Re: [Phys-L] 9th grade energy resource projects

[John Denker <jsd@av8n.com>]

On 12/13/2014 06:11 PM, Chuck Britton wrote:
> You might want to consider using an Automotive Alternator (with it’s
> Regulator included).
>
> These will output a constant (nominal) 12 volt supply - actually
> closer to 13.8 volts - but enough to light up a series of 55 watt
> headlights for instance.

Ooooh, be careful with that.

Such a thing might OR MIGHT NOT work without a battery.
If you have a specific model in mind that is known to
work, please let us know.  If you can find one where
the spec sheet says it is guaranteed to work without
a battery, that would be even better.

*) Hypothetical failure mode #1:  The field coils might 
 require excitation.  Otherwise you get zero output.

*) Hypothetical failure mode #2:  The regulator might 
 use the enormous effective capacitance of the battery
 as part of the feedback loop.  In particular, it 
 could send out just the right amount of current to 
 hold the battery at the desired voltage.  Without
 the battery, the RC time constant goes to zero and
 the feedback loop goes unstable.

 There's a corollary to Murphy's law that says
  -- amplifiers oscillate,
  -- oscillators latch up, and
  -- latches act like amplifiers

 That's amusing, but it's actually not a joke.

 I've seen reports of alternators that put out 175
 volts under no-load conditions.

*) Et cetera.

------

Let's be clear:  I don't know whether it will work
or not.  I would be willing to bet that /some/ of
them fail spectacularly.  I would not be surprised
to learn of some that work OK.

There is tremendous price pressure on the things,
so the designers make all sorts of optimizations, 
exploiting every assumption they can think of.
The KISS principle does not apply to such things.
Sometimes simple is cheap;  other times weird and 
sneaky is cheaper.