I've reviewed many of the inputs submitted on this forum regarding charging capacitors for the 912ULS. It seems errors in BHP-Powertrain Installation Manual 24-00-00 describing on pg 7 installation of a 22,000uF/25v is correct. Fig 11 #14 describes a 1uF capacitor, and as many others described, it's an obvious error.

I'm planning on installing capacitor rated at 105C, 80V 22000uF. Since I'm not confident in Fig 11's uF error, is wiring diagram on pg 19 showing #14 correct? Is installing Positive to "C" terminal and Negative to ground ok? I don't understand the "2 capacitor" function. Am I ok by installing only the one capacitor?      TKS Gang, Bob

7754_1_22000uF 80v 105C.pdf (You do not have access to download this file.)
  • Re: Capacitors

    by » 4 years ago

    Yes one capacitor is the standard configuration. The capacitor is like a battery with positive and negative so connect as you described. 

    Use this on the connections - https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/81150.pdf

  • Re: Capacitors

    by » 4 years ago

    Thanks for advice Garrett, and your heads-up on dielectric grease.

  • Re: Capacitors

    by » 4 years ago

    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.

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

  • Re: Capacitors

    by » 4 years ago

    The topic of capacitors in the charging system has been heavily discussed on the Matronics Aerolectric forum. The result of that discussion calls into question whether or not the capacitor adds anything to the system.


    Joe, a regular contributor to the Aerolectrics forum has an RV-12 with a 912 and he actually suffered a battery failure, giving an opportunity to test the effectiveness of the capacitor if there is no battery. Here's what he found:


    "Even if the battery is disconnected, I do not think that the capacitor does much good. When my PC680 failed open, the alternator voltage was unstable. However, Rotax documentation calls for a capacitor. It is best to follow manufacturer's instructions. They might know something that I don't."


    Bob Nuckolls, author of the book "Aerolectric Connection" said this:


    "These capacitors have been included as part of many PM alternator systems with rectifier/regulators for a long time. Never got to talk with an original designer but the general consensus amongst those who have offered opinions is that it 'reduces noise' and/or 'aids alternator-only operations'.


    Limited testing I've conducted on smaller systems(10 amp) showed no benefit with respect to noise on the bus . . . DC busses in vehicles are
    generally very noisy anyhow and everyone worth his salt as an appliance designer will know how to 'live with it'.

    Here's a family of plots I took on an SD8 alterantor on B&C's drive stand about 15 years ago:


    Page 1 is a picture of a fully loaded SD8 with capacitor installed. Measured noise in the low frequency spectrum was about 250 millivolts peak
    to peak, 77 mV RMS.

    Page 2 shows what happens when the capacitor is removed . . . 760 mV pk-pk and 72 mV RMS.

    In other words, no significant change. Alternator noise on the bus in an airplane with a wound field is MUCH higher and broader spectrum . . . but still nothing that your electro-whizzies should care about.

    I fiddled with some no battery ops too on pages 7, 8 and 9 . . . but didn't see how it performed with no capacitor. I'm thinking that planning for no battery ops is a capitulation to poor battery maintenance practices and/or errors of conducting FMEA in deciding architecture.

    Short answer is I don't think the capacitor adds any useful benefit. If anyone runs across some testing data that contradicts these findings, I'd be delighted 
    to know about it."

    Thank you said by: Bill Hertzel

  • Re: Capacitors

    by » 4 years ago

    I love that sort of testing!

    The One test that was missed was No Cap - No Battery - No Load;  In other words, the voltage at the regulator with NO Cap and the Alternator switch OFF.

    This removes the Alternator from the Airframe Power Bus so even if the regulator loses its mind it will do no harm to the panel, Only to itself.

    This is also not something that will appear on the panel voltage indicator, as the Alternator would be disconnected.

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

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