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Topic: Plasma Speaker Protection (Read 1 time) previous topic - next topic

Catcher

Hello,

Sorry nothing to do with an arduino. I'm interested in studying the schematics for a plasma speaker and I have one question about protecting the MOSFET from back emf of a flyback transformer.

Would a flywheel/flyback diode protect the mosfet? And will the plasma speaker still function as normal?

This is the circuit I'm studying: http://www.instructables.com/files/orig/FJH/CRW6/GYUQTKOF/FJHCRW6GYUQTKOF.gif
With this idea in mind: http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Flyback_Diode.svg/500px-Flyback_Diode.svg.png

There's probably a good reason it's not included on the dozens of 555 schematics I've seen.

Thanks for helping me!

jwatte

MOSFETs already have a built-in counter-current diode. It comes out of how they're made -- they will conduct going the "other" way. The one thing to watch out for is super high voltage spikes; don't go over the rated voltage for the MOSFET!

Catcher

Thanks!

Suppose there was a super high voltage spike and the MOSFET was in its offstate.

In a situation without a flywheel diode I would expect the mosfet to look like an open circuit and current would try to flow through the mosfet into ground, potentially killing it. What would be the benefit of having MOSFET with a built in diode conducting the other way in this situation? Wouldn't that imply the current was moving upwards? (I thought inductors hated change in current, so they should conduct in the same direction). It seems to me that the diode would only be beneficial if it was parallel accross the inductor itself, so the current would flow to the lower potential which is the positive terminal of a battery.

Sorry if I'm not understanding. Thanks!

cjdelphi

Cap 2/3 what's with the parallel capacitors?  - is it purely to save costs on a larger cap? or some other reason?

jwatte



Suppose there was a super high voltage spike and the MOSFET was in its offstate.



Use a TVS and/or a Zenger and/or a high voltage rating MOSFET?
Where is the spike coming from?
Kickback from a motor may come from the inductivity, but turning a motor will also generate current (like a generator/alternator)
A solenoid has less of that effect (but still some AFAICT)

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