Caution - InterAv "Spike Guard" Capacitor

MikeremlaP wrote:

If you have a "Spike Guard" filter capacitor on your alternator output,

Mike, according to the Interav installation documents, the capacitor is
supposed to be wired across the "Diode-Trio" output, and serves to filter
the ripple out of the voltage which the voltage regulator senses. The field
current is effectively supplied from this capacitor, so without it, the
VR sees a lot of ripple on what it is "sensing". Why the fuXX they dont
just sense the main bus like 100s of Millions of other automotive and
aircraft alternator systems is beyond me??? Maybe to get around Motorola's
patents???

you may
wish to inspect it immediately and perhaps remove it.

A year of two ago, I got into it with InterAv about why they think that this
capacitor is even needed. They could only say that they don't know why its
there. They bought the STC from a third party, who developed the STC with
the capacitor and got it approved with the FAA. In Interav's view, removing
it would invalidate the STC, and they would have to redo the STC application,
which they are not willing to do.

I think that the Interav alternator STC and the way they are sensing the
system voltage is brain dead, and as you just pointed out, it may actually
be dangerous. Putting a 60,000uF Mallory electrolytic capacitor in an
aircraft engine room is stupid, and almost guarantees a heat/vibration related
failure of the capacitor. The FAA had its head in a warm, moist place when
they approved it in the first place. Gives you a warm, fuzzy feeling about
other FAA approved STCs, doesn't it?

Wiring any big electrolytic across the main output of the alternator
compounds the problem! Such capacitors have a "Ripple Rating", which will
likely be exceeded when it is connected across the main bus. The excessive
ripple will cause a meltdown in the guts of the capacitor. That is what
your picture shows...

MikeM, PhD EE
Skylane '1mm
Pacer '00z