Re: [DML] Auto trans governor problems

["Mark Hershey" <dmcinfo@xxxx>]

Good news, Matthew! I checked with a friend of mine who designs capacitors,
programmable gate arrays, and other stuff for Lucent / Bell Labs and he said
that your caps are probably OK.

How is that possible?

Not sure I caught all the detail, but it seems that applying reverse
voltage, even as low as 20 volts or so, will indeed often cause the
dielectric inside electrolytic and tantalum caps to "flash over" (he had a
more technical term, but I didn't catch it). At that instant, the dielectric
is essentially shorted and in conventional power supply filtering circuits
the full available current from the power supply would flow through the cap,
creating heat that would literally melt the insides and boil the dielectric
(the electrolyte in an electrolytic cap, not sure what the powder dialectric
in a tantalum cap would do) causing rapid expansion and potential explosion.
I can personally vouch for that one, back in high school electronics lab I
blew up an electrolytic cap on a lab project-- it was a big 1 inch or so
diameter and it shot the insides out the end and popped the instructor on
the back of the head. Didn't get an "A" on that one.

Anyway, in small signal circuits such as the one in the Governor there is no
appreciable current available and the cap doesn't heat. Often, but not
always, the point inside the cap where the flashover occurred is not further
damaged. In most cases removing the source voltage is all that's required.
If your car shifted at all after reinstalling the caps properly they aren't
permanently shorted.

He also mentioned that any tantalum cap rated at 50 volts should take a
reverse hit of 20 volts (about all you'd get from the governor unless you
drag raced it) without flashing over at all.Guess that's why mine still work
OK after all these years.

I'd say that if you are in there anyway to resolder eveything, go ahead and
replace 'em.

Oh, and in respect to all those folks who e-mailed me personally about the
dangers of using paste flux and the long term effects of corrosion--don't
use acid flux, (I thought I already mentioned that one) and clean the flux
off after you resolder everything.

\\ Mark

----- Original Message -----
From: "Matthew L. Walker" <mwalker@xxxx>
To: <dmcnews@xxxxxxxxxxxxxxx>
Sent: Wednesday, February 07, 2001 4:08 PM
Subject: RE: [DML] Auto trans governor problems


> Well to be safe I will pull the caps and replace them. It's not that
> expensive.
>
> Ralf - The capsI used are 4.7uF 50v tantalum caps from nte. Suggestions?
>
> Matthew L. Walker
> VIN 1219