Rex’s Guide to Dynamo 12V Conversions 12 volt conversions are an attractive proposition. The benefits are better, more reliable lighting and a greater choice of battery and bulb options. The Lucas E3L dynamo delivers about 60 watts of power, either in 6 volts or 12 volts. The difference is that with 12 volts, more of that power actually arrives at the bulb. This is due to the fact that at higher voltages less power is lost in the natural resistance of the wiring and switches. It is possible to simply use a 12 volt voltage regulator to achieve a 12 volt conversion. However you must choose a regulator that will not over drive the armature windings, most of the cheaper options have no current monitoring and will allow the dynamo to run dangerously hot, until it burns out. The second problem is that the engine needs to turn faster before battery charging begins when a dynamo is regulated to 12 volts. On a BSA A10 the dynamo is spinning at +10% engine speed, it seems the designers thought about the effects of slow riding in town with the lights on and produced a well thought out system. The regulator that Rex’s supply (P-DREG or N-DREG) has a protective feature that ‘turns down’ the dynamo output if the current starts become excessive thus protecting your dynamo windings. Because of this you will get superior results when choosing to simply replace the regulator to achieve a 12 volt conversion. You should always ask if a regulator is the type that protects against over current, a fuse is not good enough as the damage will have been done before a fuse blows! Of course it is also possible to buy a kit that alters the drive speed of the dynamo which will resolve the problem of low output at slow engine speeds. Whilst the Lucas dynamo is robust the armature is not well balanced and the dynamo is subjected to considerable heat and vibration, spinning it faster is not in its best interests. While cruising the dynamo will be happy enough but you rev the engine hard the dynamo will be under considerable stress. The bearings are small and if using the original type that are open they will soon lose their grease. BSA designers made calculations to achieve a charging system which is effective whilst putting the lowest strain on the dynamo. That said we see more dynamos in for repair from BSA models and those fitted with kits to increase the dynamo’s running speed, the bearings are invariably worn out and the unit shows signs of working very hard. While these kits do allow you to cruise at lower speeds the dynamo certainly does not appreciate being driven at higher speeds, especially as it starts to wear and becomes tired. So what is the answer? While using Rex’s dynamo regulator will give good results, superior in fact than all other ‘regulator only’ conversions (and with the added benefit of protection for the dynamo’s windings) there is a better solution. For the enthusiast who wants to do the job properly the answer is to change the windings inside the dynamo. This avoids having to place extra stress on the unit by increasing the speed its driven at. With windings designed to for 12 volts the dynamo will start to charge at the same speed as they did in the 6 volt unit. This is what Lucas would have done had dynamos not been replaced by alternators. 12 volt field windings and armatures are available for many of the common 60W Lucas dynamos and while they do cost a little more than the common 6 volt versions, if incorporated with a full dynamo rebuild the extra expense is hardly noticeable. Best of all, if you already have one of our electronic dynamo regulators on a 6 volt unit, you don’t need to buy another. The very same part will regulate to 12 volts by simply cutting the green wire ‘loop’.

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