Automotive Circuit Protection - Part 1

[Bernd Felsche]

Garfield Willis tapped away at the keyboard with:

> Another responded thusly:

> >The alternator's control circuitry should protect against such an
> >over-voltage situation anyway by manipulating the excitation
> >current.  There's typically a Zener diode to sense over-voltage.
> >Such a circuit can react quickly enough to protect the rest of the
> >vehicle's electrics.

> Then, same respondent further states:
> >When the connection between battery and alternator fails, the
> >alternator's unsmoothed ripple voltage is seen by electrical
> >consumers. Note that most alternators supply at least 3 phases (6
> >diodes) and the resulting ripple is only a couple of volts; enough
> >to confuse ECUs, but hardly enough to cause damage to electrical
> >consumers designed for automotive use.
> >
> >If you're looking at transsorbs to smooth the ripple, you should
> >probably be looking elsewhere for something more useful.

> Instead of treating what are mostly misconceptions above, I'll just run
> down both cons & pros all together.

> (1) "Load Dump" is a POSITIVE over-voltage transient out of the ALT, so
> it's in the same DIRECTION as normal ALT output (rounds about 70V), and
> therefore *doesn't* usually provoke a failure in the diode bridge.
> Diodes normally fry from thermal cycle failures these days.

> (2) Contrary to the second person's glib dismissal of LoadDump as merely
> minutae ripple on the output of the bridge (which is complete nonsense),
> it IS a very REAL threat that SAE and ISO have sped'd out extensively.

I said no such thing. I was responding to the initial question of
the transorb being capable of smoothing the ripple in the absence of
a battery.

> Such dismissal shows that the person isn't in a position to comment in
> what often sounds like an authoritative tone, when they don't even know
> WHAT Load Dump is, let alone that it even exists! Always better to ask
> questions and learn something, than wax authoritative but clueless.

I apologize if anybody took my response to be "authoritative".  My
background is in mechanical engineering so I'm no expert on the
electrical/electronic side. I do happen to know just a little more
than average on the subject - which could be construed as dangerous
:-) As I understand that to be the case, I lean on my reference
books for support.

You weren't around to respond, and I think some answer (even if it's
not authoritative) is better than no answer at all.

If we all knew everything, this list wouldn't exist.

> What Load Dump IS, is a deregulation-type failure in the ALT, where a
> large enough change in load causes the ALT's regulator to be unable to
> *quickly enough* cut back the ALT's field coil current (that's the
> rotating part of the ALT, BTW). It's not a design flaw or failure of the
> electronics (usually), but rather an intrisic weakness in the
> method/means of regulation of ALT output; namely, controlling the ALT
> output thru regulation of it's FIELD winding current. Basically, it's
> like an inertia effect. You're controlling the ALT output via it's

You're suggesting that a *Zener* diode at the ALT output is
insufficient protection to prevent the bridge output from exceeding
safe levels during such transients?

Why would Bosch claim that it's safe to drive a motor car under
restricted conditions with such protection - when the battery has
been disconnected?

You have discussed the nature of load-dump. I agree with your
suggestions which I understand to be:
1. sensitive consumers should protect themselves;
2. inductive loads should ensure that they don't dump significant current;
3. the alternator should have output protection to prevent damage
   resulting from an internal failure.

Of course; that's not authoritative.

Thank you for your diplomatic comments.
-- 
Bernd Felsche - Innovative Reckoning
Perth, Western Australia
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