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Subject: Re: (Avionics) How can this circuit produce an "inductive surge"?
Date: Fri, 27 Sep 2002 00:08:23 +0100
NNTP-Posting-Date: Thu, 26 Sep 2002 23:07:20 +0000 (UTC)
X-Newsreader: Forte Agent 1.8/32.548
Don Tuite wrote
It mentions the load dump:
>The load dump overvoltage is the most formidable transient
>encountered in the automotive environment. It is an
>exponentially decaying positive voltage which occurs in the
>Table 1 shows some sources, amplitudes, polarity, and
>energy levels of the generated transients found in the
>event of a battery disconnect while the alternator is still
>generating charging current with other loads remaining on
>the alternator circuit at the time of battery disconnect. The
>load dump amplitude depends on the alternator speed and
>the level of the alternator field excitation at the moment of
>battery disconnection. A load dump may result from a
>battery disconnect resulting from cable corrosion, poor
>connection or an intentional battery disconnect while the car
>is still running.
AFAIK none of the events in the last sentence are likely. Turning off
the master switch disconnects the battery but also disconnects the
field current from the alternator.
A "load dump" is also reportedly possible when the alternator load is
substantially reduced, quickly. The voltage regulator cannot,
apparently, work fast enough. Let's take a steady state condition in
taxi, plenty of load all on, and you turn it off prior to turning off
the engine - exactly as you are supposed to do. Why is this permitted,
when a rapid alternator load reduction is exactly what is not wanted?
The one interesting upshot of this bit is that setting up the voltage
regulator so that the alternator does not quite charge the battery
when the engine is running at recommended idle (as is the case in my
a/c) would prevent the alternator seeing any load reduction at engine
idle - the very time when most loads get disconnected - so a load dump
Next, the Littlefuse appnote mentions a jump start - again not the
case in normal operation.
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