From: Humphreys, David (david.humphreys_at_honeywell.com)
Date: Wed Jun 20 2001 - 08:58:38 PDT
Marco opined:
> Hi David,
> you commented on my post but I do not completely agree.
Ok. But that means that you don't agree with a physics
phenomenon that I can prove; magnetostriction.
> You're right the
> effect has to be verified. If encapsulation doesnt make the
> thing completely
> silent it's probably not worth the work and risc.
I will save you some time and effort and tell you
it won't work.
> > When a Ferro-magnetic material (transformer core) is
> > placed in a changing magnetic field, the material will
> > change length slightly.
> I don't believe that the iron core will change length when placed in a
> magnetic field.
And I don't believe Elvis is dead, but length change is
something I can prove. ;)
> 50Hz sheeted metal cores of standard mains
> transformers make
> 50 & 100Hz noise because the single metal sheets vibrate but
> don't change
> length.
Power transformers are laminated to reduce the eddy currents
that flow in the core. Breaking the core into smaller pieces
increases the resistance to these losses. In most transformer
designs, the laminations are lacquered, but more importantly,
they are held securely either by screws or the outer case.
The sound is still produced, primarily, by magnetostriction.
A real length change occurs e.g. in piezoelectric
> materials like
> Quartz or similar crystals and glass-like materials.
Aha! Now I see your confusion. Certain materials such
as quartz exhibit the piezoelectric effect. This is not
the same as magnetostriction.
Piezoelectric materials will mechanically deform as an
electric field is applied across two opposite faces of
a crystal of that material. The converse is also true.
A potential will develop across two opposite faces when
a mechanical deformation occurs.
>
> >... that accounts for the noise as it travels through the
> > surrounding parts.
> That's a very important point: If the noise can't be avoided
> than it might
> be sufficient to screen it.
But that's the point, my friend, you can't screen it.
> > Any encapsulent will, not only move with the core, but will act
> > as a conduit for the sound, coupling it to the case and making
> > more noise!
> The core itself in the case of the Newton consists of two
> single ceramic
> ferrite magnets (an extremely hard and unelastic material).
Not correct.
The core is made of what is loosely termed 'ferrite'. But this
is simply finely powdered iron coupled with a binder and shaped
under pressure. It is not a magnet, but it is magnetic.
It IS hard and inelastic but this has nothing to do with magnetostriction.
> I think only the
> wires are causing the noise in our case.
They too will undergo magnetostriction and contribute to the noise.
> I tried
> encapsulation under vacuum
> and found that it is possible to get the epoxy resin into
> each space left
> between the wire turns. The atmospheric pressure will press the resin
> everywhere, provided that it is put over the transformer
> under vacuum and
> the whole thing is vented as soon as the transformer is completely
> surrounded by the resin. Anything else will not work.
Did you do a microsection? That would be the only way you could be
certain the epoxy resin totally filled all the voids.
>
> > The shape of the pulse being fed to the transformer plays a big
> > part too.
> I measured the pulse shape of the approximately 400Hz pulse. It looks
> similar to a sinewave with a slight sawtooth distortion, but no sharp
> corners nor extremely fast rise times like with a rectangular pulse.
> See: http://newted.dyndns.org/users/mmailand/www/Newton.html
Ok. Then cross that off the list.
>
> To produce these 40V/20mA backlight drive a charge pump is
> very likely not
> strong enough or needs to large capacitors. However, this
> technology (w/o
> transformer) would be not so noisy.
True. Switched capacitor solutions do exist.
Maxim has a number of IC's that might be useful. But, even if
you could find a solution, it probably not fit in the case.
Also, the transformer does multiple duty. It supplies other
voltages. You could not simply remove it.
> With best regards / Viele Gruesse
>
> Marco Mailand
> http://slsbd.psi.ch/timing
Whatever you decide to do, Marco, have fun doing it!
Regards,
David Humphreys
Designer
Honeywell Inc.
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