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# Automotive 6 Volt Generator Transistor Voltage Regulator

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#### Danwvw

##### Active Member
Could use some help to design a 2 or 3 Transistor voltage regulator for my 6 volt automotive generator.
Given 7 Volt 45 Amp Bosch B circuit Generator,
1000 AH AGM 6 volt Battery, (Ideal charge voltage 7 volts)
Full Load with 2X H4 High Beam 65Watt 6 volt headlights and brake lights and ignition sys is somewhere below 180Watts,
Field current?
Field Winding measures 1 ohm of resistance,
Field Voltage about .8 volts when generator is charging the battery at 7 volts.

I have seen some schematics for 12 volt A circuit ones but none for 6 volt B circuit generators.
Note the B circuit generator has the other end of the field coil connected to the D+ which is + output of the armature. I will post a equivalent schematic once I figure out how.
One additional thing Is that I think the design will need to use a old fashion PNP 20-50 amp low gain power transistor for the driver it could be nicely mounted to a grounded heat sink with it's emitter connected to the field and it's base to the rest of the circuit which would regulate voltage (7V) and current (25A) of the generator.
Thanks.

I have built a simple one for a 230v DC generator that should work on the same principle, just uses a LM311 comparitor and a power Hexfet output. for 6v you could use a Logic level Mosfet..
I can dig up the schematic if interested.
Max.
.

Thanks "MaxHeadRoom78" for the quick reply, Based on what your saying it sounds like it would use the LM311 Comparator to somehow tell a Power Transistor how much current to flow into the field F.
I have come up with this for a start but am unsure of the rest?

Will post what I have tomorrow.
Max.

One thing I see that may not work is the driver transistor may need to be a lower voltage for its Vce(Saturation Voltage) since the Generator is needing about .8 volts on the field to put out full current. What about a MOSFET Module? I've never played with one though. But Found this NTE121 Germanium PNP Transistor.

You should be able to adapt this circuit, you just need to maintain around 7v for the generator output.
I made a couple of changes, but the input for the LM311 needs a little tailoring.
I have used it from a 240v to a 24v generator.
This is an electronic version of the old mechanical versions.
Max.

Oh, Nice Looks like the 1N4742 is what a 12 volt zener? Some kind of circuit protection as all the other diodes too with the exception the 6 volt zener which seems to be part of the power supply for the circuit as well as a stable voltage reference. Is the Q1 a Power Mosfet?
What about feedback, I guess the top 7 volt rail is the generator output wonder if this circuit will current limit? Pretty simple though. I was hoping to come up with something with even less parts. Thanks I just need to figure out what the HEX-10 is and then I can order parts. Saw this Article about a 12 volt Alternator Regulator Interesting!

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Make that a 1n4148.
It is a voltage regulator, just as an automotive version is.
The power mosfet could be a logic gate type, IRL540 etc.
The LM311 input bias components will need some adjustment as this was designed for a higher voltage.
Max.

I appreciate the all the help Max, It sounds good. I found this one apparently new to the Classic Auto Parts for 6 volt systems. Pricey though. 6-VOLT VOLTAGE REGULATOR (NEW SOLID STATE DESIGN) my old mechanical voltage regulator doesn't do a very good job of keeping up with the Halogen Headlights. I have and extra 6 Volt Generator that I am in the process of rebuilding and a Engine on the Bench that I plan to mount it on. It should be easy enough to test and take some readings etc... I will give the design a shot and keep progress posted Thanks again.

Would the higher efficiency of a switching type regulator be worth the added circuit complexity?
With 0.8V across the 1 ohm field resistance, the dissipation in the control transistor would be about 5W, which requires a good heatsink..
A switching regulator could likely reduce that to below a watt.

A standard amazon/ebay buck-converter module could likely be adapted for the job.
You would just have to break the sense voltage loop and connect it to the battery voltage instead of the module output (which would now go to the field winding).

"Crutscrow" that sounds interesting! Not sure I know what your talking about?
Another Idea just occurred to me that would give advantages in what parts combinations that could be used to operate as 6 volt automotive dynamos. Basically the idea is why not convert a 12 volt alternator to put out 6 volts.
The first year the VW Beetle came with a 12 volt Alternator was 1973 1/2. A US Motorola Alternator design was used for the first year only before VW switched to Bosch Alternators. The Motorola Alternators have a 55 Amp rating just like the early 6 volt generators did so they should handle the added load of a 6 volt system ok. The Motorcycle Alternator Schematic above could be adapted to operate at 6 volts and cause the 12 volt VW Motorola Alternator which I just happen to have several of to only develope 7 Volts DC at 55 amps. Thoughts? Motorola Alternator Wiring Via Speedy Jims http://www.speedyjim.net/htm/elec.htm

"Crutscrow" Found this in a Buck Converter Thread: It's Kind of like what I am thinking to run a 12 volt Alternator at 6 volts although it's on the alternator output! An armature current regulator regulator would probably be the way to go however MOSFETs could be used to replace the schottky alternator output diodes.

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It would be Much simpler IMO to control the 12v alt. field for 6v ouput, leave the output pure sine wave.
Max.

Thanks Max, Actually that was what I was thinking for that idea, Motorcycle circuit kind of fooled me. Still going to build a 6 Volt VW generator Regulator. I already ordered the parts for your schematic.
Found this interesting Video: https://www.gener-nator.com/

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Would the higher efficiency of a switching type regulator be worth the added circuit complexity?
Max's circuit is functionally a kind of switching regulator; the field control transistor is driven by a comparator IC so either on or off.

It's not constant frequency PWM, more like a hit-and-miss governor.
But the flywheel diode across the field coil means the field current should decay relatively slowly between kicks, keeping a pretty constant output voltage.

The power dissipation should be quite low.

Originally it was designed for a 240v DC self exciting generator, and worked well, also adapted for 24vdc but never tried it as low as 7v.
It is an electronic version of the old style automotive mechanical generator regulator.
Max.

Originally it was designed for a 240v DC self exciting generator, and worked well, also adapted for 24vdc but never tried it as low as 7v.
It is an electronic version of the old style automotive mechanical generator regulator.
Max.

If it's not broken, why bother fixing it

The old mechanical regulators worked OK, you've just improved it by removing the contacts that wear out.

If it's not broken, why bother fixing it

I made and produced these for railroad mobile magnet cranes that used a carbon pile regulator, of which they could no longer source, hence the 'New' design!
Max.

I made and produced these for railroad mobile magnet cranes that used a carbon pile regulator, of which they could no longer source, hence the 'New' design!
Max.

Interesting story, hope you made some money out of it?

But of course!
Max.

Just a thought - Max's regulator will also need a high current diode to prevent the battery feeding the armature when the engine is stopped or the output is below battery voltage.

Electromechanical regulators has a second relay for that function, double wound for both voltage control and current sensing, if I remember right?

The voltage coil would pull in once the output reached near maximum, then the current winding countered that if the battery started to feed back and dropped the relay out.

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