Rewire Bukh starter panel- Alternator excitation question

Long post, but short questions!! Decided to rebuild my starter Bukh panel for obvious reasons


Was going to get original Bukh, however costly and not my style and I just don't get the complexity of the diode/resistor block... keep it simple.



Going to change to Push button start/stop, add a tacho/temp gauge and of course the Alt/Oil/Temp lamps in LED.








I think I've got it worked out mostly, i'm going to put the piezo alarm buzzer in parallel on the positive side with all new LED lamps as they are ground switching( but I don't know about the Alternator), however the Excitation resistor on the Alternator lamp confuses me....
So I need to add at least 2watts for the Alternator to kick in, and the LED won't have this, so do I put a resistor in series with the Alt LED, or Parallel with the LED lamp?
I'm guess Parallel so the load is always there, however I don't quite understand how the excitation terminal on the Alternator works? How does the Alt lamp turn on/off? I'm guessing if the Alternator has no excitation, then it would be negative and Lamp comes on, when excited, goes to positive, and no potential difference on Lamp as positive both sides, so switches off????

I don't have my new wiring diagram on me as away travelling, however hoping someone, auto-elec would understand the question.

Other question is, if the Piezo buzzer is in parallel with all LED's on the positive side, should I put a zenor diode in series with the piezo as if there is any minor leakage to negative, I'd have the buzzer annoying me the whole time?

Any cautions I should take not to destroy the Alternator/Regulator etc while playing with this?

Does the 'To Do' list of things on boats ever get shorter...

No AutoElec's out there??? Or is the long winded post confusing?

Ok, if no resistance/load is on excitation terminal on Alternator, would it act like a negative/ground terminal? Then if resistance/load is added and Alternator is the right RPM, will it become a positive/12v+ terminal?
Im just unsure what the excitation terminal does inside the Alternator???
Only know it's needed and google isn't my friend on this one

Drunken Rant!!
Well, project #37 complete..... well almost.
If anyone is googling around looking for info for Alternator excitation, after a couple of attempts.... by having a 2W 68ohm resistor in parallel and series with the LED on the Amp light with no success, turned out I needed to switch 12vdc+ (Aux on/off) with the resistor inline into the excitation terminal on the Alternator .... completely different from what I expected, so the new motor start panel complete with new wiring loom to all functions in engine bay and sensors and separate fuses on everything.... only fix is, I need a diode on ALT lamp, as if the oil, or temp lamps come on, the ALT also lights up.
Between fault finding the resistor/excitation thing, turns out the original piezo alarm has some sort of issue that sent me down a rabbit hole(half day lost).... luckily a $10 Jaycar piezo sorted it out.... apart from needing a 50cent diode, all good..... project #37 complete. Now no dodgy worn key, faded warning lights, flukey piezo, new tacho, temp gauge that works... happy camper all for 1/5 of the price Bukh wanted for a new panel without tacho/temp, and push button start/stop!!( Made so she can use it too)


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Project #38
Evo1 wheel pilot already fitted to pedestal & Wheel , P70s, Accelerometer, CPU and wiring to go.

Preping for #39
Standing Rigging, Mast/Boom paint, Anchor/Running/Deck lights and all new wiring... ( if anyone knows a good cheap rigger and shipwrights let me know)

To Do list, must get shorter one day!! When and if I sell someday , hopefully someone will appreciate the effort to renew the old and replace the average attempts of others.

Brand new Second Hand Sailing!!

Select to expand quote

AzureF305 said..
Nice work! How did you print the board? I saw no sign of stickers.

Was CNC engraved into anodised aluminium. Lots of companies do it, I had a contractor through my work do it. I think I overpaid... others cheaper.
Thinking of clear nail polish or paint on engraving to stop any corrosion?? Though probably a overkill?? Not sure, never done on a boat before. Any experience from anyone?

I'm not the auto electrician but I'm to the understanding that the exciter terminal is positive.
So what you have got is + feed to the ex coils which is then grounded via a regulator.
You turn your key on,, the dc power flows through the lamp,,energize the ex coils,, and goes to ground via the reg. The energised coils in the rotor now produce a magnetic field.

When you start the engine the alternator spins and starts to produce ac current. which is rectified and some is fed to the regulator by tapping into the lamp circuit , so the reg now feeds the exciter coil with dc+ voltage from the alternator. The alternator is now feeding positive power to the other end of the lamp circuit which puts the lamp out.
A couple of things to remember here
1.. The lamp also serves as a resistor to reduce the voltage to the coils to protect from hard starting the charge, so the wattage of the lamp can be important.
2.. The reg controls the voltage to the exciter coils. this allows the voltage produced by the aleraternator to remain stable through a wide rpm range, Eg fast spinning = less voltage to the ex coil= less magnetic field,,, slow spinning= more voltage to the ex coil = stronger mag field. This is very different to a permanent magnet type alternator in which the output increases with rpm.
3.. The key point to your question is the lamp goes out because the feed from the alt joins the ex coil circuit ahead of the lamp so you have now connected a + charge to both ends of the lamp.
I hope that helps.