Gapping Pot Core

Well, I have used a drill press with a very good grinding bit in there. But be very careful. It can easily start to vibrate and then you have stuff flying about at high speed. If you use a Dremel in a drill press stand don't run that at full speed. Use eye protection and make sure nobody stands close. Of course the liquid carries it's own risks. So be careful, proceed at your own risk ...

Oh, BTW, if the drill press is one of those el-cheapo deals and you can feel the bearing slack forget it, then you need a better one.

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Regards, Joerg

http://www.analogconsultants.com/

Dremmel tool with diamond wheel?

How steady is your hand? How good your eye?

How many cores you got to waste? :-|

Regards, JS

"WJLServo"

** Consider using push pull drive to the transformer - obviates to need to create gaps.

....... Phil

Yep, that's what I always do. And don't forget the inductor after the diodes, especially if it's a big one.

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Regards, Joerg

http://www.analogconsultants.com/

My advice: During the prototype phase, add the spacer and don't worry about the flux leakage and eddy-current losses with the shield -- they will be small. Go ahead and measure the inverter's performance. Take heart in the thought it might do better with a proper ground core. Second, add the shield to get a handle on magnetic-field leakage, etc., which at any rate will be less with the production pot core. What's the problem? If you're happy with the prototype design, tested this way, the production one will be a bit better! So... what's the big problem?

Oh, more advice: just forget grinding your ferrite core.

Yep, You can buy them already gapped.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

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

Heh! Think your advice may be best! Only reservation is wondering how easily the gap generated with spacer shim can be translated into a production value. Had thought that a modified core with center post ground to match desired AL could then be measured using dial indicator to give an exact gap spec for vendor to use for production cores. At the end of the day, though, may just use spacer, measure AL rather than gap, and ask vendor to take AL value as their production spec...

W Letendre

firstly, if you gap across the whole core face then the total gap is 2x the gap thickness (this is apparent if you think about the path the flux traverses, it crosses the gap in the centre leg, and again in the outer limbs). so centre-leg-gap = 2 x shim-thickness

Secondly, you can always calculate it, if you know the material permeability. its pretty easy to do.

Cheers Terry

Tried calculation, first up. Values I came up with were close to vendor's published values, AL versus gap, for the half dozen stock gaps they offer. Close... but not close enough for design use, especially as I need to go ~ 2X out from largest stock gap. Think I do need to test design with actual gap.

2X rule of thumb (center post versus dore face gap) may also be close enough; would like to think that it is. Think, though, that I will trust the measurements over the math!

W Letendre

for "large" gaps (> 0.1mm or so) then assuming all the energy is concentrated in the gap is a very good approximation. for small gaps, this is not very accurate, but again (assuming you have access to Snellings Ferrites book) this is fairly easy to calculate.

because you want a big gap, you may also want to take fringing into account, which increases the effective pole area.

even without the finer considerations, the all-energy-in-gap approach will get you very close to the desired gap.

its not a rule of thumb. follow the flux path, and you see where the 2x comes from. the maths is based on some pretty sound (and well understood) physics.

for power converter work, IME the exact value of L isnt *that* critical, provided you allow for sufficient margin in the overall design to suck up tolerances. Heck, just look at a graph of relative permeability vs temperature! or look at the spec'd tolerances in pre-gapped magnetics.....

Cheers Terry

That's right, except I suggest you begin comparing your A_L measurements to the limited standard A_L values available for that ferrite, and regap/rewind your core until you match one, then test your setup. Continue this process until you like your design, then order that AL ferrite core.

Measure A_L, not gaps, and compare to the catalog.

A_L is easy to measure, wind 5 - 10 turns and measure the inductance. A_L = L / N^2, where L is in nH.

Then just give it to a really good machine shop. They'll do it professionally and precise.

--
Regards, Joerg

http://www.analogconsultants.com/

to need

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. Would hate to give a ferrite core to a machinist who might try to cut it with an end mill!

W Letendre

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.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

--
Regards, Joerg

http://www.analogconsultants.com/

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.

--
Mark

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.

--
Regards, Joerg

http://www.analogconsultants.com/

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

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com