Category: 912 / 914 Technical Questions 6 months ago
  • Re: Capacitors

    by » 6 months ago

    Bill Hertzel wrote:

    A 22,000μF at 25-volt Capacitor is the correct value.

    The Positive Capacitor Terminal connects to the B+ terminal on the regulators, and the Negative terminal connects to Frame/Battery Ground

    The Caps Labeled #14 in the diagram are mislabeled as 1μF and should be 22,000μF.

    A 25-volt cap will be sufficient. 80v on a 12v system will be overkill and will be significantly larger in size.

    A 25v Cap operating at less than 16 volts is already reasonably/safely over-rated.

    80v might not be needed but as it will not improve, but also, not degrade anything.  If it makes you sleep better, so be it!

    The 1μF Cap labeled #22 is also correct.  It is for Radio Frequency (RF) Noise suppression of the electric Fuel Pump.

    The diagram shows Two Capacitors.

    One for each Generator, items #5 and #10.

    If you do not have the Secondary/External #10 Generator installed, you will only have One Cap #14 on the Primary Generator #5.

    The reason for two Caps is because they need to be located close to the Generators/Regulators and before any Switches or fuses.

    If you only had one Cap installed at the primary Regulator, and the Secondary Generator was to be isolated from the Battery, its regulator would not see a load and you would risk damaging the unloaded regulator.

    - - -

    It is interesting to note that neither Capacitor is needed nor performs any function as long as the Generators are connected to the Battery.

    So 99.999% of the time they do nothing.  They are just $10 insurance policies for the $300 Regulators.

    This is why every now and then someone will declare that they have been flying for years without a Cap and never have a problem.

    I can vouch for not having an issue without a capacitor until.....

    I'm the 4th owner of my plane. It had 600+ hours and one day while flying along I saw the low volt light come on and the voltage drop to just below 12. It would go back to normal then drop again after a few minutes. Subsequent "testing" revealed when I'd first start everything was fine then after a few minutes I would have the low volt issue. I researched it and came to the conclusion it was probably the voltage regulator. I also learned that there was supposed to be a capacitor installed. Hmm. All the people that had worked on this plane over the years never noticed? So I ordered a new regulator and the capacitor and my current A&P and I installed it. Working fine since.

  • Re: Capacitors

    by » 6 months ago

    Thanks Bill, I appreciate your comments. I couldn't find a Cap rated at 105C so that was why I was considering the 80v Cap. But as you mentioned, it seems overkill and is certainly larger, so went with the 25v Cap at 22k.

  • Re: Capacitors

    by » 6 months ago

    105°C is not necessary.

    Rotax does not offer any Capacitors for its 912ULS engines.

    It does, however, supply Capacitors within the Fuse box of its 192is and 915is engines.

    The Fuse Box and ECM of these engines are temperatures limited to 80°C.

    This would seem to imply that the internal components are rated at the industry standard of 85°C.

    Your 912 ULS Rectifiers are also limited to 80°C and the Carbs and Fuel lines to 45°C.

    Keep in mind that 105°C ( 221°F) is hot enough to Boil Water and 85°C (185°F) would not only be hot enough to Fry an Egg it is also hotter than you need to cook any cut of meat and certainly Way Hotter than you would be willing to touch.

    It should be trivial to find a location that will keep the Cap well under 85°C

    Sleep Well! smile

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated.

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