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22 microF Capacitor

in 912 / 914 Technical Questions 6 years ago

  • Re: 22 microF Capacitor

    by » 6 years ago

    I would agree they are there to soak up noise.
      
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  • Re: 22 microF Capacitor

    by » 6 years ago

    Thank you for your responses, experience wins out for me and I'll go with the one capacitor. I'm disappointed with the lack of clarity and errors on this subject in the 912 installation manual E2 Rev1 E. In the text it clearly states the requirement for a 22000uF capacitor on the output for both the voltage regulator (24-00-00 Page 7) and the internal voltage regulator of the Alternator (24-00-00 Page 14). It is silent on the topic of using both the alternator and generator together other than in the diagram on 24-00-00 page 19 where it shows both capacitors (item 14) but in the key to this diagram page 20 indicates these two capacitors are both 1uF! hence my initial confusion. The issue of using both generator and alternator together is another source of confusion. There are articles that state you should not do this under any circumstance eand others that say it is perfectly OK. The manual suggests that it is OK and garrett your experience confirms this so i'm going with that setup.
    I haven't got my engine yet and i'm just drawing up my electrical wiring diagram and list of part I need to order. I'm going to use the vertical power PPS, although it is not for sale yet, it should be tried and tested before I start my engine for the first time.

    Alan
      
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  • Re: 22 microF Capacitor

    by » 6 years ago

    I agree one is enough. The 22 capacitor is the minimum. You can go larger. Some use a 33 and some larger. Of course the size will get bigger to. Make sure you get the proper heat range of 105C. They make them less. The connections on the larger ones are far more substantial than the 22 cap. to.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Alan. Hunter
      
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  • Re: 22 microF Capacitor

    by » 6 years ago

    The capacitor is there to stabilize the voltage regulator in the times it is not connected to a battery.
    The battery acts as an ultimate capacitor.
    You can pump 10's of amps into it and the voltage will only change a few hundreds of a volt per second.

    Without the battery, the voltage regulator would raise the current slightly and the voltage would immediately go too high with only the wiring to work against.
    It would then reduce the current due to the overvoltage and grossly undershoot.
    The cycle would repeat hundreds to thousands of times a second.
    The resultant voltage waveform would be interpreted as a noisy regulator.

    The presence of the capacitor slows the regulation rate down to a level equivalent to having a battery attached.
    It has been determined that 22,000uF of capacitance is sufficient to stabilize the regulator without a battery attached.
    Any additional capacitance is unnecessary but will do no harm.

    The 1uF Capacitor called out in the Installation Manual is an obvious typo error.
    There is a 1uF cap across the Fuel pump that is reasonable.

    If it is possible to isolate each alternator from the battery individually, (Two alternator switches) you will need one capacitor for each regulator for the times the switches are OPEN.
    Notice that in the installation diagram that the capacitors are BEFORE the alternator switches.
    The Capacitors remain connected to the regulators when the Alternator switches are Open.

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

      
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  • Re: 22 microF Capacitor

    by » 6 years ago

    Hi Bill, thanks for your input, your explanation is they way I understood it. One last question if I may? The capacitors shown in the manual are also connected before the fuses for the generator and alternator. Does that mean that it is better to have the two capacitors connected in case one of the fuses blows, (same effect as opening the alternator switch for the associated regulator?

    Alan
      
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