Automotive Circuit Protection - Part 2

[Garfield Willis]

On Fri, 23 Jun 2000 16:35:40 -0700, garwillis at msn.com (Garfield Willis)
wrote:

Dang, here's another errata:

>Name				transient V			frequency/cause
>---------				----------------			-------------------
>
>Load Dump		pos/~70V			each load disconnect
>ALT Field Decay	neg/~100V			shutdown of elec. sys.
>Ind. Load Decay	pos/neg/-+50V		varied
>Reverse Bat		neg/14V			screw-up
>Double Bat			pos/28V			jmp-start/boosters

>Load Dump		pos/~70V			each load disconnect

the above line SHOULD READ:

Load Dump			pos/~70V			batt disconnect

That snagged at least one person into wondering if the bus could make
that big a blip with the battery still connected; Load Dump occurs when
you have (1) a major load change, and (2) the battery isn't there to
absorb the extra ALT current. Just so happens that if the battery is
under high charge when you disconnect it, you have BOTH conditions
satisfied.

My pretend example of the load dropping from 100A to 10A was misleading
because I manufactured an case where the battery if still connected,
could still very likely absorb the 90A extra transient current without
going all the way up to 70V. If the discussion has confused you in like
fashion, just concentrate on the summary that followed that fake
example:

>You might think, well hell's bells, isn't that going to be happening any
>time you turn off some major load? Ahh yup, BUT first off, you don't
>normally have transients as big as above, PLUS the BATT is a tremendous
>capacitor of juiceful proportions, and normally happily absorbs any such
>transients. Whilst it's charging, the BATT also presents a pretty decent
>load of it's own to the ALT. Take both of those away at once by
>inadvertant disconnection of the battery and some other major loads, and
>presto: "Load Dump".

Note the "both" in the last sentence. Sorry for the confusion. Battery
disconnect AND a high load state on the ALT are BOTH assumed parts of a
standard Load Dump event.

Hope there aren't any more gremlins lurking in that tome. :)

Gar


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