Here's my off-hand idea for a Self-Switching-Regulator,
( random Frequency depending on Engine-RPM and Field-Coil characteristics ).

I lazy, and not willing to thrash my brain enough to calculate the Temperature/Voltage curve.
Maybe someone here will have enough interest to verify the stability of this Circuit.

The biggest advantage is that it will never get "Hot".
I'm just not sure about the Output-Voltage-variation vs Ambient-Temperature.
( the desired Temperature characteristics are in the chart below )
( the Voltage-Chart below is for Maintenance-Charging, normal-"Short-Term" Charging should be at least 1-Volt higher )
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The OPs circuit has the rotor grounded, so the above circuit is not going to work.

 

If the Grounded-end of the Field-Winding can't be accessed,
it's a simple matter of changing the FET to a P-Channel
and using a "Non-Inverting" FET-Driver to drive it.
It works exactly the same either way.

N-FETs usually have better specs, are cheaper, and more plentiful, than P-Channel FETs.
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Here's my off-hand idea for a Self-Switching-Regulator,

With the normal tolerance on the Zener voltages, I don't see how the regulation voltage will be what is needed(?).

 

I pondered over the value of a Trim-Pot.
One could easily be added.
My bigger concern was with Temperature-Voltage-Drift.
Both the Zener, and the FET-Driver, have a small amount of Thermal-Threshold-Drift.
And, both use complex-Math-Formulas,
( that I am not willing to learn how to interpret ),
rather than a simple Graph.
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These are the wave forms on the Rotor of Bosch altenator under 2 different charge rates looks like.View attachment 289344View attachment 289345 Shouldnt be to hard to build.

So, this configuration can be used for any coil resistance?

 

This is the circuit of a deconstructed Bosch altenator regulator. Its quite simple & works.View attachment 289339

So this configuration can be used for any coil resistance? I think the 3A fuse may be too limiting for a 3,9 ohm coil...
About the capacitors, what should be the rated Voltage?
One question that I have.
Once the voltage adjustment is made with the potentiometer in the left side, could it be replaced with regular resistors?
It should look like this right?


I am not sure if I am interpreting the diagram right. Can the 6,8k, 1,8k and x ohm resistor be replaced by a (1,4k+x) one?

 

The current draw from the rotor is not a lot as its pulse width modulated, not DC. Why alter a circuit that works?

 

The current draw from the rotor is not a lot as its pulse width modulated, not DC. Why alter a circuit that works?

Ah! Now I see. Thanks!!
About the resistor power rating, 1 watt resistors will do? Any advisable voltage rating for the capacitors??

 

Resistors all look about 1/2W capacitors 100V.

 

Resistors all look about 1/2W capacitors 100V.View attachment 289577

Thanks! I´ll give it a try

 

What do you guys think about this two circuits by the way?

CIRCUIT 1:
Full credits to https://schweizerschrauber.ch/mot/voltreg.html

Looks similar to the one @debe put (RE55)

CIRCUIT 2 Credits to Gary Lecomte http://chemelec.com/Projects/Alternator/Alternator-Reg.htm



@LowQCab

 

Any of the Circuits presented here will probably provide very reasonable performance.

I prefer the Circuit that I provided because it will generate the least amount of Heat.

The easiest solution is to simply purchase a common '70 thru '75 Chrysler-Alternator-Regulator-Module.
Just 2-Wires and 2-Screws to hold it down.
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What do you guys think about this two circuits by the way?

Neither of those allow ready adjustment of the charging voltage, which I think is important to have, to minimize under or over charging the battery.

 

Here's my Circuit with a Trimmer added ............
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Here's my Circuit with a Trimmer added ............
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View attachment 289700
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Thanks a lot @LowQCab

It is indeed of great help!
Regarding the application for this particular motorcycle, which has the coil to ground, I have followed your example and sketched this other one, would you mind having a look?

I have searched for P type MOSFET to similar characteristics and found the following one:
- SUP90P06-09L-E3

About the non-inverting fet driver, there are two similar options:
- IXDN630MCI
- IXDD630MCI
Any of them will do? I'm a little bit lost here

Thanks again!!

 

Why reinvent the wheel? You can buy replacement alternator regulators to use in the project, they are a proven off the shelf product. Just one example - https://www.amazon.com/DB-Electrica...ocphy=9051531&hvtargid=pla-348408066910&psc=1

 

Revised Schematic for Grounded-Field-Coil, ( P-Channel FET ).

Lower-Temperatures help to insure long, trouble-free, Service-Life.

Under "Short-Circuit" conditions, ( shorted Field-Coil ), ( wiring shorted to Ground ),
the P-FET could be required to dissipate around ~1 Watt of Heat before the 10-Amp-Fuse blows.
1-Watt is probably the maximum amount of Heat that
the FET can dissipate, with no Heat-Sink, and still survive with complete reliability.

Under worst-case "normal" circumstances,
(~5-Amps continuous Field-Coil-Current, Engine not running, Ignition On ),
the P-FET will dissipate around ~0.35-Watts,
so, while a Heat-Sink of some sort is always recommended, ( small Cast-Aluminum-Box ),
it's not really required for "normal" operation.
Keep in mind the sources of Heat from the Engine,
cooler-Air is always better, even if it's not strictly required.

The chosen P-FET does not have a Screw-Hole for mounting,
but it does not require an Insulating-Mounting-Pad-Kit,
as it comes in a completely insulated Package.
After proper operation is verified,
it should be secured to a small Aluminum-Box with J-B-Weld-Epoxy.
Be sure to clean all surfaces before gluing, lightly Sand the Aluminum, but NOT the FET.
Various small "Clip-On" Heat-Sinks are also available, but they only work in free-Air.
Alternatively, the entire Circuit and all components can be assembled "Dead-Bug" style,
and then the assembly can be "Potted" in a small Cast-Aluminum-Box.
The coolest mounting location is usually behind the Headlight,
but anywhere is OK, as long as it's away from the Exhaust-Heat.

When Bench-Testing,
remember that the FET-Driver has an "Under-Voltage-Lock-Out", (UVLO), feature
which will disable the FET-Driver if the Power-Supply-Voltage drops below ~9-Volts.
This feature will prevent your Battery from being completely ruined
if You should accidentally leave the Ignition in the "On" position overnight.
This feature also turns-off the Regulator during
Engine-Cranking, if You have Electric-Start and a low-Battery.

Free-Wheeling-Diode,
The Free-Wheeling-Diode should be mounted as close as possible to the Field-Coil to
reduce the possibility of radiating "Electrical-Noise", ( EMI ), from the Wiring-Harness.
( some people don't care about EMI, it might not be an issue for You )

The Free-Wheeling-Diode has a "Full-Pak", completely insulated Package,
but the insulation is very thin to allow for good Heat-Dissipation.
This means that if the Diode is not Screwed-Down securely, ( and insulated ),
it should not be allowed to touch, or vibrate against, any other metal surface.
Vibration will quickly wear a hole in the Insulation and cause a Short-Circuit.
Vibration will always cause a failure eventually.
The Diode only dissipates around ~1 Watt of Heat, (worst-case),
so You can further protect it with Heat-Shrink-Tubing without fear of over-heating it.
It does not require a Heat-Sink.
An extra-long piece of Heat-Shrink-Tubing will create a convenient,
vibration-absorbing, insulating, Mounting-Tab, for a Screw, or Bolt, with a Washer,
or even a high-quality Nylon-Ty-Rap can be used with a longer Heat-Shrink-Tab.
Don't use only a Ty-Rap to hold down the Diode, extra insulation is required.
Don't let it just vibrate in the wind, use RTV-Silicone to mount it in place if that works for You.
Use only FINE-STRANDED Wires for connecting the Diode, and keep them as short as practical.
Don't let the Wires vibrate either,
tie them down, and/or, insulate them with Heat-Shrink-Tubing.
Vibrations and excessive Temperatures are ALWAYS a problem on Motorcycles, or
when mounted directly to a Car-Engine.
Don't let it be your problem.
.
.
.

.

 

Why reinvent the wheel? You can buy replacement alternator regulators to use in the project, they are a proven off the shelf product. Just one example - https://www.amazon.com/DB-Electrica...ocphy=9051531&hvtargid=pla-348408066910&psc=1

Indeed... I did actually find something that may suit my needs, but it took me a long time. At first I didn't even know what to search for hehe.
Apart from that, designing and building a thing on your own can be fun as well righ???

 

Revised Schematic for Grounded-Field-Coil, ( P-Channel FET ).

Lower-Temperatures help to insure long, trouble-free, Service-Life.

Under "Short-Circuit" conditions, ( shorted Field-Coil ), ( wiring shorted to Ground ),
the P-FET could be required to dissipate around ~1 Watt of Heat before the 10-Amp-Fuse blows.
1-Watt is probably the maximum amount of Heat that
the FET can dissipate, with no Heat-Sink, and still survive with complete reliability.

Under worst-case "normal" circumstances,
(~5-Amps continuous Field-Coil-Current, Engine not running, Ignition On ),
the P-FET will dissipate around ~0.35-Watts,
so, while a Heat-Sink of some sort is always recommended, ( small Cast-Aluminum-Box ),
it's not really required for "normal" operation.
Keep in mind the sources of Heat from the Engine,
cooler-Air is always better, even if it's not strictly required.

The chosen P-FET does not have a Screw-Hole for mounting,
but it does not require an Insulating-Mounting-Pad-Kit,
as it comes in a completely insulated Package.
After proper operation is verified,
it should be secured to a small Aluminum-Box with J-B-Weld-Epoxy.
Be sure to clean all surfaces before gluing, lightly Sand the Aluminum, but NOT the FET.
Various small "Clip-On" Heat-Sinks are also available, but they only work in free-Air.
Alternatively, the entire Circuit and all components can be assembled "Dead-Bug" style,
and then the assembly can be "Potted" in a small Cast-Aluminum-Box.
The coolest mounting location is usually behind the Headlight,
but anywhere is OK, as long as it's away from the Exhaust-Heat.

When Bench-Testing,
remember that the FET-Driver has an "Under-Voltage-Lock-Out", (UVLO), feature
which will disable the FET-Driver if the Power-Supply-Voltage drops below ~9-Volts.
This feature will prevent your Battery from being completely ruined
if You should accidentally leave the Ignition in the "On" position overnight.
This feature also turns-off the Regulator during
Engine-Cranking, if You have Electric-Start and a low-Battery.

Free-Wheeling-Diode,
The Free-Wheeling-Diode should be mounted as close as possible to the Field-Coil to
reduce the possibility of radiating "Electrical-Noise", ( EMI ), from the Wiring-Harness.
( some people don't care about EMI, it might not be an issue for You )

The Free-Wheeling-Diode has a "Full-Pak", completely insulated Package,
but the insulation is very thin to allow for good Heat-Dissipation.
This means that if the Diode is not Screwed-Down securely, ( and insulated ),
it should not be allowed to touch, or vibrate against, any other metal surface.
Vibration will quickly wear a hole in the Insulation and cause a Short-Circuit.
Vibration will always cause a failure eventually.
The Diode only dissipates around ~1 Watt of Heat, (worst-case),
so You can further protect it with Heat-Shrink-Tubing without fear of over-heating it.
It does not require a Heat-Sink.
An extra-long piece of Heat-Shrink-Tubing will create a convenient,
vibration-absorbing, insulating, Mounting-Tab, for a Screw, or Bolt, with a Washer,
or even a high-quality Nylon-Ty-Rap can be used with a longer Heat-Shrink-Tab.
Don't use only a Ty-Rap to hold down the Diode, extra insulation is required.
Don't let it just vibrate in the wind, use RTV-Silicone to mount it in place if that works for You.
Use only FINE-STRANDED Wires for connecting the Diode, and keep them as short as practical.
Don't let the Wires vibrate either,
tie them down, and/or, insulate them with Heat-Shrink-Tubing.
Vibrations and excessive Temperatures are ALWAYS a problem on Motorcycles, or
when mounted directly to a Car-Engine.
Don't let it be your problem.
.
.
.
View attachment 289738
.

THANKS A LOT for such a comprehensive answer!!
It is very helpful! I'm thinking about trying multiple solutions and doing a comparision, it is promising!