M201 electronic regulator project

Prototypes now under road test in my jeeps - see bottom of page

The idea of designing an electronic regulator for the M201 had been hanging around at the back of my mind for several years but only now having retired from teaching (a few years early I hasten to add) have I had the time to proceed. Originally it was to be simply a challenge out of interest but with the availability of replacement regulators dwindling and prices soaring ( 270 for a new unit in France) it seemed appropriate to proceed, the aim being to design a completely electronic regulator with no relay contacts which would mount inside a standard regulator case with no additional wiring and look, from the outside at least, totally original.

It has been at least twenty years since I last did any serious electronic design work and a search of the Internet produced only one design for a 12 volt regulator which came with the advice that adapting it for 24 volts and the type of dynamo (earthed field winding) found on an M201 would involve significantly redesigning the circuit so in the end the best thing seemed to be to start from scratch. The principles of electronic regulator design have been well known since the late 1960s and the regulator chip in a modern alternator, whilst highly advanced, still works on the same overall principle. Sadly it isn't practical to adapt an alternator unit as it's only function is voltage control. An alternator generates A.C. and is self-limiting in terms of current output by design due to the 'reactance' of its coils. On the other hand a dynamo is a D.C. generator with no self-limiting effects (other than burning out) and will simply produce ever more output under certain conditions damaging itself and / or the batteries unless controlled in some way.

Without going into technical details the design proved to be quite a challenge but I have produced a circuit that:

  • can be adjusted to regulate the voltage to between 28 - 29 volts
  • can be adjusted to regulate the current limit to between 18 and 23 amps.
  • draws next to no current from the batteries when the engine is not running (much less than 1 mA)
  • uses no coils or relays and includes fail-safe protection.

Having finalised and tested the design the next stage has been to build some prototypes into regulator cases for testing on my jeeps rather than on the test rig in my garage. The pre-production prototypes have been built on matrix board before finally moving to a printed circuit board construction method. The photo below shows two Paris Rhone regulators in the process of being converted. For testing purposes I am also building two prototypes into Marchal regulator cases.

I will keep you posted as the project develops further but for now a little known piece of electronic M201 history.

The idea of developing a more reliable electronic dynamo regulator also crossed the minds of the French army and a design was produced most probably by an ERGM Automobile establishment during the 1980s.

The defunct example opposite was sent to me by Will Gallier of William Galliers Sports Cars. It is what I would describe as one of a small batch of pre-production prototype units built for testing and evaluation in jeeps and other 24 volt vehicles. I have no idea how many were made but I have seen one other example still working on an M201. I have tracked down the jeep it was on and hope to have a photo of the complete unit soon as this one is missing the lid which had the mounting brackets fixed to it.

 When I get the time I will dismantle and repair it and at the same time extract the original army circuit for the M201 archive. It is actually not entirely electronic and uses a relay to excite the dynamo before connecting it to the batteries. Apart from this I can see it uses the same basic regulation principles as my design although the components used are rather old school now.

Does anyone out there have an M201 with one of these electronic regulators? If so please do let me know.


Testing under real conditions (rather than on the test bench) began at the beginning of May 2010 with me replacing the old mechanical regulators in each of my M201s.

Shown in the photo above with the cover removed for final adjustment. So far the performance of each has been perfect including hot days, cold days wet days, short runs, long runs and even a long run with headlights and windscreen wipers on. Testing will continue throughout the season to confirm reliability before getting printed circuit boards professionally made for the production model. I'll keep you posted.