Runaway Generator ROTAX 912ULS

RW,

Do you think it’s possible to see an elevated output voltage when the battery is disconnect even WITH the capacitor installed?  It’s an honest question, since I’ve not observed it. However, the placement of the capacitor in this circuit is essentially a high pass filter, bleeding off higher frequencies to chassis ground.  While this may protect the regulator from an initial voltage spike I can’t see it doing much for the maintained voltage once the battery is disconnect.  This is the reason for my earlier comment suggesting the battery may have dropped out.  

Hi Jeff

Thats a good question.  I would have to ask one of my contacts to find out, it has been a long while since seeing one and watching what the voltage did.  I do know that without it the regulator will simply fail inside.  I don't think there is any overvoltage protection (crowbar overvoltage protection) in the factory wiring recommendation.  In my opinion that would perhaps be the best solution in the long run.  

I will look into it some more.

Cheers

Hi Jeff...I did look into this and can confirm that there is NO overvoltage protection in the existing regulator for the 912/914 series.  As you know the injected engines are 3 phase and very different.  In a failure they kind of just melt the wires.....not nice.  

I do have some schematics of crowbar overvoltage protection one can make for the 912/914, it is not simply anything you can bolt on as I recall however.  If you want copy I can post it.  You can find great material from AeroElectric Connection  Website: aeroelectric.com

Cheers

Robert Nuckolls over at the AeroElectric-List forum started working on an updated design for overvoltage protection last year.  He posted his draft final schematic and circuit board layout in February and stated that he had circuit boards on order to produce a few prototypes for testing.  Unfortunately, Bob has been largely absent from the forum since then, so the current status of his project is unknown.

I built and bench-tested my own adaptation of Bob's new design and found that it works as it should.

EDIT:  A couple of notes about Bob's schematic and board layout.  1. To correlate the components in the schematic with the part locations on the circuit board, simply ignore the letters in the reference designators for each part.  For example, the LM431 is labeled U3.  Drop the "U" and simply match that part to item #3 on the board layout.  So, U3 in the schematic is part 3 on the board.  2. Q15 is a surface mount SCR that is soldered to the back of the board (bottom layer in Bob's document).

In case anyone is interested in this sort of DIY electronics project, here's a diagram showing how to apply Bob Nuckolls' overvoltage protection circuit to a Rotax 9-Series engine.

If you're not confident in your ability to construct and install this system in a robust manner, I'd suggest leaving things as Rotax intended.  Attack this project only if you know what you're doing, as the consequences for errors are not trivial.  Needless to say, this is for Experimental aircraft only.

Do not attempt to implement overvoltage protection by interrupting the rectifier/regulator's output to the battery or Fuse Box instead of its input from the stator.  A properly functioning rectifier/regulator can be destroyed by the load dump that results from a nuisance trip of an output-side overvoltage protection system.