Dan Michaels wrote:
>
> Hi Roman,
>
> Thanks greatly for the comprehensive overview of protection
> cktry you use on automotive devices. This is a great wealth of
> practical information, useful for design, reliability, and
> testing. It took me a while to digest it all. Anyone who has
> seen the underbelly of a transistor knows electronics design
> involves at least as much practical as theoretical.
>
> Good rules of thumb - simplicity, overkill, bipolar devices
> to buffer CMOS. I favor using the "smoking finger test" for
> determining whether a device is running too hot. If I can keep
> my little pinkie on it, then it is probably ok. Otherwise, I
> re-design for cooler operation. [sounds like you would favor
> using a baby's pinkie - figuratively speaking, of course].
>
> I have one question. You mention using zeners in several places
> for spike suppression. Have you any experience using transzorb
> TVS devices [low-inductance, fast-breakdown diodes] to protect
> I/O lines? Should be superior to std zeners there.
>
> thanks greatly,
> - Dan Michaels
> Oricom Technologies
> http://www.users.uswest.net/~oricom
> ===================================

Thanks Dan, them's nice words! And I have got 8 years
of "theoretical" too hee hee!

Re the transzorbs, no I don't think I have ever used them.
I would guess I've replaced some of the dead ones in TVs with
big zeners though! ;o)

Seriously, we currently buy the big 1w/5w zeners in bulk, and
I have used them for enough years to be real confident with
what they will take and what they won't. I know roughly what
mass of metal is inside them and what to expect. I feel a bit
unsure of new technology stuff, often in my work I find that
what should be a higher performance part due to new operating
principles is often "exploited" in a manufacturing attempt
to save silicon, make it smaller etc, that the benefits get
a bit foggy. ie, "yes it's a whiz-bang new design that will
safely conduct 10 times the current, so we will use 1/20th the
size of silicon inside it". Mosfets especially fall prey to this
kind of thinking. Even more so when they put huge current/power
specs in a tiny package. Laws of physics just don't work like
that...

Now I don't know if this relates to the transzorbs, but to me
they are still an unknown... If I had been using them (or working
with them in other peoples designs) for 10 years I would know
pretty much what they take and what kills them. Now the resistor,
zener, cap solution I know well and it works. We use beautiful
high quality metal film 2w resistors, good caps and big zeners of
a known brand. At bulk quantities we get these for a few cents
and THEY WORK. Really well. Not only do they protect well, but
there is no reason to think they will ever fail. Now with the
transzorbs, do you use a large series resistor? With the R/Z/C system
it may not clamp it as tightly, but the BULK of the power of any
spike is dissipated by the resistor, which as a piece of ceramic
with metal film wrap, is probably my part of choice if something
has to take the strain and dissipate the power, especially
intermittant power with the associated heat/cool expand/contract
that semis really don't like.

Ideally if you can run a part at 10% of its ratings (I,V,and P)
and also allow in P for the part to run cold, it will last forever.
I have stuff that was built by me 20 years back that has never
had a part failure, save for replacing some electros as the
rubber caps perish after 5 to 10 years. Dissipating heat itself
is not bad, I often make linears that give off 100W+, the trick
is using enough mass so that temperature rise is minimal. You
can have four big transistors giving off 100w at a temp rise of
3 degrees celcius, and they will last forever as they run cool.
However a small switching chip that gives off only 5w and
runs at 10 degree celcius rise will fail in a year or two,
as it is running hotter AND heating/cooling thousands of times
every second. Then you get an engineer who says "great, this
chip is rated at 2A, and I only need 1.2A"

I work all day fixing 2 year old TVs with multiple blown semis,
then I sometimes get a 15 year old TV with dry joints. Why has
it failed with dry joints? Because in 15 years it has NEVER had a
component failure. The weakest link was the actual metal properties
of the solder itself. There's an engineer that did his job! :o)
-Roman

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