RM12 core - high temperature.

Anyone know if it's possible to get RM12 cores - say Al = 160nH or thereabouts - which will work at maybe 250'C ? Those I've seen have a curie temp of 210'C which is a bit too close to my ambient 180'C for comfort.

Cheers

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Syd
Reply to
Syd Rumpo
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Have you looked at the materials available to you?

N49 from Epcos has Tc > 240°C

Your bobbin will probably not withstand that.

Reply to
Spehro Pefhany

Thanks, I'll look. The bobbin can be PEEK. ... Just did - 'Optimum frequency range' is 300kHz - 1000kHz and I want

10kHz - 80kHz. I don't know enough about this to know how the optimum is determined and how it behaves outside this.

Any recommendations for an idiot's guide to magnetics? So many variables.

Cheers

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Syd
Reply to
Syd Rumpo

If you get stuck talk to Solvay.

You are almost on the turf of tape cores. I'd talk to these guys:

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As for literature, Amidon is pretty good but there's also lots of others. Lots of variables to deal with and if you've never done a transformer design like this I suggest to get in contact with a good vendor.

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Reply to
Joerg

a

es.

Google is a good place to start. I don't think there is any problem going to a lower frequency. (But I know little about magnetic materials.)

What are you going to solder it with? (I always start to worry if the temp gets above 150C...) Hey, what about wire insulation?

What the bleep are you doing at 180C?

George H.

Reply to
George Herold

Since you already have a core size/shape in mind, then you can look at vendors (eg. Epcos, Ferroxcube) that make that core and decide which available material (if any) fits your application from the material datasheets. The 160nH Al requirement will determine your air gap. Eg.

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etc.

Reply to
Spehro Pefhany

The minimum for a material is more like, you wouldn't want to use it for less because other materials are cheaper, with higher Bmax and mu.

Actually, Bmax is quite good for N49, typical of MnZn. It looks comparable to Ferroxcube 3F35 (or 3F3 or 3F4).

Mu of course doesn't matter here as you'll be gapping it. As long as it's enough to begin with.

Just looking at the Ferroxcube listing, while I'm there, # Tc

3C92 280C 3F45 300C 3S5 255C 3B46 255C

3B46 has unusually high mu and Bsat; with a crossover frequency around

700kHz, it wouldn't be very lossy at 10-80k.

NiZn cores appear to have Tc and mu inversely related, so you could pick a

4E1 (mu = 15) with Tc > 500C, but you'd never get your AL either.

AL seems kind of high for a choke, you might also consider using more copper and less ferrite. Not sure why really, but I suppose the tempco could be better -- less reliance on a teeny gap. Tempcos suck all over, of course.. copper ain't as pretty at 200C (about double R).

It's probably also worth mentioning average transformer varnish softens below 200C. In fact, cooking parts at ~200C is a good way to salvage the cores.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

IIRC I have used N49 for a transformer prototype at 80kHz. If its for a switching regulator you'll get more ringing at lower frequencies because the material isn't so lossy.

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Failure does not prove something is impossible, failure simply 
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Reply to
Nico Coesel

e a

les.

That could actually be an advantage in his application - lossy ferrite gets hot because of the current circulating in the ferrite core itself, and if the ambient is already 180C you want to generate as little heat as possible inside the core. The windings are at least made of copper, and can conduct the heat they generate without too much of a temperature rise - though he probably needs to use forced convection (fan cooling) to minimise their running temperature.

--
Bill Sloman, Sydney.
Reply to
Bill Sloman

"Syd Rumpo"

** For use in a 180C ambient, you will need Teflon bobbins and wire.

Use an oversize core to allow for loss of mu as the Curie temp is approached.

And of course, practically no electronic components made will stand it for long - with the exception of all glass valves.

... Phil

Reply to
Phil Allison

Hi Phil

Usually custom made PEEK bobbins are used as it's easy to machine, and these are very small quantity instruments, but Teflon would work too. If someone already makes such, that would be very useful.

High temperature wire is usually 'enamelled' with polyimide (Kapton) and is good for 250'C.

Electronics at 180'C needs careful selection and testing, and doesn't last too long. That's not always a bad thing :-)

Cheers

--
Syd
Reply to
Syd Rumpo

Using lesser windings may also be an option (increase flux swing IIRC). The core will run hotter but it has more outer surface than the copper inside the transformer.

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Failure does not prove something is impossible, failure simply 
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Reply to
Nico Coesel

There are a lot of materials that are still okay well over 200°C. We use polyimide insulated wire, as well as PTFE. If you had to go really high, for the bobbin, Macor is good to something like 1000°C. More reasonably, the OP mentioned PEEK.. there are also things like Vespel.

Depending on the atmosphere, I'd worry about fine wire lasting very long. I've seen failures of fine Cu wire occurring at pinholes in the insulation. The resistance will increase quite a bit at the high temperatures too- more than +50% compared to room temperature.

On N49 anyway (Tc> 240°C), the mu goes _up_ almost monotonically from room temperature to about 225°C, after which it plummets. There's a little dip between 80°C and 140°C, but nothing significant. A bit surprising.

There are some semis that are claimed to last in excess of 10 years at

180°C (5 years at 225°C!). Expensive, but required in some applications, many of which are okay with limited lifespans. Honeywell, for example.

Changing the subject more than a bit, but I wasn't aware that so-called "all glass" valves require stopcock grease.

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Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

a

I've seen open open air coil forms made of teflon, where the teflon 'flowed' before the wire insulation failed. I have no idea the temperature, perhaps it's better when enclosed in the transfomer.

George H.

r
Reply to
George Herold

Actually, mu always goes up with temperature. Stochastic resonance -- think of it as white noise dithering the magnetic domains' hysteresis.

At low temperature, the domains just stick around. The B-H loop is wider (I think... I'd like to see some data though), and around B=0, it's much flatter (low initial permeability). When looking at a B-H curve, you have to remember it's not actually a nice sigmoidal loop: real materials often have pronounced flat spots around the axis ("butterfly curve"), which manufacturers typically ignore.

At higher temperatures, thermal energy scatters the magnetic domains, raising the ground state energy level, reducing hysteresis but leaving less room for magnetic energy. You can still force the domains into alignment, but you don't get as much flux from doing it -- so, saturation Bmax drops linearly as Tc is approached, hitting zero at Tc.

Presumably, materials with a gradual, progressive curve are a blend of many Tc's, and materials with two spikes are mainly two Tc's. And accordingly, Bmax would drop off in a sum-of-parameters slope. I don't know if this is a correct interpretation or if there's more subtle physics going on.

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

At 120 Celsius, the highest characterized temperature for dissipation, at

100kHz, and 100mT, N49 dissipates 115kW per cubic meter.

Initial permeability at 180C, the OP's operating temperature, is 1820.

Figures from Epcos ferrite magnetic design tool.

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Reply to
Fred Abse

a

for

Various people here, but not all the time have made high temperature "down hole" devices that operate up to 250 C for a short time. Sometimes it is cheaper to make them expendable. They have their methods. I might recognize one if posting to this thread, or i might not.

?-)

Reply to
josephkk

You may have to do some math: by a generally applicable susceptibility law, the Al value will drop as ( T-Tc) **gamma in the vicinity of the Curie point (with some minor correction because the Al value isn't exactly zero with only an empty space for a core). Gamma, for iron, is 1.33; I don't know about ferrites.

Reply to
whit3rd

Thanks for all the suggestions, that's very helpful. I have a fair bit of experience with high temperature work, but not with magnetics.

Cheers

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Syd
Reply to
Syd Rumpo

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