Re: 12V source for lighting

[Alan Cordeiro <cordeiro@avlna.com>]

At 03:09 PM 1/5/96 -0800, you wrote:
>>The "system" *DEPENDS* on the battery to filter the voltage surges, which
>>can easily be in the multiple *HUNDREDS* of volts without the battery
>>there to suck up all those excited electrons. Without the battery in place
>>to smooth out those voltage transients, you start frying electronics in
>>very short order . . .
>>
>>                                        -RDH
>

RDH is talking about a phenomenon known as " load dump", where a sudden change
(reduction ) in curent draw caused by a turn off of some electrical load
needs some relief because the current in a highly inductive device like
the alternator will not shut off immediately.

>The battery is really performing 2 major functions.  The first is that the
>alternator puts out AC, which is then rectified (by the main diodes).
>Without the battery in place, you would see a complex wave form consisting
>of three phases of rectified AC.  The voltages wouldn't be that high (I'd
>guess @20 VDC).  The other function of the battery is to absorb voltage
>transients caused mainly by electrical motors being shut off.  These
>voltages are very brief spikes, but these could get into fairly high
>voltages (I won't bother guessing).

the ripple on the top of a rectified three phase is so small that only
electronic devices will " see " it. The thermal time constant of the
lamp filament is typically 10 to 100 times ( depends on engine rpm )
longer. In fact, the battery has a hard time decoupling these 
frequencies ( 2000 hz. and up ).

>Richard Funnell,
>San Jose, California
>'83 urQ
>'87 560 SL
>
>
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