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Electrical Load Currents - Peak and Continuous

in 912 / 914 Technical Questions 8 years ago

  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Rob or Michael,

    So what are you calling continuous, peak or max draw? I see what the book says is max, but that isn't usually continuous?

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

      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Roger: The 22A "Max" is a continuous current rating.

    * Based only on standard electronic ratings: when a single rating is given, it must be assumed that said rating is "Absolute Maximum Rating - Continuous".

    I'm not an electrical engineer but I have a hunch that the longevity of these regulator units (aside from heat dissipation and vibration factors!) is directly proportional to the quality of the 22,000uF Capacitor that smooths the ripple from them.
      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Good to know. Thank you.

    Maybe a 33K uf or higher would be better? Like the 912iS with it's 33K.
    Maybe publishing or letting MFG's and owners know that the coolest spot for mounting is the better choice over hot spots? Just a thought. I know Vans was having an inordinate number of failures on the RV12 because of the original location. On a metal shelf right behind #4 exhaust pipe and with a metal cover over it and then a tube that was supposedly allowing cooler air to move over the reg. When I put Temp strips on it they were too hot. I helped several by repositioning the tube to allow the air to be forced through the hose and over the Reg. The tubes original angle just wasn't getting the job done.

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

      
    Report

  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Well, the 912iS "Generator B" (the one that normally runs your avionics, etc) is nominally rated at 420 Watts, which works out to 30 A @14.2V
    So it looks like the ROTAX engineers are using a rule of thumb of 1000uF per amp to get the desired ripple smoothing. (Rounded up to standard component values, you would pick a 33,000 uF cap.)

    And I would say in the case of this vital capacitor, it's really QUALITY, not quantity that matters. Electrolytic caps are well known for not being the most robust of components, I'd certainly rubber mount it and keep it away from heat - and make sure it's a high-quality one, not some cheapo unit. And don't neglect to check the manufacturing date! Electrolytic caps degrade over time...

    I do agree we should look at publishing some more explicit guidelines about component placement, since it seems that sometimes people miss the 'obvious' (subtle?) things, like how the airflow through the cowl isn't always going to be "intuitive" - and the fact that once you have landed and the prop stops turning, your engine is now just a great big heat sink - have you made adequate provisions for that heat to escape, or is the cowling then just a big oven, cooking your electronics? We touched on this subject in episodes 5 and 9 of these videos ...
      
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  • Re: Electrical Load Currents - Peak and Continuous

    by » 8 years ago

    Asking for a QUALITY capacitor is marketing jargon.
    Engineers translate this word into "Ripple Current" rating and dielectric material properties when specifying capacitors.

    Be careful making claims to increase the 22,000uF cap to 33,000uF as the inrush currents will go up 50% which impacts the rating of wiring and circuit breakers/fuses and electronic components in-line with the higher current surge.

    The BUZ60 is a 5.5A MOSFET and a very careful analysis is required regarding pulse widths for higher peak current loads. The heatsink to transistor case thermal resistance and case to junction thermal resistance for the package die are key to the thermal performance of the transistor. Finally, the airflow and heatsink design plus the ambient temperature contribute greatly to the maximum current rating.
    Thank you said by: RotaxOwner Admin
      
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