12v conversion for BSA Bantam D1 with direct lighting system.

Click on drawing for larger image

Owners of British Lucas alternator-equipped machines know that there is a simple upgrade to 12 volts by connecting a pair of output cables together and connecting in a Zener diode to regulate the output voltage. But what about owners of more down-market British and most Italian lightweights? When I finished restoring my '58 D1 Bantam I had enough bits left over to build another machine. The second project was the more common 'direct' lighting model and the lights were definitely in the 'glow worm in a beer bottle' category.

Now, the theorists will tell you that the output voltage of an alternator, for that is what these Wipac flywheel 'generators' really are, will rise with speed and is balanced by the equipment it is powering, up to some threshold. Owners of Bantams, Honda 50's and the like will know that this provides lights which dim and brighten with engine revs. Very handy when you're waiting at that unlit junction. The flip side is that bulbs blow just as you're enjoying one of your rare blasts down your favourite dark lane. Designers of the 50's and 60's played all sorts of tunes with coil switching just to avoid a proper regulation system and the mass of wires sprouting from your headlamp switch is the result of all this complicated 'balancing'.

The success of a 12v conversion relies on two things. If you can get the alternator to provide twice the voltage at any given output current, you have twice the output power. Neat eh! The next piece of magic is the zener diode. This is a clever but very crude piece of kit that will dump current straight to earth if its input voltage rises above a certain level. This is where the Lucas 'connect all the output wires together and throw away the current you don't want' strategy comes from. The alternator gives maximum oomph at all times and the battery is saved from being boiled dry by the helpful zener. The zener obliges by turning all the unwanted electricity into heat, necessitating the ubiquitous heat sink mounted under the bottom fork yoke of countless BSA's and Triumphs.

If the Bantam's puny 6v Wipac S55 Mk8 alternator could be encouraged to provide 12 volts, its meagre 30 watts would grow to a whopping 60 or so. My first experiment consisted of simply replacing the 6v 30/24w headlamp bulb with a 12v 40/50 bulb which I had to hand. Surprisingly, the generator could maintain 12 volts across this and gave a brilliant light. Suitably encouraged, I set about finding the other bits that were necessary to bring about this revolution in Bantam electrics. With a 50 watt headlamp bulb there was still 10 watts or so to play with to drive a tail lamp. A battery would provide for the occasional use of horn or stoplight.

I had a Lucas Zener diode lurking in my spares box, so this was bolted to a piece of aluminium plate under the fuel tank where it could dissipate its heat unnoticed, but still receive a draught through the front forks. The same plate was home to a solid state rectifier and a small electric horn. There is already a redundant coil bracket under the Bantam tank which can be used for mounting a large heat-sink. With care the whole installation can be totally invisible.

In order to convert the two coil Wipac a.c. 'generator' to work in a d.c. system, it is necessary to remove the earthing screw from the generator backplate underneath terminal '4' so that the a.c. system is isolated from the Bantam's chassis earth. Once this is done, a second wire can be connected to terminal 4 to carry the alternating current up to the rectifier. The d.c. output is then regulated to 12v by the zener and can be hooked up to the battery and other ancillaries.

My revised wiring diagram includes all the necessary connections for a working system and is simpler than either a direct lighting system or the standard 3-coil battery lighting system originally used. This simplification is due, largely, to the Zener diode which provides voltage control without the need for clever coil switching in the generator.

The Bantam flywheel mag has its own ignition circuit, so the battery can be installed or removed as required with no ill effects. The only problem is finding one which can be secreted away. My Bantams use a small sealed Yuasa battery hidden in the toolbox. With a 12v conversion you will have a Bantam that can sail through an MoT test and can see and be seen in the dark! I now have two 12v Bantams running. My 150 Gilera, with a Marelli system that makes the Wipac generator look beefy, has also been given the 12v treatment, but needed rather more surgery in the flywheel generator. But that's another story.................

Bulbs, Wire & Connectors
Merv Plastics, Nottingham - 01159 9305454

Zener diode
Local Brit bike shop - Mine came from Wilemans in Derby (about £15).

Rectifier and Battery
Maplin Electonics 01702 554161 -
Order code BH48C at £3.95 is a 25 amp bridge rectifier with standard spade connectors

Further Reading on Bike Electrics

Restoring Motorcycle Electrics by Roy H. Bacon

 

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