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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Piezo Ignitor to power an LED -- any ideas?
References: <email@example.com> <firstname.lastname@example.org> <email@example.com> <3DDD8241.5D571789@webaccess.net> <firstname.lastname@example.org>
Date: Fri, 22 Nov 2002 14:20:46 GMT
NNTP-Posting-Date: Fri, 22 Nov 2002 06:20:46 PST
John Fields wrote:
> On Fri, 22 Nov 2002 01:03:14 GMT, Chuck Simmons
> >I don't see any need for high voltage diodes. (I actually don't see the
> >need for the bridge at all because the main pules is DC anyway.) The
> >capacitor will insure low reverse voltage on the diodes. You are a
> >little sensitive to current in that the charging current may be high.
> The bridge (or some sort of rectifier) is needed because the output
> from the piezo is oscillatory about zero volts. You're right about
> the high voltage diodes (in this application) because of the high
> impedance of the piezo and the low impedance of the cap, but only
> because a bridge is being used. Consider this:
> I don't think the piezo can source enough current to worry about,
> because of its high impedance, but I'll try to measure it today and
> post what I find.
There are a number of cases where piezo device outputs are DC. The one
most commonly seen is the bender bimorph. If you bend it and hold it
bent, there is no oscillation. The electric field would be constant if
it did not slowly bleed away. A piezo device under constant acceleration
is DC. Since the oscillation you mention is purely mechanical (not
electrical), whether oscillation occurs or not depends strictly on the
mechanics. This is perfectly clear if you have ever used piezoelectric
positioners such as PZT stacks or bender bimorphs. They are DC
positioners and the effect is the inverse of the effect due to bending
or applying external force. I'm not sure what impedance you are talking
about. If you measure the devices with a charge amplifier, it is
difficult to make sense of impedance. If you are talking about the
effective series resistance in the equivalent circuit, this resistance
is usually just a few ohms if that. The fact that this resistance is
very low makes piezo devices useful as accelerometers. Back in the
1970s, I had accelerometers in the lab with 40 MHz response using quartz
(quartz was selected for mechanical properties as well as its excellent
piezoelectric constant) and a charge amplifier. The RC time constant due
to the effective series resistance did not limit bandwidth. The limit
was in the charge amplifier itself. Charge amplifiers are usually
bootstraps and so there is almost unity positive feedback.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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