Announcement

Collapse
No announcement yet.

Large Capacitor in Generator charge circuit ?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Large Capacitor in Generator charge circuit ?

    After 8 years 912 UL ownership I am informed the genny wiring should have a 22,000 micro-Farad capacitor across the 12 VDC output. It's never had one and the specified one is a chunky great lump.

    I agree after studying the Rotax diagram they do show one - BUT:-

    a) Does anyone else run without one ?

    b) What's the problem of using a much smaller/neater electrolytic condenser instead. I realise there 'must' be a reason but i'd appreciate knowledge on it.

    mike hallam.

    P.S.
    Since posting above query I found a guide note - not Rotax' - which mentions using electrolytic 25 V 85 C. so have one order to connect up.

  • #2
    Large Capacitor in Generator charge circuit ?

    Mike,
    As I understand it:-
    The output from the regulator is not smooth it is in fact more like a shallow wave (at best!) that is at a voltage above zero (13 volts ish) and is therefore DC. The battery accepts this wobbly output and 'to some extent' smoothes it out. The key here is 'to some extent' - if your system is able to handle this. Ripple DC then all is well, however, any change in voltage will cause a radiated magnetic field thus if you have equipment on board that is sensitive to stray emf the equipment will exhibit 'interference'. Such equipment as rpm gauge, radio, nav aids, glass panel etc can be affected in this way.
    The capicator is used to further smooth and try to eliminate the ripple in the DC.
    On my ssdr Chaser I have by design put the regulator as close to the engine as possible, screened to earth the generator and rpm gauge outputs and have battery and electrolytic (24v 14700 mf) capacitor.

    Comment


    • #3
      Large Capacitor in Generator charge circuit ?

      Thanks Paul,

      Deeper study of what I could find in the literature indicates this large capacitor's main function beyond normal ripple smoothing is the avoidance from a spike killing the rectifier/regulator should the battery suddenly go disconnect.

      All makes sense, except the builder of my a/c 15 years ago left it off I didn't know till last w/end). He also chose the simplest wiring which uses the "ignition" switch normal run contacts (not 'start') to connect the 12 VDC bus to the battery. If, and it happens once in a while, I switch off with the switch but don't kill the two ignitions first, the engine of course keeps running.

      As far as I can see in this circuit configuration the genny and so, on loses the soothing effects of a large battery and the voltage could spike having no load. Hence my odd query.

      However though so far it's not done any damage I'm aware of, e-bay is winging a hefty capacitor to me to shove in somewhere accessible for luck.

      mike.

      Comment


      • #4
        Large Capacitor in Generator charge circuit ?

        It's an optional fit - many Skyrangers run without them, I haven't heard if it causing problems.

        Paul

        Comment


        • #5
          Large Capacitor in Generator charge circuit ?

          Thanks for that useful and comforting feed-back Paul.

          As one BMAA Inspector was horrified not to find a condenser installed last week-end I bought an inexpensive 22k MuF electrolytic capacitor to spec'n and the Rans has one now.
          I'm pleased to learn my 'plane wasn't a solitary "offender" and no harm is likely to have been done by lack of it over the last 15 years.

          mike hallam.

          Comment


          • #6
            Large Capacitor in Generator charge circuit ?

            From memory, when applying for initial permit on an LAA kit-plane I was involved in there was a specific tick-box for "is a capacitor fitted?". So it may be that the LAA mandate a capacitor, although on what evidence I don't know. The Rotax 912 installation manual certainly shows one, so maybe the LAA insist simply on that basis, hardly unreasonable. My guess is that you're right, it's mainly about spike suppression, particularly under light generator loads if the battery disconnects. So nothing very critical about choice of capacitor provided it'll handle a reasonable voltage above normal system operating voltage, and will be happy at the ambient temperature wherever it's mounted. I can't believe there's any technical reason it needs to be a "chunky great lump".

            We purchased a small-ish 22,000uF 25 Volt electrolytic capacitor, decent quality with screw terminals and a decent temperature spec, mounted on the firewall next to the regulator, and it's been absolutely fine.....

            Comment


            • #7
              Large Capacitor in Generator charge circuit ?

              Thank you John,

              That agrees with my actions and capacitor of the identical spec, except mine needed a quickly applied soldered on pair of leads. On the Rans it was fitted where it's cooler and easy to connect behind the cabin central instrument console.

              My principal lack of charge problem only went away after buying a new Ducati rectifier/regulator, off the shelf from ConAir at a sensible price to replace the suspect old one. Then had to wait for a nice day to haul the a/c outside, which here was last Friday, light wind & good vis. She's been on the solar charger so started nicely and as soon as rev's were allowed to rise the volt meter went from 13.3 quiescent to a steady 13.9, where it stayed for 3/4 hour jolly until engine off.

              My thanks to Louise & Conrad at ConAir, they still stock essential spares & mine arrived within 48 hours. So handy despite the Rotax new engine business being gifted away that they are willing and so far able to support our kind of ownership.

              I can heartily recommend their service with a smile.

              mike hallam.

              Comment


              • #8
                Large Capacitor in Generator charge circuit ?

                Paul Dewhurst wrote: It's an optional fit - many Skyrangers run without them, I haven't heard if it causing problems.

                Paul
                Out of interest Paul what makes you say it's optional? Rotax maintenance manual (Ed 1/Rev 2 : Jul 1/2008)has it in section 3.7 (fig 74-12) and doesn't say "optional"

                Mine blew today whilst Frogden was flying. But it was only a 2200uF one that had been fitted which Steve U found is listed as standard for Quantum (at least at some point).

                Perhaps not having one is better than having one (or having a small one!)

                Comment


                • #9
                  Large Capacitor in Generator charge circuit ?

                  The capacitor is really only important if you have no battery fitted, or if you have a small (less than around 7AH) gel-cell fitted. Any aircraft with an electric starter, and appropriate sized cranking battery will happily smooth out the ripple from the rectifier/regulator. It might be useful to fit one in case the battery fails - though whilst the ripple from the regulator without a battery or capacitor looks horrific, it should never exceed 15v, so avionics ought to be safe. I have noticed a tendency for manufacturers to fit (if they fit) electrolytic capacitors of dubious quality (mainly because the decent ones are expensive). These are usually found to be defunct after a couple of years (they fail low capacitance). They need to be 22,000uF (though that's not critical), at least 25vdc rating, and if you go for the 110C over the 85C temperature rating, you are getting a better quality cap. Needs to be mounted securely - I've seen one or two where the leads have fatigued.
                  Dave

                  Comment


                  • #10
                    Large Capacitor in Generator charge circuit ?

                    Capacitor will smooth better as it's electrical whereas battery is chemical and slower. That's my understanding anyway. My cap exploded because the cheap fuse holder failed and the battery disconnected and the cap wasn't man enough. The p&m wiring is not mean enough for the charging circuit. 30A fuse and lower rated wiring and cheap and nasty fuse holder that will never take 30A. P&m have not responded to questions about the specifications.

                    Comment

                    Working...
                    X