Chuck Dunlap wrote:

What waste of power? The resistors do not dissipate much - the originals

are just 5 Watts resistors, and I do not think they even dissipate that
much. I think the reasons there have been failures in aircraft is from high
engine compartment heat. The resistors are wire wound and I think the high
peak pulse current fatigues the wire. The 25 watt resistors use a larger
diameter wire, not as prone to failure. My original units are still working
fine after 175 hours with cool air blast. I once flew my Mazda Rv6 with EFI
for over one hour with the alternator turned off (within gliding distance of
the airport) and the voltage still did not drop below 12 volts. This was
with a standard 33 amp hour battery.

Paul Lamar replied:

OK Chuck. So you don't think the higher average current is causing the
failure? First I have heard that high peak pulse current can fatigue

wire

at about 80-100 pulses per second. Light bulbs last about 1000 to 1500

hours at

120pulses per second and the wire is incandescently hot. Of course that
is a sin wave and ignition module works off a pulse. Do you think this a

thermal

fatigue phenomena? Very interesting. Got any references on this subject
of wire wound resistor failure modes?

Paul,

I haven't read all of my email, so perhaps what I am about to say doesn't

apply.

We here at Texas Instruments have alot of experience with "interesting"

failure modes of high current devices (we're currently the #1 supplier of
analog devices in the world).  What Chuck may be referring to is a
phenomenon called "electro migration" failure.  It happens in small
conductors at high current.  A small pre-existing defect in the conductor
causes a narrowing of the conductor at some point. As the electrons go
whizzing through, that small narrowing gets made smaller as atoms are slowly
shifted "downstream".  This, of course, only makes the narrow part narrower,
so you can see that a failure is in the making.  I've got some
photomicrographs of this failure mode.  Here is one.  The official failure
mode coclusion for this device is:

     The unit failed opens testing due to metal migration caused by high

current flow while the bump was at an elevated temperature. This caused high
resistance in the bump and cracks.

The "bumps" are contacts from the silicon device to the chip carrier.

[Image]

That happens enough in power semiconductors that we have special chip

design rules to try to prevent it.  I don't know if this phenomenon is the
cause of the failures in the relatively macro scale of the wire wound
resistor.

This is a SEM (scanning electron microscope) picture of what happenes when

one thermally shocks a device from -65C to +150C.  The leafy stuff is silver
that has migrated on the semiconductor and evetually caused one of the
electrical wires to lift off the contact.

[Image]

* Russell Kent

That brought back memories! I used to operate the SEM for Western Digital in
'81-83 in Newport Beach (was a QA Specialist). Back then they just made many
various Semiconductors, no hard drives. Was great fun to stick insects in
there too! (I was 22)

Happy New Year All!

JET

The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org