BMW D12 Marine Diesel rectifier/regulator project

[Pauli]

Hi, this is my first post.

I have an old sailing boat with BMW D12 marine diesel. The engine has a generator (or magneto) with brushless permanent magnet rotor and stator with coils. The problem is the rectifier/regulator that blows up about after three years usage. The part costs 200€ and it's hard to find thanks to the engine's old model. I have planned to make it myself.

The rectifier/regulator parts are in a small aluminium heat sink box filled with epoxy. It's impossible to fix or even identify the components. I have find the circuit from the engine's workshop manual. The components are shown without values. The generator's charging capacity is 14 V 350 W - 25 Amps.

Can someone kindly help me to find right components for the rectifier/regulator? I have not very good skills in planning electronics but I can make and solver the unit myself if I get some help with finding components.


Magneto rectifier regulator circuit:

Stator AC output voltage between different wire pairs and rpms:

Original BMW electric diagram:

Picture of the stator:

Charging components, a picture from the workshop manual. The rectifier/regulator is number 15

 

[duke37]

The circuit can be split into two parts.
1. The high current rectifier D1 and D2. You will need rectifiers of 100V and 25A
2. The control circuit. It is not easy to give component details, The Zener will need to be about 12V. The output will be the zener voltage plus the drop across the voltage divider plus the voltage necessary to turn on the SCR.
I would try 100R for the resistors.
The SCR could be 100V at 5 to 10A.
The capacitor is to reduce interference, perhaps 1 microF.

You should be able to test which section is faulty and replace only that section.
Some measurements could be made of the components in the good section.
The diodes will need to be bolted to a substantial heat sink (the box) but need to electrically insulated from this.

 

[Pauli]

Thanks for your answer, Duke37! The heat is a problem, as you told, should find a suitable heat sink.

I got an answer at a finnish electronic forum, too. The electronic hobbyist advice me to ask professionals before I begin to solder parts together. He was not sure of R1 value and SCR, in particular SCR's gate sensitivity.

What do you think about this proposal?

D1, D2 - STPS80170CW STMicroelectronics | 497-4815-5-ND | DigiKey
- Dual center tab Schottky rectifier, 2 x 40 A, 170 V

D4 - S6070WTP Littelfuse Inc | S6070WTP-ND | DigiKey
- SCR, 70 A, 600 V, gate sensitivity 50 mA

D3 - 1N6002B-TP Micro Commercial Co | 1N6002B-TPMSCT-ND | DigiKey
- Zener diode, zener voltage 12 V, 42 mA, 500 mW

R1 - 47-100 Ω
R2 - 1 kΩ
C1 - 1 µF

 

[duke37]

The Schottky rectifiers will have a lower voltage drop than standard diodes so will generate less heat, a good thing.

The SCR is a bit over the top, a smaller one may be more sensitive.

The Zener has a tolerance of 5% so you may have to trim the voltage in some way.

As I said previously, this unit is in two parts and you only have to replace the faulty part. If it is only the rectifiers that have gone, then it will be a fairly simple repair as you will not have to calibrate it.

Measure the forward and reverse resistance between the white and each of the black wires.

The regulator part can be tested by measuring the voltage necessary on the white connection to turn on the SCR. Put a bulb in series with the red wire and connect to 12V

 

[Pauli]

I am afraid it'snt possible to change faulty components. The rectifier/regulator parts are in a small Al heat sink case filled with epoxy. And filling stuff is hard like concrete.

But thanks for your comments! SCR is oversized, as the guy who made the proposal said. Should find smaller.

 

[duke37]

You can test each half of the circuit without digging out the epoxy. If you find the faulty half, that is the only piece that you will have to make but you will need to keep the original box also.

 

[duke37]

Consider using a programmable zener for D3 (e.g. AS431) so that it can be trimmed to the correct voltage.

 

[Pauli]

I ordered parts from Digikey and LARGE heat sinks! . Included also a couple of LM341 shunt regulators to make an adjustable model.

First I gonna make a basic model with a 12 V zener and I hope I can show you results and pictures before midsommer eve.

 

[duke37]

Did you find if the power diodes or the regulator were at fault?

 

[Pauli]

I haven't test the old rectifier/regulator yet.

But new one is made! I really appreciate your help!!!

Parts:
D1, D2 - STPS80170CW STMicroelectronics | 497-4815-5-ND | DigiKey
- Dual center tab Schottky rectifier, 2 x 40 A, 170 V

D4 - S8065KTP Littelfuse Inc | S8065KTP-ND | DigiKey
- SCR, 65 A, 800 V, gate sensitivity 50 mA

D3 - 1N6002B-TP Micro Commercial Co | 1N6002B-TPMSCT-ND | DigiKey
- Zener diode, zener voltage 12 V, 42 mA, 500 mW

R1 - 10 ohm
R2 - 270 ohm
C1 - 1 µF film cap.

Aluminium housing (a piece of a cable tray) with a processor cooler (I found these parts in my carage). Using screw terminals give it flexible way to change components.

Charging works fine on slow engine rounds (about 1000 RPM).

Regulator generates a little bit too much voltages on high (2500 RPM).

15 V is too much. An ideal would be 14,5 volts. Any ideas??

 

[duke37]

Zener diodes can have a wide tolerance of voltage. You could get many and find one to suit you although if they are from the same batch, they might all be similar.
The zener may be dissipating more than it should with such low resistances. How hot does it get?

There are adjustable zener diodes (I have never used one), the first one I saw on a search was AS431.

If the original control is OK then you could try that or even incorporate it in your new, posh box.

 

[(*steve*)]

431's are not an adjustable zener (although the circuit symbol used kinda makes it look that way).

They're actually an adjustable shunt regulator.

In some respects they act very much like an adjustable zener, and s moire perfect one in many cases.

 

[vincentdelft]

Hi Pauli,

I have also a sailingboat with a BMW D12 as engine.
The alternator is not working and I think the voltage regulator is broken.
Did you get your own build regulator working properly?
And if so are you willing to share you final circuit and used components?

Best regards,
Vincent

 

[PaulB]

Hello,

I also have a BMW marine diesel engine for my sailboat and was very interested by your project.

I have been contemplating this particular problem for a while and am very happy I found this post.

Would it be possible for you send me some more information about the Resistors and heat sinks?

 

[nagmat84]

The post is quite old, nonetheless I try my luck.

@Pauli: Did you figure out the correct type and value of each component? If yes, could you share your results?

Where did you get that table with the voltage values from? My own manual of the engine does not include it. Are there more information about the magneto where this table is from?