On Tue, 22 Sep 2015 19:02:33 -0700, John Larkin
wrote:
The tantalum thing is very erratic. Some batches blow up, some are
fine.
I guess I should mention that Intech had two divisions. I worked for
the marine radio division. There was also a "modular products"
division that made military grade modules (A/D, D/A, amps, etc). Both
divisions shared many of the same components including a wide
selection of tantalum caps. Most of the modules ran on +15/-15 VDC
and used 35 VDC rated tantalums. As I vaguely recall, there were no
aluminum caps used in anything that had to work from -40C to +105C. If
tantalums were that failure prone, they would never have survived in a
mil spec environment.
Some typical radio boards. This is Intech M3600 2-30 MHz 150 watt PEP
synthesized SSB radio circa 1977(?):
http://www.hellocq.net/forum/read.php?tid=226493
The boards are a mix of purple potted electrolytics and blue or orange
colored tantalums. No failures in 10 years of similar radios.
There also seems to be an aging mechanism involved. I'm the not so
proud owner of several Wavetek 3000B service monitors:
http://802.11junk.com/jeffl/pics/home/slides/BL-shop5.html
Some of the tantalums have shorted over the years. I'm replacing them
as I blunder forward. No fires, smoke, or discoloration in about 10
years of fixing these. I also have some other equipment with similar
tantalum problems. I recently repaired an M3600 radio which showed no
evidence of deteriorating or failed tantalums. Has something changed
in the last 40 years in how tantalums are made?
I know for sure that tantalums sometimes blow up at below their rated
voltages, with no overshoot spikes. It's dV/dT, namely peak current,
that can ignite tiny particles of tantalum, which then burn in the
solid MnO2 electrolyte.
I think you mean dI/dT which provides the heating necessary to ignite
the tantalum. That all sounds logical, but doesn't explain why a
similar amount of heating caused by normal ripple current doesn't set
fire to the capacitor. I've seen some heat darkened tantalums
operating normally without ignition. Like the bulging electrolytics
and burning LiIon batteries, I suspect there's been some changes in
production methods (like skipping important steps to save pennies).
The original cap is a 100uF 10v tantalum which is already 3:1 derated
in a 3.3 VDC power supply. However, that doesn't included voltage
spikes from the nearby inductor.
Could be. Or maybe there was a lot of dV/dT. Or maybe some other
failure mechanism.
Tantalums are just right for some things, but have to be used
carefully.
https://dl.dropboxusercontent.com/u/.../Caps/Bang.jpg
https://dl.dropboxusercontent.com/u/...ied_Tant_1.JPG
Nice photos. Having done post mortem failure analysis a few times, I
like to look at the damage and try to guess what was the cause. It's
fairly easy to inspect the remains and estimate the violence of the
failure. For tantalum, there's usually something left of the wire
leads or carbonized slug. It gets hot, belches flames, carbonizes,
falls apart, which finally breaks the connection.
However, the OP mentioned that:
"The first thing I notice when looking inside is that the small
SMT 100uF 10V tantalum capacitor C109 has completely vacated -
it appears to be gone, blown right off the board. There are
some little fragments rattling around in the case."
That's not what I consider to be a conventional tantalum burn failure.
The cap should have looked like the one in the above photos. Something
caused this one to explode rather than burn, which is why I suggested
that a much higher voltage wall wart was involved. I don't think the
tantalum was at fault simply because it was the first thing to blow
and was the most obvious physical failure.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558