Try this link

[Garfield Willis]

On Mon, 12 Jun 2000 21:09:12 +0100, "Gavin"
<gavston at sprockets.freeserve.co.uk> wrote:

>just found this snippet of info on the web.  Not much good, but it may be of
>somegood to someone!!!
>
>http://www.tri-m.com/tech_pages/techpaper8.html

Is this a thinly disguised troll? :)

An important and useful topic, sadly somewhat mangled...

The topic is "silicon avalanche diodes", commonly known as transorbs,
used for protecting expensive/delicate circuitry from the harsh
automotive environment.

That article above is unfortunately a case of some guy touting their
particular power supply design, and essentially saying "my dog's bigger
than your dog". In short, it's a good bit of marketing hype.

What I mean is, he says:

"Standard devices such as transorbs (P6KE or 1.5KE) will not survive the
high energy discharge of a load dump. Special 'automotive' suppressors
must be used to suck up the 20A to 30A peak currents being shunted."

What he doesn't tell you is the very device he's touting the superiority
of their power supply because of, a "Diodes Inc. part# 5KP43A", as he
puts it, is indeed a TRANSORB *just like* the ones he says above "will
not survive". Only thing about the 5KP devices is they're rated the next
step up in peak pulse power. That's ALL. So much for "special automotive
suppressors"! The fact is that there are even MANY voltage regulators
made for automotive use that are rated against load dump, according to
the SAE definition of a load dump, and I can tell ya FO SHO they aren't
rated for no 5kW PPP. See below.

Trouble with this sort of logic is that another competitor could come
along, use a 15KP device (that's the next bigger one up from the 5KP),
and then say "their dog was now bigger" :).

The other thing that's misleading about that lil blurb is the suggestion
that devices like transorbs are there as the only line of defense
against transients like load dump. That's nonsense; the main purpose of
things like transorbs is to protect electronics against FAST transients
that fuses, breakers, and OVP (over-voltage protection; for example
crowbars) are too slow to see, by absorbing enough of the energy in the
transient to provoke (and provide enough time for) the secondary line of
defense (fuses or breakers) to operate. And if necessary, destroy
themselves failed-short so that nothing beyond the transorb suffers
permanent damage. But that's absolutely worst-case scenario; albeit
another important role for these devices if need be.

Last year I heard first-hand from a famous demo pilot (Bob Hoover), of a
case where his alternator failed, dumped 100+VAC onto his main bus, and
fried $50K worth of avionics, that WOULD have been protected if he'd had
proper suppression devices installed, such as transorbs. That
overvoltage would have tripped all the transorbs hanging across every
device, and caused an effective short lasting long enough to pop most
all of his panel breakers. But because transorbs are "new technology"
for the archaic field of general aviation, he had a panel full of the
latest sensitive stuff, but would have had to have special certification
to add the transorbs (they really should be in each piece of
instrumentation anyway). He wasn't able to perform for the airshow, cuz
nearly his entire panel of fancy instruments were fried! To make things
worse, he discovered his insurance didn't cover it. Yow.

Transorbs (or tranzorbs, or TVSs, aka "Silicon Avalanche Diode", a
special high-speed, high-energy form of a zener diode), are currently
the FASTEST and most reliable form of overvoltage protection [in
comparison to thyristor-based clamps or crowbars, MOV (metal oxide
varistors), etc].

I wish I could refer y'all to a single good source of poop on Transorbs;
alas, I don't know of a good online tutorial that's specific to
automotive protection. I woulda thot there would be one, but I've never
run across it; there are many EE appnotes detailing their use, however.

HTH,
Gar


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