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Author Topic: Stacking toroids  (Read 486 times)

K8AXW

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Stacking toroids
« on: May 07, 2022, 09:44:40 AM »

I want to use stacked toroid's to make an Unun.  I see this done using a white tape to attach them.  I'm assuming it is some kind of Hi-temp tape like Teflon.  Problem is I can't find a "Teflon" tape that I recognize.

Suggestions on this topic?

Al - K8AXW
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W9IQ

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Re: Stacking toroids
« Reply #1 on: May 07, 2022, 10:03:00 AM »

I recommend not using any form of tape - it adds more thermal resistance in an application where heat dissipation is important.

Try lightly cable tying the two cores together with two or three cable ties. Don't crank down on the cable ties as this can cause the cores to fracture during thermal growth cycles.

When designing an unun, pay attention the number of primary turns as this has a profound impact on the thermal performance of the transformer. If you would like some design assistance, let me know here or via email.

- Glenn W9IQ
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God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

WB6BYU

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Re: Stacking toroids
« Reply #2 on: May 07, 2022, 10:04:12 AM »

Something like the Teflon® tape used for pipe joints?

Wrapping the cores with tape, while popular, may not always be
beneficial, depending on your application and the cores you are
using.  If the core is prone to overheating, the tape reduces the
thermal capacity because it reduces heat transfer.

Generally, tape may be handy when the core is conductive and
may scratch the insulation (such as magnet wire).  #31 material
is more conductive than many other types of ferrite, while it may
not be required at all for some other cores, or types of wire
insulation.

K0DBH

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Re: Stacking toroids
« Reply #3 on: May 07, 2022, 10:20:47 AM »

Al,
   I agree with Glenn about using the cable ties. If you were to use tape, 3M makes a glass cloth tape, #69, which will withstand high temps and insulates to 3000 volts.
73
Don K0DBH
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W9IQ

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Re: Stacking toroids
« Reply #4 on: May 07, 2022, 10:45:07 AM »

To bring more context to my point about tape...

The primary source of heat in a wide band unun transformer design is from the resistance nature of the core material itself. The RF resistive loss in the windings is minuscule in comparison - even with relatively small gauges of wire. Adding tape to the core traps the heat that is generated in the core - this is generally the opposite of the desired thermal effect.

Depending upon the power levels involved and the transformer design, it may be wise to wind the transformer with PTFE insulated wire or similar instead of basic, insulated "magnet" wire. This will provide a very robust, high temperature winding that is most resistant to chaffing or cutting. But since most HF unun designs are based on type 43 material, the conductivity of the core is generally a minor concern should the insulation be compromised.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W7CXC

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Re: Stacking toroids
« Reply #5 on: May 07, 2022, 11:51:39 AM »

Seems like one could use ties to start then when there are enough turns to hold the cores in place remove them. Here is a question.....IF there is concern about a bit of tape causing a heat problem would it not be prudent to go to a bigger core? It would seem that the thermal resistance of a single layer of Teflon plumbers tape would be of no consequence unless the design was marginal from a power handling standpoint in the first place.
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W1VT

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Re: Stacking toroids
« Reply #6 on: May 07, 2022, 12:37:55 PM »

3M Glass Cloth Electrical Tape 27, ¾ in x 66 ft, 1 Roll, Non-Corrosive Adhesive, Pressure Sensitive, High Temperature, Corrosion Protection, 7-mil Woven

Maybe this is what you saw?
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G3RZP

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Re: Stacking toroids
« Reply #7 on: May 07, 2022, 01:28:54 PM »

Over here, you can get 'plumber's PTFE' tape, which is  a very few thousandths (mil) of an inch thick, and is cheap. In general, for transformers  to operate at high impedance, I avoid toroids  like the plague. Seen too many catch fire in professional transmitters....
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W9IQ

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Re: Stacking toroids
« Reply #8 on: May 07, 2022, 03:21:02 PM »

Seems like one could use ties to start then when there are enough turns to hold the cores in place remove them. Here is a question.....IF there is concern about a bit of tape causing a heat problem would it not be prudent to go to a bigger core? It would seem that the thermal resistance of a single layer of Teflon plumbers tape would be of no consequence unless the design was marginal from a power handling standpoint in the first place.

Like so many options in engineering you trade off one thing for another. Larger or multiple cores increases power handling but also increase "interwinding" capacitance which lowers the self-resonant frequency.

Most FT240-43 transformer designs overheat due to too few primary turns. Most sites simply copy another's two turn primary but then play with bifilar vs twisted or even silly 14 gauge primaries without ever understanding the effect of complex permeability. Proceed at your own peril.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

GRUMPY2021

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Re: Stacking toroids
« Reply #9 on: May 07, 2022, 03:25:48 PM »

4 drops of super glue.    Youtube Steve Ellington has a wealth of information on the topic.
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K8AXW

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Re: Stacking toroids
« Reply #10 on: May 07, 2022, 10:11:20 PM »

I've been reading info and watching Youtube videos on the suddenly popular HWEF wire antenna.  While I have no immediate plans to construct such an antenna at this time, I am adding all the info I can to my ANTENNAS file in case I lose my beam.  Whenever possible I have a fallback plan...for almost everything.

If ever built it will follow the popular design of using the 240-43 toroid.  The design is rated at 250w.  If I build one I plan to use two toroids to handle 500w.  I have an amp.

I noticed while watching a couple Youtube videos that the 500w version uses stacked toroids held together with a white tape wrapped around the circumference.

Plumbers tape isn't sticky and isn't made to hold or attach anything.  It's a filler.

The wire wraps look like a good option.  Might consider super-glue to hold them while winding.

Glenn, I was going to go with #14 wire.  What do you suggest for 500w?

Al



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W9IQ

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Re: Stacking toroids
« Reply #11 on: May 08, 2022, 02:51:49 AM »

Glenn, I was going to go with #14 wire.  What do you suggest for 500w?
18 or 16 gauge is more than sufficient.

You might get away with 250 watts voice SSB through a single core but that is pushing it. As a frame of reference, a well matched transformer with a two turn primary on a single FT240-43 core at room temperature can easily reach its Curie temperature running only 100 watts of FT8.

Just like using tape, sealing the transformer in a box adds another source of thermal resistance. Consider a well vented box or not using a box at all.

Good luck with the project.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W1VT

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Re: Stacking toroids
« Reply #12 on: May 08, 2022, 04:11:35 AM »

It is hard to scale up transformers for more power without sacrificing something.  More turns is a pain because folks don't like winding all those wires around the ferrite donut.  Even worse is the reduced bandwidth.  10M is a really good band when it is open and folks want to run the amp there too.  As well as on 80M where you often need the amp to deal with noisy band conditions.
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K4IBC

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Re: Stacking toroids
« Reply #13 on: May 08, 2022, 08:17:51 AM »

Like so many options in engineering you trade off one thing for another. Larger or multiple cores increases power handling but also increase "interwinding" capacitance which lowers the self-resonant frequency.

Most FT240-43 transformer designs overheat due to too few primary turns. Most sites simply copy another's two turn primary but then play with bifilar vs twisted or even silly 14 gauge primaries without ever understanding the effect of complex permeability. Proceed at your own peril.

- Glenn W9IQ

I had read this before elsewhere but hardly much discussion about it. I am particularly interested in stacking cores and the effects and what to do to compensate. If you stack cores does changing the amount of windings raise the self resonance frequency. Due to the lack of commercial true QRO continuous carrier chokes and baluns there is room for development. Yeah I know expensive and heavy doesn't fit into the typical ham equation. Time to stop playing with toys and get serious. 
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W1VT

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Re: Stacking toroids
« Reply #14 on: May 08, 2022, 09:42:48 AM »

If cost isn't an issue figure out ways of getting heat out of the cores.
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