Hi Bruce, the impedence of piezos will vary with frequency but what's more
important is that they can also generate a voltage when they are
mechanically flexed. (Same principle used in lighter ignitors) It'll be
good practice to include a series resistor and protection diodes from each
terminal to VCC. (Same as the back EMF diodes for relays)

Some microcontrollers have protection diodes on the outputs, they're speced
at about 14V (PIC12Cxxx) Depending on the size of the piezo, mechanical
force and speed of flexing, the generated voltage could damage your driver.

The final decision is yours.

Terry



At 02:10 PM 7/25/99 -0700, you wrote:
>Hello all:
>I've been developing a device which includes a piezo element driven
>differentially by two IO lines, no other components.  It works great and
>draws very little power for so much sound.  Too easy, I wonder?  I've looked
>over past PICLIST discussions of driving piezo elements with the PIC (and
>have searched the web for other info as well to no avail), and have seen
>mention of a series resistor, annd occasionally external protection diodes.
>1. Can anyone give me a nutshell explanation of why these elements might be
>neccessary?  I understand that there is an average current at the given
>frequency, but is there also an instantaneous (or do I mean periodic?
>inrush?) current and/or voltage which I must consider as well (and which is
>dangerously higher)?
>2. Can anyone recommend a source for additional reading to help reduce my
>ignorance?
>Thanks,
>
>Bruce Cannon            Style Management Systems
>bcannon@jps.net         http://www.jps.net/bcannon
>(510) 787-6870          1228 Ceres ST Crockett CA 94525
>
>Remember: electronics is changing your world...for good!
>