'll have to go back and look at tempco. But interesting, and I suppose fairly cheap.

 

[AnalogPackrat]

[quote author="bcarso"]
The 850F Curie point is at least a second point on the curve! :grin: I suspect the transition to µ of 1 is fairly abrupt though, so a straight-line approximation will likely be pessimistic. [/quote]

One other tidbit from the link I provided:

"Roll off of permeability begins as operating temperature nears (within 25ºF) the Curie temperature..."

Of course no plot or parameters are provided which makes it all a bit murky.

 

[bcarso]

Well that almost suggests that things don't change much below that threshold, which would be good news.

One article on magnetic materials says Permalloy powder cores are high stability, although doesn't say what kind of stability.

 

[AnalogPackrat]

Hmmm. I read it to mean that the tempco is linear until you get near the Curie point. Maybe I should have quoted a larger section:

"MUMETAL® has a positive coefficient of permeability with respect to temperature. This is a quick way of saying that the permeability increases as temperature increases. This occurs until operating temperature nears the Curie temperature (850ºF.), defined as the temperature where permeability goes to 1 and the alloy becomes magnetically transparent. At operating temperatures above the Curie temperature, the shield will not provide any attenuation of the magnetic field. Roll off of permeability begins as operating temperature nears (within 25ºF) the Curie temperature listed, so a safety factor for allowable operating temperature should be used."

 

[bcarso]

Yes, as Emily Latella would say, Well. That's very different then...