Gapping Pot Core

[Joerg]

I doubt that would happen- most machinists are pretty canny. If it's
an RM type pot core your machinist may be able to handle it on an
ordinary surface grinder with a narrow wheel and a magnetic chuck.

Yes, most of them sure are good. The best was a guy who was clearly
under the influence of lots of booze. Gives him a steadier hand, he
said. Got my parts back later that day, looked mil-spec. When trying
whether the coax gear really fit you could hear the air hiss when
removing the stuff. Top notch precision and not a single scratch anywhere.

 

[Spehro Pefhany]

Win's method is what I have used in the past. Beware that the junk
spitting out from the core can get into other electronics and cause
problems if your gap is large enough. I've used business cards as
shims. Seems to be the best use for them. For proto purposes, you can
use black electrical tape to hold the core halves together. Since you
are gapping your cores, the offical hardware isn't necessary for your
experimental stage.

You need diamond tools to gap your own ferrite. Best to use the shim
method.

Once you have a gap you like, simply calculate the AL of your core set
by measuring the inductance of a coil with a known number of turns. Be
sure your bobbin is nearly full as the AL value is more sensitive to
winding height with lower AL vaules (a 0.2 relative winding height
will give an AL value 6% lower at and AL of 160, 2% lower at an AL of
600 for many pot cores). When ordering custom gapped cores, all you
need to give is the AL value you want and they do the rest. However,
see if you can use a standard gap as that makes life much easier and
cheaper.

You can buy a set of non-conductive shim stock from McMaster for about
$30 which has color-coded thicknesses from half a thou to 30 thou.

For production, epoxy with glass spacers can be used to hold the core
halves together.

Best regards,
Spehro Pefhany

 

[Allan Wilson]

Working on eval design for power inverter; will use a pot core for
transformer. Have picked a core size, based on allowable hysteresis
loss. Also have a pretty good idea what magnetization inductance
needed. Requires an AL value a good bit smaller than that of largest
gap core in vendor's catalog...

Thought of buying a few core sets with largest available gap, then
modifying them to test for proper indutance, and get vendor to make a
special order core, same gap, for production. But, damned if I can
figure out how to remove material from core center posts. Could just
use gap shims, I guess, but that would open up gap at periphery as
well as center of core, and I would not then be able to put core in a
shield cover without incurring eddy current losses in metal cover.

An E core would be easier, I think: just come in from the side with a
grinding wheel and shave down the center post, using a bit of flood
coolant to make a nice, smooth ground surface. Getting to the center
post of a pot core would almost seem to require a jig grinder, which I
don't have. Any suggestions?

Thanks!
W Letendre

You should use the largest standard gap core then add shims to get AL.
This will minimize the periphery gap.

 

[Clifford Heath]

True enough. Does require a shop with at least a little experience
with brittle, high Rockwell materials, though. Ferrite, as a ceramic,
almost certainly requires grinding rather than machining.

I had some ferrite magnets made by these people:
<http://www.alphamagnetics.com.au/>, and they used
a wet diamond saw to cut blank ferrite blocks.
The diamond blade went through them like butter.
This was non-magnetised hard ferrite (hard meaning
it can be permanently magnetised IIRC).

I reckon you have nothing to lose by trying a
Dremel and a diamond burr. Set the Dremel up on an
angle in a stand if you can, so you can start it with
the burr in a gap and just slide the core around until
all the center section is ground to the same height.

Clifford Heath.

 

[Ecnerwal]

But, damned if I can
figure out how to remove material from core center posts.

die grinder, foredom, dremel - small grinding burr, patience. If lacking
patience, be prepared for things to fly about and break. No need for
that to happen with a bit of care.

 

[Joerg]

die grinder, foredom, dremel - small grinding burr, patience. If lacking
patience, be prepared for things to fly about and break. No need for
that to happen with a bit of care.

- Cats, coffee, chocolate...vices to live by
^^^^^^

And none of this stuff before firing up that Dremel ;-)

 

[Ecnerwal]

- Cats, coffee, chocolate...vices to live by
^^^^^^

And none of this stuff before firing up that Dremel ;-)

Speaking for yourself. Takes me a couple of full pots to get all
jittery, and I haven't been at that level for years (spent a while
there, backed off from it to 2-3 mugs of half-caf). Not being able to
think does not help my work, so the coffee goes in regardless. Otherwise
my day starts all wrong and continues not too well.

I can solder TSSOP and 0603 in that condition, and I can grind in that
condition. Regardless of coffee intake or lack thereof, some work is
aided by having a place where you can rest your hand or wrist set up, so
that you are mostly holding steady with your fingers, rather than
needing to hold your hand and arm steady all the way back to the elbow
or shoulder.

Doing a lot of heavy lifting or the like is about the worst thing I can
do for fine motor skills - takes several hours before things return to
normal. I get the BIG twitches from that - either letting go of small
things I mean to hold onto, or jerking a good 6 inches to a foot when
intending to make a small adjustment (or both, so the item is flung
rather than merely being dropped).

 

[Joerg]

Speaking for yourself. Takes me a couple of full pots to get all
jittery, and I haven't been at that level for years (spent a while
there, backed off from it to 2-3 mugs of half-caf). Not being able to
think does not help my work, so the coffee goes in regardless. Otherwise
my day starts all wrong and continues not too well.

I can solder TSSOP and 0603 in that condition, and I can grind in that
condition. Regardless of coffee intake or lack thereof, some work is
aided by having a place where you can rest your hand or wrist set up, so
that you are mostly holding steady with your fingers, rather than
needing to hold your hand and arm steady all the way back to the elbow
or shoulder.

I can get that done as well but at about 0402 sizes my eye don't want to
comply, at least not for a long stretch of soldering.

Doing a lot of heavy lifting or the like is about the worst thing I can
do for fine motor skills - takes several hours before things return to
normal. I get the BIG twitches from that - either letting go of small
things I mean to hold onto, or jerking a good 6 inches to a foot when
intending to make a small adjustment (or both, so the item is flung
rather than merely being dropped).

That's kind of unavoidable around here. There is a wood stove ...

 

[ChairmanOfTheBored]

Push pull drive would work, and work well. But, believe design I've
got in mind will be ~ $1.50 lower part cost. Target production volume
definitely high enough for part cost savings to pay off R&D labor.

In fact, had rather hoped that target volumes would be high enough so
that we could sweet talk vendor into making up an eval lot of cores
with assorted gaps "bracketing" intended AL value. Will still pursue
this, but, vendor is in Far East; they make good cores, and they make
them cheap, but they have not generally given us very good lead time
for prototype lots. So, thought I would try "fiddling" gap in our lab,
then giving gap spec to vendor for production order. May try Dremel
tool; do worry about cracking core!

W Letendre

Silly rabbit! You buy some UN-gapped cores, and use fish paper to
achieve the right gap. Remember that when gapping the entire core as
this paper gap process does, you have to double that gap value when you
go to order the properly tuned (gapped), gapped core.

Instead of paying for a set of cores with incremental gaps, you should
get un-gapped core sets, find out what your gap needs to be by gapping
the entire core (inner post is one path, and the outer ring is the
other)(note how the surface area of the outer ring is the same as that of
the center post). Then the gap you order is a mil or two less than
double the paper gap tuned figure. That way, you can have it closer, or
do some fine, flat plate abrasive sanding on the center post (the dremel
is a BAD idea) to make changes. You can easily pull off a mil or two or
ten, if you wish with the flat plate method.

Your proto tuning point which you arrive at should transfer over well
to a center post only gapped cores set, where you take a single, non
gapped core half, and mate it to a core half that you have had gapped on
the center post. You would double the gap you used in the proto session
to get the gap thickness you would order from the core presser. They
usually remove the media abrasively.

You can buy 1 mil (fish paper) or find "Nomex" high dielectric
transformer paper (that's the brand you google for), which comes in
precision thicknesses. Try Farnell for a source.

You can get a mil, 1.5 mils (important for making half steps),
2 mils, 5 mils, and ten mils in thickness, and I'll bet that they would
send you a sample pack. With media like this, you can buy un-gapped
cores and gap everything yourself, as this paper keeps its physical size
well, even after impregnation. (I had to use "the I word" :-] )

You can also use transformer tape to gap with for lab proto, as it
also comes in precision thicknesses. A razor knife edge trims
transformer tape excess well.

 

[ChairmanOfTheBored]

he already explained that - gapping across the core allows fringing flux
to escape, which defeats the purpose of using pot cores, which is to
keep the fields contained.

To use full face gaps for simply arriving at the needed value, however,
is ok, and then THAT value gets doubled for the center post gap to match
it.

That center post, and the outer ring are exactly the same surface area
in size. It is like the source and return of a magnetic circuit.

So exactly double works both mathematically and in real practice.

Trick is that your gap measurements have to be precise to below 0.5
mil., which means that precision gapping media "transformer paper" or
"transformer tape" must be used. That way, it is easy to tell how much
gap you have in at the moment.

Also, if you want it to keeps its characteristics over a long period of
time, you will want to vacuum impregnate it with a good transformer
varnish, and bake it out at like 400 degrees F for a few hours. Let it
settle for a day, and THEN, all of your measurements will always be the
same on that transformer for years to come, short of the introduction of
a failure mode through improper utilization.

One also has to be very careful when installing a gapped core
transformer as since the center post is no longer a mechanical force
bearing face due to the gap, you can not add very much force at all to a
screw used to secure the transformer that goes through the center of it.
Unless you place a bushing at the gap interface to link the force
through. Otherwise, broken cores will result. Impregnated transformers
suffer from this less. They are truly non-serviceable, however.

 

[ChairmanOfTheBored]

Use a shop that is experienced in handling ceramics. They should be able
to grind it down precisely to within a few micrometers.

It isn't really a ceramic. It is a sintered plug essentially, and acts
like a very brittle, hard packed clay. Not end mils, but bladed cutters
can be used on it, though abrasive grinding is far easier, safer, and
cheaper.

 

[ChairmanOfTheBored]

You can buy a set of non-conductive shim stock from McMaster for about
$30 which has color-coded thicknesses from half a thou to 30 thou.

For production, epoxy with glass spacers can be used to hold the core
halves together.

It's called transformer paper, and the most common brand I know of is
"Nomex". Specifically for use in transformer applications, including
precision gapping.

 

[Joerg]

It isn't really a ceramic. It is a sintered plug essentially, and acts
like a very brittle, hard packed clay. Not end mils, but bladed cutters
can be used on it, though abrasive grinding is far easier, safer, and
cheaper.

And not the slightest wobble. I tried cutting toroids with a diamond
blade and in a pinch I only had a wet tile saw and the blade had been
used on porcelain before, despite repeated dressing attempts. Not good,