Dominator alternator

As the bike has a magneto you don't need a very high output alternator (who does?) Your existing alternator is sufficient and can be used to run at 12V and charge a 12V battery. Usually the voltage rise is controlled and limited by installing an appropriate Zener Diode the max voltage rise to around 14.5v will prevent overcharging the 12V battery....that's all you need to do apart from changing the bulbs to 12V.....Loads of info on the internet how to do this but so simple you don't need it! The Zener needs a decent heatsink in a cooling breeze and is connected anywhere that is permantly connected to the live side and the body must have a good earth (discrete preferred) .....Les

PS: I note you favour a voltage regulator with combined rectifier, if so, just install as per instructions otherwise buy a 25A bridge rectifier for about a couple of quid and fit the Zener.....All wiring will remain the same then.

Your current setup uses the ignition switch to add alternator coils into circuit as you add the load of the lights etc. When you add the Zener this setup is redundant and you want all the coils in circuit all the time, there are examples of the wiring changes needed on the internet, the C15 and B40 conversion is popular and easy to find. It entails changes to the connections inside the rotary switches.

Hi John...If peter's wiring loom and switches are all good I would leave the whole wiring system the same. The reason being that the original system as you said, switched the coils to provide a higher current when the lights had been switched on. The Lucas Zener was not rated to fully sink and dissipate the full alternator load but with the coils permanently summed together with mag ignition (no coil) and no lights switched on and a fully charged battery, the poor old Zener will have the full output continuously flowing through it to ground (about 80W or 7A) and I don't think the device is rated for that sort of continuous power dissipation so will burn out. So best to leave the wiring the same or fit a modern electronic control unit but you'll have wiring changes to make too and it is more expensive ....Les

I had to go down a similar electrical conversion route with my 650 when it was first put back on the road 6 years ago. So my recommendation is a little different.

I also had a RM18 alternator but after checking with the Club expert decided to upgrade. His opinion being that although my RM18 was rated at 80W, I would be lucky to get 65W from it due to aging of the rotor and losses in the Lucas Rectifier, Zener Diode and general circuitry. Converting the RM18 to 12V was no problem but fitting modern a 55W front headlight coupled with a 5W/18W rear plus instrument bulbs would over-tax the system. Bearing in mind this was pre-LEDs taking over the world of lighting.

So I opted for an RM21 (p/n 47205) plus an electronic rectifier and voltage controller. The RM21 is a 2 wire alternator rated at 12V & 110W that connected straight into the controller which itself was mounted between the battery box and oil tank. There are just 4 wires to connect. 2 to the battery and 2 to the alternator.

The controller was cheaper than buying a new rectifier and Zener Diode by half was very easy to install and has worked perfectly since fitted. Checkout photo.

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RM21 is this good enough for decent lights?

Definitely!!! My Bike as follows.......

Front Halogen + Side + Rear + Brake + Instruments @ 12v =

55+5+5+18+4 = 87W

An RM21 will give 110W at over 4000rpm and enough at 3000rpm to stop the battery going flat. If I had electronic ignition then I would go for more alternator ooomph and fit a bigger RM23 Single Phase or the better RM24 3 Phase. Both give out up to 180W.

Peter - are you making sure you checked the timing on full advance? If it is at 20 deg static but in its normal rest state thenit might end up 50 deg or thereabouts after it has gone to advance whenthe engine speed goes up. You need to wedge the auto-advance open when checking timing. It ought not to pink all that easily.

I put white paint lines (typing correction fluid) on the rotor and stator so I can check with a strobe. More for curiosity than anything.

My 60 99 with orriginal alternator converted to 12v has happily powered my coil/Ei bike for 25 years the headlamp is 50/40 watt Halogen and it has indicators and 3 brake lights.If I leave the side lights on all night its still ok in the morning.

Electrically, a Zener is a resistor which cuts off the alternator output above a certain voltage, and converts it to heat. That is not what converts a 6V alternator system to 12V. The difference is a bridge rectifier, which enables the full wave to be used, instead of only one half..

I would recommend getting a rectifier / regulator which cuts off over-supply electronically instead of converting it to heat

Paul

Previously Peter Gibson wrote:

Hi

i have a 1958 dominator with a 6 volt Rm15 or 18 alternator which I want to change to 12 volt,the bikes ignition is by magneto my questions are

1,do I need single or three phase

2,two or three wire

3,do I need a zener diode

4,I'm going to fit a regulator/rectifier (any perticuler model)

any help with wiring would help

A Zener is not a resisitor, it is a form of diode. In this application it shorts current to earth above a certain voltage, usually around 14.4 V. You have two main choices. 1, Get a modern combined rectifier/regulator, OR 2, Get a modern full wave rectifier (around £6) and a Zender (£20 isg).

Either method shunts excess voltage/current to earth creating heat. A good modern system arguably does it with more finesse.

If you go route 1, make sure you get a branded quality item (e.. Al osborne?) as the cheap £20 ones are just trouble. If you go route 2, make sure you get the correct polarity according to whether your bike is Positive Earth or Negative Earth and ensure it has enough cooling. Remember the less current you use, the more work the Zener has to do.

Personally I use a Zener system as it either works or doesnt work, no messing around with black boxes. I use 23w pilot light to give the Zener an easy life when not using the headlamp.

Rob.