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From: John De Armond
Subject: Re: One Wire Alternator Questions
Date: Mon, 28 Feb 2000 19:41:46 EST

JHowell297 wrote:
> Tran:
>    That's a great input and I thank you--will probably do it this week.
> However, I'm still curious--why doesn't Detroit do this?  Is there some
> disadvantage that I should be aware of?

There are several reasons and some of them will affect your use of
the 1-wire alternator.

First off, realize that the alternator needs field power in order to
generate.  Unlike an older generator, the alternator's tiny bit of
residual field is not enough to get things rolling.  In a
conventional setup, power is applied to the voltage regulator from
the ignition switch.  The voltage applied to the voltage regulator
and field is also the reference voltage that the regulator controls
to.  The regulator excites the field at whatever level it takes to
keep the voltage at the input terminal at 13.8-14 volts.  That means
the output terminal voltage of the alternator may be (much) higher,
depending on the voltage drop in the wiring between the alternator
and battery.  If the vehicle has an ammeter, then there will be
voltage drop across that in addition to the drop through the

The one-wire alternator must - by definition - regulate its output
voltage.  The designer of the regulator has to allow for some
defined amount of voltage drop through the wiring to keep the
battery at the desired 13.8-14 volts.  If the resistance in the
alternator circuit is higher than what was anticipated, then the
battery will be undercharged.  If the resistance is lower, e.g., a
short, fat wire directly from the alternator to the battery, then
the battery may be overcharged.  My experience with a number of 1
wire alternators is that the terminal voltage is set to 14.2-14.5

Two issues for manufacturers here.  One, while an individual's
mildly over or undercharged battery may not matter much in the big
picture, for an OEM, having this happen to thousands of cars would
be a warranty disaster.  Second, to keep the charging voltage at the
battery correct, a different regulator with different wiring
compensation would be required for each model.  A logistical and
cost nightmare.  

The other major consideration is field current control.  A
conventional alternator draws full field current when the engine is
stopped.  The reason it doesn't drain the battery is that the field
supply is switched off with the ignition switch.  Since the switched
field power isn't available to the 1-wire alternator, engine stop
and start to turn the field off and on must be inferred from other
parameters.  The 1-wire regulator detects engine stop by the
cessation of AC from the stator.  This is reliable.  Engine start
gets a bit more complicated.  Since the alternator is not generating
until the field is applied, engine start must be detected by other
means.  With the regulator I commonly use, this is done by looking
for the dip in voltage associated with engaging the starter motor. 
If it sees a dip in voltage, it applies field and looks for stator
output.  If no stator output, the field is cut off again.

The problem is, to be sensitive enough to detect engine starts under
all conditions (such as when the car is rolled off without engaging
the starter), the voltage dip detector has to be pretty sensitive. 
In experiments I have done, I've discovered that the small dip
caused by switching on a single 50 watt driving light will trip the
field on.  That means that the field will be momentarily turned on
from a wide variety of conditions other than engine cranking.  It
won't be on long but it does consume some battery power.  

Again, for the individual user, this isn't much of an issue.  Most
1-wire alternators end up on hotrods and old cars with few
accessories and usually none that draw impulse current with the
ignition off.  At most, if the dip detector ended up too sensitive
or the car has some load that trips it regularly, the only
consequence would be an occasional dead battery.  Having this happen
over millions of cars would, for the OEM, again be a warranty

The 2 wire setup neatly addresses all these issues and so the OEMs
stick with that design.


From: John De Armond
Subject: Re: 3 wire Alt to 1 wire
Date: Thu, 07 Sep 2000 07:24:41 -0400

Mark C Olson wrote:

> There ARE '1-wire' alternators out there, how EXACTLY they work I do
> not know, but they are not direct current generators, they are indeed
> alternators with diode rectifier bridges.
> I imagine they have a small amount of residual magnetism in their pole
> pieces, which is enough to bootstrap them into self-excitation.

I've instrumented my 1-wire alternator and figured out how it work.
It is thus.  The regulator detects the dip in battery voltage when
the ignition is turned on.  It is sensitive enough not to require
starter engagement.  Actually it detects any rapid CHANGE in voltage
of either polarity so it will start generating even after a push
start.  When it detects the change in battery voltage, it applies
field for a moment.  If it detects output via the half-bridge that
feeds the internal sense terminal, it leaves the field on and
commences regulating normally.  If not, then it turns the field off

If the alternator is connected to a very stable source of DC, say a
battery sitting on the shop floor, the generator will NOT start
generating when it is spun up.  However, even the most minor
electrical disturbance, such as touching a brake light bulb to the
battery terminals, will trigger the regulator into operation.

I have my 1 wire, 200 amp alternator mounted on a small gas engine
that I use for rapid charging my RV batteries.  I have to remember
to connect the leads AFTER starting the engine or else the thing
won't start charging until I either wiggle the clips a little or
turn on a light.  Once triggered, it works wonderfully.

BTW, most any alternator shop can install a 1 wire regulator in any
alternator, usually for under $50. Lots cheaper than paying the
rip-off prices of the outfits that cater to the car stereo guys.  If
you gotta have chrome, buy a chrome alternator from JC Whitney and
install the regulator.


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