In article <e95c9102-be08-4eb6-898d-e8c39e133c6an@googlegroups.com>, 
pallison49@gmail.com says...
> 
>  Mike Coon wrote:
>   ===============
> > 
> > > But for an inductor you want a gap, and R-core (or any tape-wound core) 
> > > won't give much of that SFAIK. 
> > >
> > Why? I thought the point of core designs was largely to minimise any 
> > gap, but I haven't been following the whole argument...
> 
> ** FYI  :
> 
>  any "gapless"  magnetic core saturates very easily, losing its magnetic properties when used as an *inductor". 
> Transformers are a different story, since primary off load currents are small.     
> 
> For a compact inductor that  passes significant AC or DC current, a carefully sized air gap is essential.  
> This apples to laminated steel or solid ferrite cores -  but not so much to powdered iron cores as the air gap is intrinsic. 
> 
> Not a simple topic, bore your self to tears looking it all up. 
> 
> 
> 
> .....   Phil 
> 
> 
>             

No need, that was sufficient unto the day, thanks!

On Saturday, May 29, 2021 at 7:31:27 PM UTC+10, palli...@gmail.com wrote:
> Mike Coon wrote: 
> ===============
> > 
> > > But for an inductor you want a gap, and R-core (or any tape-wound core) 
> > > won't give much of that SFAIK. 
> > > 
> > Why? I thought the point of core designs was largely to minimise any 
> > gap, but I haven't been following the whole argument...
> ** FYI : 
> 
> any "gapless" magnetic core saturates very easily, losing its magnetic properties when used as an *inductor". 

It saturates at lower ampere turns than a gaped core, and the permeability of an ungapped core tenbds to be a bit less predictable.

> Transformers are a different story, since primary off load currents are small. 

That does depend on the application.

> For a compact inductor that passes significant AC or DC current, a carefully sized air gap is essential. 

It can useful, but "essential" depends on the details of the appllcation.

> This apples to laminated steel or solid ferrite cores - but not so much to powdered iron cores as the air gap is intrinsic. 
> 
> Not a simple topic, bore your self to tears looking it all up. 

There 's a shortage of good texts, and a liberal supply of bad ones.

https://www.amazon.com/Soft-Ferrites-Second-Properties-Applications/dp/0408027606

E.C.Snelling's "Soft Ferrite" is about a bad as they get. It's got all the information you could ever need presented in way that is particularly confusing and unhelpful. 

The Siemen's Soft Ferrite data book and application notes was much less ambitious, and enormously more helpful. Siemens ferrites as now EPCOS parts, and it''s part of TDK. The late great Tony Williams e-mailed a couple of megabyte worth of most of them to his friends some time ago.

-- 
Bill Sloman, Sydney

 Clive Arthur wrote:
===============
> 
> >> "a couple of mH at a couple of amps" will be pretty big. 
> >> 
> > 
> > ** Should be smaller than a golf ball ( 42mm dia) 
> > 
> Yes, that's about the right volume. Normally we'd use two or three 
> stacked toroid cores, but this customer doesn't have the form factor 
> space for that. 
> 

** IMO ought to be possible with a powdered iron bobbin and just the right turns.

Loudspeaker passive x-overs often have such inductors in them. 


.....  Phil 

 Mike Coon wrote:
  ===============
> 
> > But for an inductor you want a gap, and R-core (or any tape-wound core) 
> > won't give much of that SFAIK. 
> >
> Why? I thought the point of core designs was largely to minimise any 
> gap, but I haven't been following the whole argument...

** FYI  :

 any "gapless"  magnetic core saturates very easily, losing its magnetic properties when used as an *inductor". 
Transformers are a different story, since primary off load currents are small.     

For a compact inductor that  passes significant AC or DC current, a carefully sized air gap is essential.  
This apples to laminated steel or solid ferrite cores -  but not so much to powdered iron cores as the air gap is intrinsic. 

Not a simple topic, bore your self to tears looking it all up. 



.....   Phil 


            

On 29/05/2021 01:56, Phil Allison wrote:
> John Larkin wrote:
> =================
>>
>> "a couple of mH at a couple of amps" will be pretty big.
>>
> 
> **  Should be smaller than a golf ball  ( 42mm dia)
> 
> 
> .....   Phil

Yes, that's about the right volume.  Normally we'd use two or three 
stacked toroid cores, but this customer doesn't have the form factor 
space for that.

-- 
Cheers
Clive

In article <s8rnro$moc$1@gonzo.revmaps.no-ip.org>, usenet@revmaps.no-
ip.org says...
> But for an inductor you want a gap, and R-core (or any tape-wound core)
> won't give much of that SFAIK.
> 
Why? I thought the point of core designs was largely to minimise any 
gap, but I haven't been following the whole argument...

John Larkin wrote:
=================
>
> "a couple of mH at a couple of amps" will be pretty big.
>

**  Should be smaller than a golf ball  ( 42mm dia) 


.....   Phil 

On Fri, 28 May 2021 21:38:00 -0000 (UTC), Jasen Betts
<usenet@revmaps.no-ip.org> wrote:

>On 2021-05-28, Clive Arthur <clive@nowaytoday.co.uk> wrote:
>> I need to make an inductor - a couple of mH at a couple of amps.  It has 
>> to fit in an awkward space, roughly oval toroidal and nothing off the 
>> shelf. Nothing I can do about the space.
>>
>> So this tape - I know nothing of it except that it can be used for 
>> transformer cores.  Can I buy a roll and wind my own core on a former? 
>> Anyone used it?
>
>Roughly oval, tape wound core, sounds like R-core.
>
>But for an inductor you want a gap, and R-core (or any tape-wound core)
>won't give much of that SFAIK.

Good point. A few tiny permalloy or kool-mu toroids could be wound in
series and stuffed into an odd-shaped volume. Powdered iron if the AC
current is low.

"a couple of mH at a couple of amps" will be pretty big.

On 2021-05-28, Clive Arthur <clive@nowaytoday.co.uk> wrote:
> I need to make an inductor - a couple of mH at a couple of amps.  It has 
> to fit in an awkward space, roughly oval toroidal and nothing off the 
> shelf. Nothing I can do about the space.
>
> So this tape - I know nothing of it except that it can be used for 
> transformer cores.  Can I buy a roll and wind my own core on a former? 
> Anyone used it?

Roughly oval, tape wound core, sounds like R-core.

But for an inductor you want a gap, and R-core (or any tape-wound core)
won't give much of that SFAIK.

-- 
  Jasen.

On Friday, May 28, 2021 at 10:10:56 AM UTC-7, Clive Arthur wrote:
> I need to make an inductor - a couple of mH at a couple of amps. It has 
> to fit in an awkward space, roughly oval toroidal and nothing off the 
> shelf. Nothing I can do about the space. 
> 
> So this tape - I know nothing of it except that it can be used for 
> transformer cores. Can I buy a roll and wind my own core on a former? 
> Anyone used it? 

I think you're talking about Metglas; some quick-quench alloys that have
a non-crystalline amorphous microstructure, so cannot be usefully
cold-worked or reheated without destroying its virtues.

It can be produced as a thin tape, suitable for winding into toroid shape,
and a crimped spool can hold it there.   Presumably, a shaped core and epoxy
could make an oval, but applying a winding would be a challenge (just
as with any toroid).   It's springy stuff (remember, you cannot anneal it to
make it soft) so there's some force required to keep it together.   As for
getting a sample, just buy a toroid and cut off the spool; it'll unwind into
a long strip .  There may be some glue, though; that's a chemistry problem.

Elna and Hitachi make such cores.