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PostPosted: 05 Dec 2014, 18:16 
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Hi forum

I had a small chat with Ko Tilman ( radiofun232 ) about converting old transformers to nice chokes. I want to share this trick with you guys to. In my situation i've used some old scraped toroidal transformers ( 220 V / 87 V ) and unwounded the secondary coil away from the transformator. After this i had access directly to the core. The primary coil is still figuring on the transformator and it's used for the choke. But it's not that easy ...because we need an airgap, to convert it to a choke. In the illustrations a saw is shown, but i actually didn't use that. Instead i used an electric saw ( also a grinder could be used ). After this gap is made you'll be able to find out if it's useable. I tried it in a 30 VDC PS setup ( as a CLC configuration, with 25000 uF Spraque caps as well as 66000 uF for testing ). There was a perfect magnetic flux around the gap ...it was attracting a big piece of metal and was able to hold it. The inductance of the transformer was 1 H ( now as a choke, 838 mH ) and has a DC resistance of 3,9 Ohm. In the setup i was drawing 2 A.

Note: I leave about 1 mm material ....which means i didn't saw all the way through the core !

See the illustrations below ...and hopefully you guys also will try making some good chokes out of scraped transformers 8-)

PS: As a bonus the secondary coil you've unwounded, can also be used for a choke. I've made some on plasticspools, with cores normally intended for loudspeaker crossover filters.

I wish you luck !


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PostPosted: 05 Dec 2014, 21:11 
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That's one HUGH air gap. :eek:

Air gaps are usually measured in mils (1/1000").

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PostPosted: 06 Dec 2014, 08:16 
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Great idea. But as Matt said the air gaps are usually very fine. But love the concept.

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PostPosted: 06 Dec 2014, 16:39 
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Thanks Mark and Matt

Okay, then i'll go to buy a lasercutter Image ( just kidding ).

It was actually just for fun i tried this, but the result was much better than expected, so i will do this trick again, when a choke is needed.

I've been testing the two shown coils up against each other in a PS. Only the one with the airgap does a ripple free PS. The other one without a saw track is only working as a 3,9 Ohm resistor. The difference i've been watching on the scope tells the truth. :wizard:


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PostPosted: 06 Dec 2014, 17:04 
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After Matt was bringing this isue about 1/1000 mm ( 1 my ) airgap ( Thaks Matt ), i had some thougths about removing the last millimeter of material in the airgap. After that i will put some heavy glue into the gap, then carfully wrap the choke into some rags, then put it into the vices and tighten it until the airgap is nearly disapered. After the tightening i'll leave it there over night, until the glue is dry !

A good idea Matt ...or ?

PS: Just hope for the coil not will be damaged in the vices.


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PostPosted: 06 Dec 2014, 18:11 
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The other way would be to slide in thin magnetic material with a a paper spacer. Maybe just tap it in.

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PostPosted: 07 Dec 2014, 10:31 
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mwhouston wrote:
The other way would be to slide in thin magnetic material with a a paper spacer. Maybe just tap it in.


Exactly Mark ...i've been around that idea too ( a paper spacer ). My thougths was cutting two pieces of metal from a tuna can and place a slice of paper in between ( like a metal/paper/metal sandwich ). Then just hammer this sandwich into the gap. :wizard:

Nice to get that idea confirmed ...thanks Mark :up:


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PostPosted: 07 Dec 2014, 18:19 
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Tuner can, that sounds a little fishy!

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PostPosted: 07 Dec 2014, 21:30 
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mwhouston wrote:
Tuner can, that sounds a little fishy!


*budda-boom, ching!*

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PostPosted: 08 Dec 2014, 02:43 
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Well now I must show some intelligence. Get an old EI tranni and take the plates out and pack your crack with them. With thick paper spacer of course.

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