Caution - InterAv "Spike Guard" Capacitor

Okay guys, I cut the cap open.

See foto at

http://www.fotolog.net/palmer_mp/

No foil that I can see. In fact, I'm not sure how this thing works. I thought
the idea was to create maximum surface area.

I probably should have buzzed the negative terminal to the can to see if
connected. (Can do that tomorrow).

Black stuff has a strong order to it...

In any event, it seems that, with vertical mounting, lugs down, either
component could rattle its way to short the inputs. I'm surprised it didn't
happen.

Mike Palmer
Excellence in Ergonomics, but apparently obsolete in Electrical Engineering.

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

On Wednesday, July 16, 2003 at 7:13:14 PM UTC-4, MikeM wrote:
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

Mike - I know this is a very old forum post, but I, too, am an EE and having problems with the InterAv system, and am hoping you will contact me for a discussion.

jwills8606_at_gmail.com

Firstly, I can make NO sense out of the big cap. I haven't checked mine, but it's at least twenty years old, and I bet it's dead. InterAV shows the cap between the "regulator" terminal and ground - I assume that terminal is some kind of voltage output sensing wire - it shows that terminal can be hooked to a "warning light" also. I had the alternator rebuilt about 7 years ago, so I don't remember exactly where that wire is hooked inside the alternator.

What I DO remember is that the 3-phase diode array output then all feeds into ANOTHER diode, which is then connected to the alt. output lug. WTF? Nice single-point of failure - a single diode for all the alt output.

On my 182, I have developed an alternator whine that is worse with increasing electrical load. I put an oscilloscope on it, and interestingly, it is not the alt. output - it doesn't increase with rpm or change frequency with engine speed - it's the square waves being fed into the "field" from the voltage regulator. I wonder if somehow the big cap has something to do with that. I am told that reg is a very poor design and should be replaced whenever possible.

And another thing: I've never seen a spike snubber made with such a big cap. It's usually a smaller tantalum in parallel with a very small ceramic. I think somebody doesn't know what they are doing, even though it (somewhat) works.

Thoughts?