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Dave Moore
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 Posted:
Sat Dec 10, 2005 9:35 am Post subject:
Capacitive transformers ? another ponderance for ye experts |
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I just read an article that reminded me that if
one could somehow change the capacitance
of a charged capacitor, the voltage would change
accordingly. So methinks to meself, hmmm
plates layered in piezo?
So what if it'd be the size of a house :-)
Any thoughts on this?
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Dave Moore
Guest
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| Posted:
Sat Dec 10, 2005 9:35 am Post subject:
Re: Capacitive transformers ? another ponderance for ye expe |
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"Tim Williams" <tmoranwms@charter.net> wrote in message news:1otmf.3841$ES.1775@fe05.lga...
: "Dave Moore" <valveless@last.net> wrote in message
: news:lBsmf.70847$Y82.15037@bignews4.bellsouth.net...
: > I just read an article that reminded me that if
: > one could somehow change the capacitance
: > of a charged capacitor, the voltage would change
: > accordingly.
:
: This is done often in RF (read: narrow bandwidth), where you put two (or
: more) capacitors of various value in series and the voltage divides like a
: resistive divider, except because the reactive current is canceled with an
: inductor (the total series capacitance is the capacitor side of a tuned
: circuit), you can divide it with little power loss
True, not to mention there are already piezo transformers
in existance, even high powered ones such as the "Transoner"
which I'm hoping will actually become available soon.
http://www.transoner.com/
But I'm thinking of some sort of stacked plate device
with terfenol or possibly some kind of MEMS membranes
that vary the capacitance of a single flying capacitor.
Of course, the switching circuitry would consume some
power, hopefully minimal.
Switch the cap to the source in the expanded low capacitance
mode, then switch it over to the load and contract the plates.
But then again, I reckon this is DC/DC conversion and not
necessarily the equivalent of an AC transformer and would
require rectification of an AC input.
I'm simply proposing a device. It's up to ye experts to
determine what if anything could be done with it,
apart from a door stop or paper weight :-)
-DM-
:
: I thought about this problem once (that is, an electric rather than magnetic
: transformer), but I don't remember my thought process. I don't think it'll
: work the same because electric fields have monopoles, whereas magnetic
: fields are always loops. It also wouldn't work as you'd expect, since
: voltage and current are reversed: instead of a low impedance (constant
: voltage) source, you need a low conductance (constant current) source to
: make an equivalent circuit.
I think I see what you mean. IOW for near 100% efficient power conversion.
But wouldn't it work for wideband lossless voltage conversion as apposed to
a narrow band response from a resonant LC?
Certainly there's gotta be something useful there in some way shape or form :-)
:
: Tim
:
: --
: Deep Fryer: a very philosophical monk.
: Website: http://webpages.charter.net/dawill/tmoranwms
:
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Tim Williams
Guest
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| Posted:
Sat Dec 10, 2005 9:35 am Post subject:
Re: Capacitive transformers ? another ponderance for ye expe |
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"Dave Moore" <valveless@last.net> wrote in message
news:lBsmf.70847$Y82.15037@bignews4.bellsouth.net...
| Quote: | I just read an article that reminded me that if
one could somehow change the capacitance
of a charged capacitor, the voltage would change
accordingly.
|
This is done often in RF (read: narrow bandwidth), where you put two (or
more) capacitors of various value in series and the voltage divides like a
resistive divider, except because the reactive current is canceled with an
inductor (the total series capacitance is the capacitor side of a tuned
circuit), you can divide it with little power loss.
I thought about this problem once (that is, an electric rather than magnetic
transformer), but I don't remember my thought process. I don't think it'll
work the same because electric fields have monopoles, whereas magnetic
fields are always loops. It also wouldn't work as you'd expect, since
voltage and current are reversed: instead of a low impedance (constant
voltage) source, you need a low conductance (constant current) source to
make an equivalent circuit.
Tim
--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
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Guest
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| Posted:
Mon Dec 12, 2005 1:35 am Post subject:
Re: Capacitive transformers ? another ponderance for ye expe |
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Dave Moore wrote:
| Quote: | : "Dave Moore" <valveless@last.net> wrote in message
: news:lBsmf.70847$Y82.15037@bignews4.bellsouth.net...
: > I just read an article that reminded me that if
: > one could somehow change the capacitance
: > of a charged capacitor, the voltage would change
: > accordingly.
|
| Quote: | True, not to mention there are already piezo transformers
in existance, even high powered ones such as the "Transoner"
which I'm hoping will actually become available soon.
http://www.transoner.com/
But I'm thinking of some sort of stacked plate device
with terfenol or possibly some kind of MEMS membranes
that vary the capacitance of a single flying capacitor.
Of course, the switching circuitry would consume some
power, hopefully minimal.
|
What sort of capacitance would you need to do useful work? 10s of uF?
How much plate area is that? You now have to provide that much piezo
area, or a fair fraction of it, and vibrate it all at audio frequency.
Easy to roll capacitor plates into a can, but for this you'd need them
flat, which is none too convenient. And you'd need power input to the
piezo element.
| Quote: | Switch the cap to the source in the expanded low capacitance
mode, then switch it over to the load and contract the plates.
|
that would drop the V. V boosting is worth more money, though has many
less apps.
| Quote: | But then again, I reckon this is DC/DC conversion and not
necessarily the equivalent of an AC transformer and would
require rectification of an AC input.
|
a diode is no problem. I guess youre thinking of replacing wall warts.
| Quote: | I'm simply proposing a device. It's up to ye experts to
determine what if anything could be done with it,
apart from a door stop or paper weight :-)
-DM-
|
Let me propose another equally problematic design option, one that
bypasses the need for costly piezo or drive power.
The capacitor uses a domed film as the dielectric. This behaves a bit
like the rubber mebrane on a keyboard: with no applied pressure it
stays thick, when force is progressively applied it suddenly collapses.
It springs back when applied force is removed.
Now, apply a charge to the capacitor through a diode. At some point, as
V increases and the interplate forces rise, the domed film collapses,
the plates get much closer, and C rises considerably. You now have a
lower V with the same charge as before. Yes, youve reduced v, but you
havent added i, and this is the problem in both applications. No
transforming effect. Just a mechanical loss.
Theres also no isolation like this, so no use for wallwart apps.
FWIW discharging the cap remove interplate forces, allowing the film to
recover.
So the problems are high cost, large size, operating energy
consumption, no transformation, no isolation, noise and vibration, and
the realiability issues that tend to go with mechanical devices. R&D's
a b--ch!
NT |
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