Hi All,

I'm looking to upgrade my fixed frequency PWM to an adjustable one for resonnance experiments.

Right now I'm using a Kelly dc motor controller, wich has proven to be very reliable so far. 

I'm thinking about a new cell design with overhanging plates, to allow for mechanical / acoustic resonnance, as well as L/C resonnance, serie or parallel, depending on the design chosen.

From there I thoughts about several options, including:

1 Buy an off the shelf unit with adjustable frequency. I remember seing one from a guy in Germany, I think his company was "Inovation Shope" Don't know if he's still in buisness. It was fairly small scale and artisanal..

2 Buy an audio amp, car audio amps are cheap, powerful and reliable, and very efficient, especially the Classs D ones.  Only things is that they must used with sinewaves only, as they are not made to do square waves. 

3 Try to hack into the program of the Kelly motor controller, to be able to adjust the frequency..

These are made with PIC micro controllers that have been factory programmed to a specific frequency, so I don't see why this could not be changed..  I've seen this made with another controller brand.. 

I gues there would be some limitation to doing this, because the components, mosfets and the like have been optimised to work at a certain frequency, and could get hot or loose their efficiency if operated too far outside their intended range..

Any idea on this? 

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Yes... I've been thinking about this one for a little while now.

How about using a H-Bridge and driving the gates with a variable frequency  square wave signal?

What sort of frequencies are you thinking about?

A BTS7960 should be good to around 100k.

Just been doing a little bit of research.

A  555 / LM358 square wave module from EBay is good to 200 khz or more. 

That will give you a 5v pp signal. More than capable of driving a H-Bridge module.

Not quite "off the shelf" but available by combining two or three easily available modules.

Yes, 

I've already bought a DDS signal generator, cheap from ebay and a opto isolated module with mosfets ( I don't know exactly how it's called ) to be able to drive bigger mosfets, and avoiding a transient current damaging the signal generator. 

I didn't tested these units yet, cause I didn't know where to start and my knowledge in electronics is very basic. 

I felt I needed something more robust, for real world testing in a vehicle.

I hope I'll find the instruction manual for these devices..

In the meantime I'm looking to find a motor controller manufacturer that can make a customized version with adjustable frequency.. Those controllers are very robust and have over-voltage and over temperature protection, and can handle lots of current at fairly high frequencies, without heating up.. Only drawback is the fixed frequency.. 

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