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Author Topic: LF low pass filter toroids, how far from metal case?  (Read 6321 times)

2E0ILY

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LF low pass filter toroids, how far from metal case?
« on: November 14, 2014, 02:45:47 AM »

I am building a LPF for my 1kW 136kHz amp, and thought it may be better to
be build it standalone for future use with other projects. It's on
T200-2 toroids as per G3YXM's 1kW amp at  http://www.chriswilson.tv/amp.doc
 Should I use a plastic box, or is alloy / steel OK? if metal is OK, how far from the walls of the box
need the toroids be please? Thanks
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Best regards, Chris Wilson.

DL8OV

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RE: LF low pass filter toroids, how far from metal case?
« Reply #1 on: November 14, 2014, 07:38:52 AM »

Two thoughts spring to mind:

1) The file you linked to talks about toroids getting hot and needing fan cooling, it therefore sounds as if you will need lots of space inside for air to circulate (and a fan).

2) The file also contains this text at the end: "Warning: If you apply 800W or so of 136kHz to a high Q aerial system, the Voltage on the wire will probably be over 20kV." Air breaks down at a rate of 30KV/cm so this means a minimum of 1cm between the case and any point in the output circuit.

Peter DL8OV
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N7EKU

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RE: LF low pass filter toroids, how far from metal case?
« Reply #2 on: November 16, 2014, 07:21:48 PM »

Hi,

I think alloy or steel would be OK.  I've read that metals do vary in their effects on coil Q.  Aluminum (being non-ferrous you only get mostly capacitive coupling) affects this less than steel.

For spacing, the general rule of thumb is to space at least 1 coil diameter.  Even though toroids are supposed to be self-shielding, I find they really aren't.  I think what the real advantage with them is that the coil diameter is much smaller than with an air-wound solenoid coil, and there is not magnetic field at the end because there is no end!  Sorry wordy, but I space toroids also at least 1 diameter of the coil (basically the thickness of the core, not its overall diameter).

I think that's a nice idea to build it separately so you can use it in different projects.  You need some serious inductance there for 136kHz though hey?!

73,


Mark.

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Mark -- N7EKU/VE3

2E0ILY

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RE: LF low pass filter toroids, how far from metal case?
« Reply #3 on: November 19, 2014, 10:47:06 AM »

Thanks for the replies Peter / Mark, some good advice there, much appreciated, will probably build standalone in a capacious box, thanks again!
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Best regards, Chris Wilson.

G3RZP

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RE: LF low pass filter toroids, how far from metal case?
« Reply #4 on: November 19, 2014, 02:26:56 PM »

According to the book 'Radio Frequency Measurements' by Hartshorn, published by Chapman and Hall, London, 1940, having a last turn in the opposite direction on a toroid with a ferromagnetic core considerably reduces the stray field outside the toroid which is caused by the winding being spread out along the core.

To get good screening, I would use tinplated steel of at least 22 SWG and spaced at least 1 inch from the toroid as well as Hartshorn's reversed turn technique.

I did an article on shielding for the 'Signal', the journal of the Vintage and Military Amateur Radio Society. If you want a copy, email me at g8on at btinternet.com - substituting '@' for 'at', of course

73

Peter G3RZP
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2E0ILY

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RE: LF low pass filter toroids, how far from metal case?
« Reply #5 on: November 20, 2014, 12:09:39 AM »

Thanks Peter, mail sent, much appreciated.
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Best regards, Chris Wilson.
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