Hybrid Car – More Fun with Less Gas

Hey do you know your car's alternator only outputs 7-10% while it's running? - Page 5

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Posted by clare on July 8, 2009, 3:14 am

Even 150 volts or more.
At just a fast idle the 35 amp alternator on my Dad's old dodge van
provided enough power to run 2 skill saws, or 2 half inch drills, or
one of each (all universal motor, non variable speed tools) at once.
That was 30 amps at a voltage that varied, depending on load, from
about 110 to 150. If you cranked the speed up the voltage just kept
climbing - but over 150 volts the rectifier diodes were getting well
up close to their maximum voltage limit, and the stator winding
insulation, designed for 12 volts, was also being run  close to it's

He wired a lot of houses using that old van as a genset (often sat
running 8 or 9 hours in a day - though more often closer to  5 or so)
and occaisionally provided power for the framers as well.

Posted by News on July 8, 2009, 9:15 am

He does.

Yep.  I don't think he knows that.  Somehow I don't think he is  tortured

Posted by harry on July 8, 2009, 7:24 pm
Automobile alternator voltage can't usually be tampered with (at least
not in the UK) as they are controlled by a Zener diode.  There used to
be an external "box" that had the thing inside but nowadays it's all
completely internal to the alternator.

Posted by Tim Jackson on July 8, 2009, 7:38 pm
 harry wrote:

It's not actually a zener diode, but it is a sealed potted and/or
integrated circuit, and it does include some sort of reference diode.
It is to all intents and purposes an amplifier which produces an output
current in the field coil proportional to the difference between the
output (ie battery) voltage and its shut off point, something like
14.5V. The lower the voltage, the higher the field current and
therefore, indirectly, the output current.

A zener diode regulates voltage by dumping excess current, and so would
a) get very hot, and b) be a significant load on your engine.

While I wouldn't expect to to take one apart non-destructively, the
entire module can be removed and replaced with a circuit of your own
contrivance, should you so desire.  It's not rocket science.  I
reverse-engineered one once for a classic car when I couldn't get the

Tim Jackson

Posted by m II on July 9, 2009, 3:51 am
 harry wrote:

Close, but not right.

English motorcycles, during the Lucas days, DID use Zener diodes to
regulate voltage. It's rather inefficient, as the surplus amperage was
dumped directly to ground. In the case of the Lucas alternators, it
wasn't a big deal, as they put out roughly 150 watts Maximum.

A finned heat sink kept the diode from destroying itself.

No self respecting car has, to the best of my knowledge, ever used that
setup. It's far more efficient to regulate the field magnetism of the
alternator. That's a hard thing to do with the permanent magnet setups
of the old bikes.

The three phase wound rotor(field) alternator is almost the universal
setup on bikes today. Harley being a notable exception, with permanent
magnets on most, if not all of their models. They use a transistorized
regulator to block excess current flow instead of dumping it to ground.


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