From: Marco Mailand (mmailand_at_newted.dyndns.org)
Date: Wed Jun 20 2001 - 02:20:22 PDT
Hi David,
you commented on my post but I do not completely agree. You're right the
effect has to be veryfied. If encapsulation doesnt make the thing completely
silent it's probably not worth the work and risc.
> 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. 50Hz sheeted metal cores of standard mains transformers make
50 & 100Hz noise because the single metal sheets vibrate but don't change
length. A real length change occours e.g. in piezoelectric materials like
Quartz or similar crystals and glass-like materials.
>... 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.
> 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). I think only the
wires are causing the noise in our case. 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.
> 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
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.
-- With best regards / Viele Gruesse Marco Mailand http://slsbd.psi.ch/timing -- This is the Newtontalk mailinglist - http://www.newtontalk.net To unsubscribe or manage: visit the above link or mailto:newtontalk-request_at_newtontalk.net?Subject=unsubscribe
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