Rotax voltage regulator

[Jan Olieslagers]

For those who are into electrics/electronics, and can make out the Dutch language, there is a quite interesting article on another forum:
https://www.circuitsonline.net/forum/view/135679
discussing the original (Ducati) rectifier/regulator, including schematics. Also giving an in-depth test and discussion of an after-market alternative from Silent Hektik. I am now trying to convince that chap to run the same tests on another alternative, the one I use, which is from one Mr. Schicke, also German.

[9 lives]

I think I can understand "Hallo" and "kapot" strikes a chord with me. In seeing a reference to "100V", I think I know the topic;

Some automotive alternator systems when used on aircraft do not include in the circuit, an external (to the alternator) voltage regulator. They use the internal one in the automotive alternator. The difference being that with an external voltage regulator may also have an airframe over voltage sensor in the circuit. In a Cessna, that's what turns on the little red "Voltage" light. If the OVC senses an OV, it will open the circuit (otherwise closed by turning on the alternator half of the master switch), and the red light will tell that it has done that. This happening saves all your avionics from being subjected to the high voltage. In a 'plane, this could really matter, in a car, much less.

If the circuit does not have an external regulator and OVC, instead having an internal regulator, there is no pilot or system control of the alternator, when it goes OV, and a very high and damaging output can result before being noticed. There is a device made by Zeftronics called a "crowbar" which may be installed in the circuit to correct this vulnerability, though I'm not aware of one in a certified installation.

So if you are contemplating installing an internally regulated alternator, understand the difference, and implications to your avionics. I did have the very expensive alternator on my Audi at the time go over voltage. I saw the voltmeter go off the scale beyond 16V, and the headlights became very bright. I pulled over, but not before I had blown a few bulbs. I unplugged the alternator, starter 'er up, and drove 'er 'ome, to then learn the cost of an Audi alternator. Instead of buying one new, I bought a whole used Audi, drove it home, and moved the alternator over - it was cheaper!

Truly understanding your electrical system is important. If I have misguessed the Dutch, as you were....

[Jan Olieslagers]

I am afraid you are not really on the track. In fact the Rotax installation is quite different from what one sees in "traditional" aircraft engines, and in cars; it is indeed closer to motorcycle habits. Actually the author is into motorbikes, he is not acquainted with aviation installations but he knows that one and the same rectifier/regulator is used in the Rotax 912 as in certain motorbikes. Rotax orginally installed a part from Ducati, which is however underdimensioned and not very well designed. Thus there came some after market alternatives, the essence of the article is to compare one such with the original.

On the flywheel, on the rearside of the 912, there are some strong magnets. By rotating under a collection of coils, they generate electric power in them. 4 coils drive one ignition system, 4 more the other, 2 in series charge the battery, the last one supplies the tachometer. The ignition system is from Ducati, too, and quite expensive. I have heard the Rotax also uses a starter motor from a motorbike.

The output voltage and power available from the coils depend directly on engine rpm and on nothing else; so that a regulator is required. All known designs apply a Graetz bridge rectifier, with two of the four diodes being SCR's that do the regulation.

As for the crowbar device you mentioned: it is my lasting surprise one sees them so rarely. One glitch in the charging circuit can be enough to blow up several thousands worth of avionics! I use a circuit published by a gentleman from California; it works - as the name says - by short circuiting too high a voltage to ground, thus blowing the fuse or tripping the breaker.