Ferrite transformer query.

I use RM8 ferrite cores as signal transformers in the range 10kHz to

100kHz. It's a half duplex system so is switching between transmit and receive.

There's quite a lot of variability in the audible noise from these, some you can barely hear, others there's a pronounced clicking as transmit starts.

These have been hand wound by me and others for prototypes. Would a 'proper' transformer manufacturer pot or encapsulate these by default?

The cores are ungapped and very sensitive to assembly - measure the inductance, take it apart and re-assemble and you get a different inductance. Is there some recommended way of putting these together, for example cleaning the faces with something, or using some sort of glue?

I don't want a toroid, thanks.

Cheers

--
Clive
Reply to
Clive Arthur
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On a sunny day (Mon, 6 Nov 2017 12:30:56 +0000) it happened Clive Arthur wrote in :

Is there not some spring loading mechanism to really push the halves together?

Reply to
Jan Panteltje

Yes, we are using the appropriate spring clips.

Cheers

--
Clive
Reply to
Clive Arthur

?

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Is the data sheet for the EPCOS RM8 cores and accessories.

It includes a pair of spring clamps that do hold the core halves together q uite tightly. It does make sense to wipe the mating faces clean with a low viscosity solvent. If you buy gapped cores, the gap is only about 0.2 mm wh ich drops the inductance by about a factor of ten, and a ten fold smaller i nadvertent gap would be enough to make a significant difference.

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Bill Sloman, Sydney
Reply to
bill.sloman

After cleaning, rub the core half mating surfaces together to ~polish off any hard or insoluble residue, before installing the (clean) bobbin.

RL

Reply to
legg

What is the inductance? How much does it change? Does it matter?

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

Are you using DC bias? Is the air gap consistent?

You can try to arrange the circuit to prevent DC bias or startup transients. Using a push-pull rather than single-ended circuit, for instance.

Ferrite has nonzero magnetostriction, so it will necessarily vibrate a little, at whatever frequency you're magnetizing it at, including startup transients or DC bias (or twice the frequency, if unbiased). This vibration can only be dampened with potting and [acoustic] shielding.

A good example is the old 15.7kHz CRT flyback, which sometimes sang terribly loud because of poor manufacture, but was always audible thanks to magnetostriction. Well, maybe not audible to old people, but us not-so-young-anymore-'ins know this well.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

Can you use a gapped core with more turns? I think you get more consistency with gapped cores.

Mikek

Reply to
amdx

Am 06.11.2017 um 20:31 schrieb amdx:

Yes, just a little bit of Kapton tape to create a small, repeatable air gap.

Reply to
Gerhard Hoffmann

No, the bobbin is pretty full, so no more turns.

Cheers

--
Clive
Reply to
Clive Arthur

There's 10mH winding and two 1mH, it wouldn't matter much if they changed a bit, but it has been demonstrated that fiddling with a transformer does change the signal quality a little. I'm hoping that properly made coils will be more consistent, but it's not something I know much about.

Cheers

--
Clive
Reply to
Clive Arthur

You can buy gapped RM8 cores with a closely specified Al.

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lists lower inductance constants at +/-3%, going up to +/-5% and +/-10% fo r smaller gaps.

Kapton tape will do the same job, but the tolerances aren't as tight.

Effectively what you are doing is making the magnetic path length longer, a nd letting the extremely stable air-gap dominate, so gapped cores do offer a much more stable inductance.

--
Bill Sloman, Sydney
Reply to
bill.sloman

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** Suspect core saturation as the cause of the clicks.

LF components of the waveform during switching could saturate the core and momentarily magnetise it - hence the click. The more perfect the mating of the opposing faces the worse this will be.

** Probably, if the clicking noise is an issue.

Pick a noisy one and super-glue the faces together for a test.

.... Phil

Reply to
Phil Allison

Saturating the core doesn't "magnetise" it - it takes it to the point where it can't get any more magnetised. The current through the winding will start changing more rapidly at this point (if it can).

The mating surfaces will be as engaged as they will ever be by the time the core saturates, so saturation isn't going to make the core "click". If it was going to click it would have done it before the core material saturated.

Hand-wound wiring might be sloppy enough to let individual turns move around as the forces on the wire change, and they could move enough to give you a click.

Coil winders frequently stick layers of kapton transformer tape between windings, which tends to make the winding more regular and stop the wires moving about.

You can do even better by winding the coil with self-bonding wire, then putting enough current through the completed coil to warm it up enough to let bonding layers fuse. Great for self-supporting coils too.

Agreed.

It will drop the inductance a bit.

Phil isn't going to like this, but I won't bother abusing him in anticipation.

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Bill Sloman, Sydney
Reply to
bill.sloman

Looney bill puked more vomit:

** So fully magnetised, like I said.

Normal operation does nothing like it.

The situation is comparable to switching ON a mains transformer and hearing the inrush surge as a bonk or thud.

** Which will, if anything, reduce the onset of core saturation.

It's worth a try and will reveal if the winding is the real cause of the noise.

.... Phil

Reply to
Phil Allison

When repeatedly switching digitally between fixed frequencies or uncorrelated sources, it's not an easy matter to control the static flux setting.

In signal-type parts, where a good portion of the flux swing was employed, I've had to begin and end pulse streams with half-width starts and finishes, in order not to stress drivers during the start of a stream.

It's one way programming firmware can squeeze the most out of magnetic parts that are meant to be at minimal cost and maximum reliability.

I think modulated information transmission can have similar issues.

RL

Reply to
legg

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** Another test the OP should do is to monitor the primary current in his tranny during operation and compere noisy and quiet examples.

.... Phil

Reply to
Phil Allison

o

and

some

it

e and momentarily magnetise it - hence the click. The more perfect the mati ng of the opposing faces the worse this will be.

here it can't get any more magnetised.

What you said was "magnetise" not "fully magnetise"

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pdf

shows a ferrite material - N87 - that satuates at about 500mT at room tempe rature (down to a bit less than 400mT at 100C), and is useful (if lossy) at up to about 200mT.

You'd normally aim for a peak field of 100mT, which is not all that far awa y.

ng the inrush surge as a bonk or thud.

That's a different problem. A mains transformer can be left magnetised in t he wrong direction, in the sense that turning the mains on at wrong instant can saturate the core even if it wouldn't have saturated if soft-started - and it can keep on going in and out of saturation for a couple of cycles, which is noisy, but usually non-damaging.

Soft ferrites can be magnetised to some extent, but they aren't iron.

Marginally. The thin layer of super-glue increase the magnetic path length around the core, but the magnetic path length through the core is roughly e quivalent to about 20 micron of airgap, and you'd try to get a thinner glue

-line than that

noise.

It's certainly going to give you extra information.

--
Bill Sloman, Sydney
Reply to
bill.sloman

LUNATIC Bill spewed.

--------------------

** I know and can see what I wrote, funny how you cannot.

FFS moron - read the *whole* sentence.

** But a comparable one - you revolting pedant.

** There is no right one and such events are rare. A fuse has to blow at the crucial moment for it to happen.
** Redundant nonsense.
** Read the whole sentence- f*****ad.

** Meaning there is no disagreement here AT ALL !!

Feel free to drop stone dead - anytime.

..... Phil

Reply to
Phil Allison

On a sunny day (Mon, 6 Nov 2017 13:15:03 -0600) it happened "Tim Williams" wrote in :

Yes, I could hear if the TV was in H sync (most would go lower than 15625 Hz if not in sync) Those are gapped U I cores, with some paper in between as gap. I have used plastic spray on those to silence those. Not sure if plastic spray on a potcore helps, or even is a good idea, never done it.

Reply to
Jan Panteltje

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