Author Topic: Spacers to gap ferrite cores with?  (Read 2136 times)

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Offline FaringdonTopic starter

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Spacers to gap ferrite cores with?
« on: January 24, 2024, 03:08:28 pm »
Hi ,
The below gapped ferrite cores, formers and former-clamps are needed
for a two transistor forward and a flyback design.

There are none on RS, Digikey, Farnell, or Mouser.
Norwe, Kaschke and Pulse Magnetics also dont have them available
to purchase.

So we will try to gap them ourselves with a plastic film
in between the cores.
The 420um gap will need us to have a plastic spacer of thickness
210um.
The 620um gap will need us to have a plastic spacer of thickness
310um.

Do you know where such precision plastic pieces can be found?
They need to be hard but cuttable.
We need approx 8mmx8mm squares of it.
______________________________ _

Ferroxcube PQ2020 gapped cores:
PQ20/20-3F3-A160
PQ20/20-3C90-A160

Ferroxcube PQ2020 core clamp
CLM/P-PQ20/20

Ferroxcube PQ2020 Former (either of below)
CPV-PQ20/20-1S-14P-Z
CPV-PQ20/20-1S-14PD-Z

______________________________ _

Ferroxcube PQ2620 gapped cores
PQ26/20-3C90-A400
PQ26/20-3F3-A400

Ferroxcube PQ2620 formers (either of below)
CPV-PQ26/20-1S-12P-Z
CPV-PQ26/20-1S-12PD-Z

Ferroxcube PQ2620 Core clamp
CLM/P-PQ26/20
______________________________ _

Ferroxcube PQ26/25 parts...(can be used if no PQ26/20)
PQ26/25-3C90-A400
PQ26/25-3F3-A400
CPV-PQ26/25-1S-12P-Z
CPV-PQ26/25-1S-12PD-Z
CLM/P-PQ26/25
____________________________ _

TDK former for PQ26/20 (can be used if no ferroxcube one)
B65878E0012D001
« Last Edit: January 24, 2024, 06:39:29 pm by Faringdon »
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Offline jpanhalt

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Re: Spacers to gap ferrite cores with?
« Reply #1 on: January 24, 2024, 03:48:31 pm »
Micron accuracy in a plastic shim will be hard to find -- even with metal (e.g, brass) that would be had to find.  How accurate do they need to be?  Can they be stacked to get your desired thickness, say +/- 12 microns?  For example, plastic in 0.008" is probably available (approx 203 um).  You might find similar shims in a machinery supply place.  Rather than separate sheets of various thicknesses, they may be peelable laminates.

Google just told me that you can use non-magnetic metals.  Accurate metal shims are easy to find and they can be ground or plated to get even greater accuracy.
« Last Edit: January 24, 2024, 03:52:13 pm by jpanhalt »
 
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Offline Slh

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Re: Spacers to gap ferrite cores with?
« Reply #2 on: January 24, 2024, 04:21:40 pm »
For the benefit of others, do not use a non-ferromagnetic metal (or ferromagnetic one). It acts like a shorted turn and gets hot as one of my colleagues discovered. One of my specifications to him should have been "non conductive" which I thought was obvious.
 
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Offline jpanhalt

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Re: Spacers to gap ferrite cores with?
« Reply #3 on: January 24, 2024, 04:46:46 pm »
I was worried about that, then saw this (https://electronics.stackexchange.com/questions/210640/adding-a-gap-to-a-core-for-an-inductor ) on StackExchange:
Quote
Just insert non-magnetic shim material between the cores. Consequence is that you'll get a bit more leakage of EMI out the sides (flux fringing). You will be adding two gaps so you should use shim material thinner than with a gap at the core post. This is commonly done when prototyping flyback transformers, in fact you can buy special 'spacer' glue to glue cores together. In your case, I suggest procuring a kit of various thicknesses of plastic shim sheets used for mechanical engineering purposes and use the usual spring clip to hold the cores.

It's easy enough to calculate the required air gap for the desired Al. See, for example, Transformer and Inductor Design Handbook, Col. McLyman 1-18~1-23.

Grinding ferrite is not particularly easy to get right, even on a milling machine, at least that's my experience.

As for grinding ferrite, I do not quite agree.  It can be easily ground and lapped for a close fit if done with water -- just like ordinary glass can be ground and lapped.  Maybe the core configuration matters?
 
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Offline jpanhalt

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Re: Spacers to gap ferrite cores with?
« Reply #4 on: January 24, 2024, 05:17:34 pm »
Glass coverslips for microscopy come in various thicknesses (see: attached Table).  Precision coverslips are available for confocal microscopy.
 
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Offline Slh

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Re: Spacers to gap ferrite cores with?
« Reply #5 on: January 24, 2024, 06:01:38 pm »
I think the link mentioning non-magnetic spacers probably meant non conductive as well. Further down that paragraph says to buy a plastic spacers kit.

When I'm prototyping I often use paper or card (business cards can be good). There's no point in getting the distance perfect to the micron as the gap will vary with temperature anyway.
 
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Offline jonpaul

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Re: Spacers to gap ferrite cores with?
« Reply #6 on: January 26, 2024, 01:26:05 pm »


Grind gaps are hard to do, need diamond charge lap machine,

Most special order, and you may have to trim.

since 1970s just used fish paper, G10 FR4 fiberglass  or mylar gaps.

Fine on RM8, EER, ETD, PQ ferrite and UU cut core up to 12 kW.

Enjoy,

Jon
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Offline FaringdonTopic starter

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Re: Spacers to gap ferrite cores with?
« Reply #7 on: January 26, 2024, 09:04:27 pm »
Thanks, it would be good if the following Mylar Film  told us the tolerance on  the thickness.

https://uk.rs-online.com/web/p/plastic-film/7850815
« Last Edit: January 27, 2024, 02:03:51 pm by Faringdon »
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Offline Kean

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Re: Spacers to gap ferrite cores with?
« Reply #8 on: January 26, 2024, 09:45:49 pm »
Thanks, it would be good if the following Mylar Film  told us the tolerancee on  the thickness.

https://uk.rs-online.com/web/p/plastic-film/7850815

That link is to mica film, not mylar film.  Mica is a mineral, and it is going to have a much rougher texture than a plastic material.
« Last Edit: January 26, 2024, 10:01:29 pm by Kean »
 
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Offline coppercone2

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Re: Spacers to gap ferrite cores with?
« Reply #9 on: January 26, 2024, 10:24:12 pm »
I wonder if you can lap thin sheets with a flat vacuum holder
 
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Offline Neilm

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Re: Spacers to gap ferrite cores with?
« Reply #10 on: January 28, 2024, 12:47:43 pm »
Plasic shim material is readily available, for instance https://uk.rs-online.com/web/p/shim-kits/0681407?gb=s.
If the OP is doing mass produciton, it would be worth talking to the core manufacturer and getting ones ground pre-gapped
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Offline jonpaul

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Re: Spacers to gap ferrite cores with?
« Reply #11 on: January 28, 2024, 01:15:34 pm »
Aha,  again the FTSS toilet bowl of time....

Mylar (TM) film is made in many thickness 0.5 mil..10 or 20 mil, by Dupont and 3M.

No ide why English disty RS fails to inform,

Get the exact mfg part no and FIND THE ANSDWER YOURSELF.

Regardless another moot and useless topic as plain old paper or fiberglass like bare G10 FR-4 has worked for 100 years...

Get your micrometer and  MEASURE some bare fiberglass sheet and business card/heavy paper stock.

If too thin, just stack a few up.
 

Where to send my invoice, $100, MC/Visa/AMEX accepted.

j

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Offline tggzzz

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Re: Spacers to gap ferrite cores with?
« Reply #12 on: January 28, 2024, 03:31:20 pm »
Aha,  again the FTSS toilet bowl of time....

Can I suggest adding the OP to your "ignore list".

Unfortunately that won't cause him to be ignored, but it will use JavaScript to hide each and every one of his future utterances. That should act as a prompt not to stoke his ego.

Then, as he emits them, you'll have to explicitly ignore each and every one of his threads.

Bring back usenet readers' "ignore sub-thread" filters.
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Offline T3sl4co1l

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Re: Spacers to gap ferrite cores with?
« Reply #13 on: January 28, 2024, 05:48:32 pm »
I was worried about that, then saw this (https://electronics.stackexchange.com/questions/210640/adding-a-gap-to-a-core-for-an-inductor ) on StackExchange:

Check it again just in case :>

I hope it's not too bleedingly obvious not to put a shorted turn into a transformer?  But, point taken, sometimes doesn't hurt to be specific.  Plastic and composite insulating materials are fine here.  Metal is not.

Even more importantly for production purposes, they need to be part of the insulation system used in the part, which will further limit selection.  That is, a range of materials that have been UL-approved (or other) to work together in a component up to the rated temperature.

As Andy said in the other answer, even paper will do, if it's just a few-off sort of thing.  Mind that's class A insulation, not rated for very much in particular, and won't exactly have you passing UL, but it's unlikely you'll make a few and seek such approval.

Quote
As for grinding ferrite, I do not quite agree.  It can be easily ground and lapped for a close fit if done with water -- just like ordinary glass can be ground and lapped.  Maybe the core configuration matters?

I mean, dry is fine... it's not a particularly strong or hard material as ceramics go.  It's comparable to glass or porcelain.  Actually let me see here... NiZn can definitely scratch some, I think this is soda-lime glass; and maybe with difficulty, glass can just barely scratch ferrite, but it might also just be scuffing it and nothing more.  MnZn ferrite seems comparable or maybe a bit harder.  Can't seem to get NiZn and MnZn to scratch each other very much, any edge just kind of crumbles and it's hard to see scratches on the textured surface*.   Anyway, the only thing you need water for, is clearing dust, generally lubricating the cut, and only at high speeds, cooling; and any other liquid will do just as well.  You'll be working very hard indeed to need cooling by hand. :P

*Even a lapped (but not polished) surface has a granular appearance due to the polycrystalline makeup, probably thanks to cleavage planes giving randomly smoother or rougher micro-surfaces on individual grains.

Personally, I've used SiC sandpaper, diamond-embedded stones, and tile saws, to work with ferrite.  Of those, the tile saw is obviously the most aggressive, and tends to chip out material in a saw cut, with finish depending upon how smooth-running the saw is, and how well you're able to guide the material (so, by hand, not very clean, lol).

Other comments for readers' benefit...

Gapping the whole core, will probably make spring clamps unusable.  Or forcing them on will crush the ferrite (particularly if you've only gapped the center or something..!!).

The EMI comment is simply about fringing fields around the airgap, which does have some impact, albeit small.  Obviously it's in proportion to distance from the component; a several-mm gap could well cook an adjacent (touching, or nearly) clip, heatsink or capacitor body, for example (induction heating), in a very tight (compact) power supply design.  That's bad news for efficiency, and another good reason to keep the gap in the center.  Unfortunately, the same is true of the wire, and litz is often required...


Thanks, it would be good if the following Mylar Film  told us the tolerancee on  the thickness.

https://uk.rs-online.com/web/p/plastic-film/7850815

That link is to mica film, not mylar film.  Mica is a mineral, and it is going to have a much rougher texture than a plastic material.

Cleaved mica surface is atomically pure, at least until you reach the edge of the next layer(s).  Note that mica is almost exclusively synthetic, so can be far more consistent than naturally mined minerals, if that's where you were going with that.

Using rocks to support rocks is rather appealing, if I do say so myself.  The challenge is really just getting it flaked accurately enough.  If you need it more accurate than, say, a skiving setup with polished carbide blade can afford, then gluing and lapping is probably your alternative.

Which...

I wonder if you can lap thin sheets with a flat vacuum holder

Vacuum pits will have to be very small in relation to the thickness of the material, otherwise the material flexes into them, and now what used to be a flat surface during grinding, bows back to a polkadot-bumped surface sticking out.  It doesn't seem like much for bulk materials, and, well, bulky low-aspect things being what they are, sure.  What comes to my mind is, I've got a chip of ferrite here that I've lapped to pretty close to parallel, around 80 mils thick; if I take it out and put it on the diamond stone, only the places where my fingers support and push on it (on the back side) take any cut (on the front side).  It's not very flexible, but on the scale of 10s-100s ┬Áin where grinding cutting takes place, yeah, it's a floppy piece of paper.

Assuming you have some way to release it -- I would recommend super-glue: it can lay down very thin indeed, and then you have a solid backing plate to reference against.  Heat and acetone will be fine here, but just to say, you might not always have that option, and if it comes down to mechanical means, peeling things apart is likely to damage them.

Tim
« Last Edit: January 28, 2024, 05:51:16 pm by T3sl4co1l »
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Offline jpanhalt

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Re: Spacers to gap ferrite cores with?
« Reply #14 on: January 28, 2024, 06:34:15 pm »
I was worried about that, then saw this (https://electronics.stackexchange.com/questions/210640/adding-a-gap-to-a-core-for-an-inductor ) on StackExchange:
Check it again just in case :>

I did a bit more reading on it from other sources and reached the conclusion not to use non-magnetic metal.  That was my intuition initially, but I was swayed by the clearly misinterpreted and misleading statement on Stack Exchange.  I stand by my statement that the ferrite I was using can be worked much as glass is worked.  Diamond works but so does carborundum for grinding and lapping with water as lubricant/cooling.

My specific project was to make a C-shaped toroid with a well fitted ferrite "latch" to close the gap (attachment) for looking at current profiles with a scope.  At the time, I was looking at saturation current in toroids, and my results were close enough to a standard toroid kindly gifted by Coilcraft to satisfy me.
 
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Offline coppercone2

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Re: Spacers to gap ferrite cores with?
« Reply #15 on: January 28, 2024, 06:59:36 pm »
maybe a graphite holder would work too, it works for air bearings in reverse. but its rather soft.

I am sure there is a eventual market for rigid superalloy sponges to allow for gripping thin materials for lapping. i wonder when.

but this depends on how the tempco of the material works. not sure how useful temperature stabilized ferrite cores are for some specific purpose (i.e. whole board flood cooling).

Maybe for achieving some ultra high efficency switching supply? is this the kind of tech you need for 99.x? (only would achieve that efficiency at some specific temp tho)

if you did manage to figure out how to super glue lap this precisely where could it get you? I am not sure what aspect of SMPSU design precise/arbitrary core spacing would improve.

« Last Edit: January 28, 2024, 07:09:30 pm by coppercone2 »
 
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Offline jonpaul

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Re: Spacers to gap ferrite cores with?
« Reply #16 on: January 29, 2024, 10:42:07 am »
just get a Tektronix current probe, P6021, P6022, CT-5 or the Hall effect DC probes.

For large guage wideband CT without jaw, we use Pearson 410, 411 CT.

Jon
Jean-Paul  the Internet Dinosaur
 
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