Pulse motor build off time.

[MarkE]

HI Mark E,

Not what I'm doing, it's more like a pump circuit design with extra diode that protects mosfets.


Tom...

Tommey, for a low-side switch such as you have drawn, the voltage rise that will appear at the drain terminal of the MOSFET will be the sum of the L_STRAY * di/dt between the drain and where the catch diode cathode connects to the circulating current loop through the inductor.  If I interpreted your video correctly you have a circuit board for the MOSFETs and the diodes mounted on a separate block.  The wiring between those two assemblies will develop L_STRAY * di/dt voltage that the MOSFETs will have to tolerate.

If you want to recover back to the power supply then the conceptual circuit below can do that by replacing the SPST switch with a high side driver.  Then you have three phases:  MOSFET ON, MOSFET OFF coil discharge into the capacitor, and then capacitor discharge back through the coil to the supply.  The more conventional method is a full H bridge that just drives the coil and then discharges back into the supply.  Any of the modern H bridge drivers will do that for you.   You only need the top transistor and diode on one side and the bottom transistor and diode on the other side of your coil, a so-called "X" configuration.

[TommeyLeeReed]

Hi Mark,

I forgot the other diode from my drawing and also my diodes are going the wrong way....

Sorry, try to answer too fast without thinking...But it works in my design...Never had any mofets blown.

Tom

[TommeyLeeReed]

Hi Mark,

This is it.


Tom

[MarkE]

The diodes on the right as shown block al current from the power supply.  If you reverse them, then they don't do anything except dissipate power.  The diode on the left dissipates power and prevents reverse conduction through the MOSFET that could occur with long wiring loops.

Generally speaking it takes two SPDT switches to be able to drive current into a coil and then recover most of that energy back into the power source because you have to turn the current around with respect to the power supply while the current through the coil will only be going one way when it flows.  An alternative is to wind a transformer and use the secondary with a diode to reclaim the magnetization energy.

[TinselKoala]

I think I might submit this toy for the PMBO just so that I can have an entry in place this year (missed last year). There are far better ones, I know, but all  my better ones are already built so are ineligible and I'm fresh out of ideas. So far my vote would be for Tommey's build, whether it pops mosfets or recharges itself or not. I love the radial engine/microprocessor/linear alternator combo idea.
But watch out for Lidmotor: he's the king of PM builders and it will be interesting to see what he comes up with.

http://www.youtube.com/watch?v=uBIs9CA6K70