Re: Capacitive transformers ? another ponderance for ye experts

Dave Moore wrote:
> : "Dave Moore" <valveless@xxxxxxxx> wrote in message
> : news:lBsmf.70847$Y82.15037@xxxxxxxxxxxxxxxxxxxxxxxxx

> : > 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.


> 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.


> 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.

> 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.


> 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

.

Relevant Pages

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