Lawn Tractor Tri Circuit Alternator

Thread Starter

EMPace1

Joined Feb 10, 2019
5
The attached wiring diagram is for a garden tractor. It uses a Briggs and Stratton engine that utilizes what is referred to as a Tri Circuit Alternator.
It has a 22 VAC stator and the single output stator lead has two diodes, one positive bias and one negative bias. The positive DC output is for charging the battery and the negative DC is for the headlights. One side of the headlight circuit is from the diode (-DC) and the other side is from the ground bus side where the battery negative is attached. I know one diode is conducting on positive side of the AC cycle and the other diode on the negative.
If the one diode is supplying -dc to the headlights why is the other side of the headlights terminated to the negative ground side and not to the positive.
I am having a hard time understanding how the headlights get the positive voltage.
 

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crutschow

Joined Mar 14, 2008
30,088
I am having a hard time understanding how the headlights get the positive voltage.
They don't.
They run fine with a negative voltage (all that means is that the current goes in the opposite direction through the bulb, which has no effect on the bulb or the light output).
 

Thread Starter

EMPace1

Joined Feb 10, 2019
5
By the way the headlights only work with the engine running and do not cause any drain on the battery. Lights will still work with the battery disconnected.
 

Thread Starter

EMPace1

Joined Feb 10, 2019
5
Thanks
They don't.
They run fine with a negative voltage (all that means is that the current goes in the opposite direction through the bulb, which has no effect on the bulb or the light output).

How is the difference in potential created to make the current flow? Is this a dumb question?
 

crutschow

Joined Mar 14, 2008
30,088
How is the difference in potential created to make the current flow? Is this a dumb question?
Nope.
The alternator rotor has a rotating magnetic field that induces an AC voltage in the stator windings (loop up magnetic induction if you need more info on that) with equal positive and negative peak voltages (as measured to ground).
The diode to the battery allows the positive peak to flow and blocks the negative.
The other diode to the lights allows the negative peak to flow and blocks the positive.
So the magnitude of the positive and negative DC potentials are the same (the only difference being polarity).

Make sense?
 
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Thread Starter

EMPace1

Joined Feb 10, 2019
5
Yes. Thanks
I think I need to stop thinking of current flow in terms of polarities and realize that current is the flow of electrons between two points with different potentials. As you stated a light bulb doesn't care what direction. I have seen cases in auto generators where it does matter because the field coils are wound in a certain manner so that reversing the flow affects the magnetic field produced.
Thanks much for the help!
 

wayneh

Joined Sep 9, 2010
17,175
Nope.
The alternator rotor has a rotating magnetic field that induces an AC voltage in the stator windings (loop up magnetic induction if you need more info on that) with equal positive and negative peak voltages (as measured to ground).
The diode to the battery allows the positive peak to flow and blocks the negative.
The other diode to the lights allows the negative peak to flow and blocks the positive.
So the magnitude of the positive and negative DC potentials are the same (the only difference being polarity).

Make sense?
That seems like an odd choice to make in the design. I hardly ever use my lights and that means the alternator capacity is largely wasted (if my tractor is configured that way). Why not just use a full bridge and make the alternator a bit smaller? Surely the cost of two diodes is not driving the design. Is it a way to keep the user from draining the battery with the headlights?
 

crutschow

Joined Mar 14, 2008
30,088
That seems like an odd choice to make in the design. I hardly ever use my lights and that means the alternator capacity is largely wasted (if my tractor is configured that way). Why not just use a full bridge and make the alternator a bit smaller? Surely the cost of two diodes is not driving the design. Is it a way to keep the user from draining the battery with the headlights?
Yes, the design does seems a little odd.
You will lose another diode drop with a bridge but that wouldn't seem to be a big factor.
Since I see no starter, perhaps the battery is so small that the lights would rapidly drain the battery with the engine off.
Of course that could be prevented by switching the lights with the ignition switch.
 

Thread Starter

EMPace1

Joined Feb 10, 2019
5
There is a separate starter. It is the circle with a "S" in the right top corner of the diagram. Briggs and Stratton has a multitude of alternator designs - DC only, AC only, Dual circuit, and this weird Tri Circuit. Stators and flywheel magnet design are different in some cases.
I think the intention of this design was to prevent the lights from draining the battery as the battery is only used for starting and for engaging the electric clutch for the mower deck or other accessories. Seems weird to me that to read voltage on positive diode the negative meter lead goes to ground and to read the negative diode the positive lead goes to ground. I think I am a little clearer on why this is now but it is was contrary to my normal thinking about DC circuits. Thanks for all the feedback.
 

crutschow

Joined Mar 14, 2008
30,088
Seems weird to me that to read voltage on positive diode the negative meter lead goes to ground and to read the negative diode the positive lead goes to ground.
Digital multimeters can read negative voltage, so you don't need to reverse the lead connection.
 

shortbus

Joined Sep 30, 2009
9,449
In every tractor I've ever worked on, the starter and the generator are the same.
What kind of tractor is that? Some of the old Wheelhorse ones, that used a generator did that a starter/generator , but were belt driven. But any of the small riding type mowers and tractors I've seen, the alternator use the flywheel magnets, and the starter is a totally different thing, a DC motor that drives a gear on the flywheel.
 

MrSoftware

Joined Oct 29, 2013
2,027
What kind of tractor is that? Some of the old Wheelhorse ones, that used a generator did that a starter/generator , but were belt driven. But any of the small riding type mowers and tractors I've seen, the alternator use the flywheel magnets, and the starter is a totally different thing, a DC motor that drives a gear on the flywheel.
I've seen this on older tractors (belt driven starter/generator), I think it was a Cub Cadet from the 1970's or 1980's, and it was the size of a modern mower, probably 10HP, but was definitely more "tractor" like than today's "lawn tractors". But all the modern Briggs & Stratton types that you find at Sears, Home Depot, etc.. that I've seen all have a separate starter.
 

wayneh

Joined Sep 9, 2010
17,175
What kind of tractor is that? Some of the old Wheelhorse ones, that used a generator did that a starter/generator , but were belt driven. But any of the small riding type mowers and tractors I've seen, the alternator use the flywheel magnets, and the starter is a totally different thing, a DC motor that drives a gear on the flywheel.
I misspoke. We had a tractor when I was a kid that was like that, and I can picture it clear as day. But yeah, the newer ones I’ve had have a little starter that resembles one from a car, just smaller.
 

MisterBill2

Joined Jan 23, 2018
12,392
It makes perfect sense to use the opposite phase to power the lights. One extra diode and free lights power. Nothing wasted. Possibly a bit confusing but a simple solution.
 
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