B.T.H Magneto-Generator Series 'A' Comet

[oexing]

So you got a mild power supply from that dynamo. I would not want to rely on unknown voltage for charging a battery so below you can find converters suitable for that dynamo at near to no costs.

Vic

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[Robert Watson]

There are lots of modern electronic bits that will convert DC from whatever to given voltages. I have mini USB plugs on my 6V Miller or Lucas to power the GPS or charge my phone. The trouble is they provide a constant power but no control to switch it off when the battery is overcharging like an old (or new) school regulator will do, although at 24 W it might take a long time and being aware, and no longer a teenager, Simon can self regulate.

By the way, I have one of those things on the shelf here, don't know if any of it works but came with a large lot of mostly NOS Vincent bits I bought a few years ago, but I can assure you it is not NOS!

 

[oexing]

The point is you want a converter that got a stable max voltage at 7 V resp. 14 V so at that voltage it will not overcharge any battery of 6 V resp. 12 V nominal - like on any car equipment. The old regulators from Lucas or Bosch will do same so I don´t see the problem here.

Vic

 

[Robert Watson]

I'll leave it to the electrickery people but I think Lucas and possibly Bosch control the output by controlling the field coil voltage, and with no field coils in this thing, only permanent magnets, as they said in Houston, We Have a Problem.

I understand that in North America it is more common to control the armature to regulate the output and hence wiring for a podtronics differs from wiring an old Lucas Voltage regulator. This is about where I find a little knowledge is a dangerous thing and I have to refer to my old Lucas handbook where it describes containing the smoke........

 

[Chris. S]

Hi all. By chance I restored a BTH, permanent magnet dynamo, during lockdown. On test, after a thorough overhaul, including commutator skim on the lathe, it would produce barely 9 volts, with no load; and could just light a 6 watt bulb reasonably brightly, at approx 2000 rpm, on the magneto shaft. The windings tested o.k, so I recharged the magnets on an old magnetiser.
It now produces 15 volts, no load; and can light an 18 watt bulb.
Whilst experimenting with cut outs and diodes, I found that the BTH cut out required over one tenth of an amp to energise it. A not insignificant part of the dynamos tiny output. Experimenting with diodes, proved that they mostly dropped the dynamo output by around half a volt. This would be the difference between charging and not charging at lower rpm. I eventually found a special diode, developed for use with solar panels, to be the best; with just a few millivolts drop. Texas Instruments SM74611, for anyone who wants to experiment.
Regarding automatic battery charge control. Because of the vey low maximum output current, a simple transistor, series regulator, should do the job; or go with Vic's solution. The modern boost/buck convertor will accept a wide range of input voltages; and provide a, user preset, maximum output voltage. Their efficiency is often over 90 per cent.
Cheers, Chris.

 

[bmetcalf]

The permanent magnet Lucas crank end alt makes AC which is rectified then controlled to 12 v nominal by a zener diode. Would a suitable zener work with a BTH?

 

[Chris. S]

It would certainly work, but, whenever the output from the dynamo was at the zener conduction voltage, all of the dynamo output current would be turned into heat. This would result eventually in a cooked dynamo

 

[oexing]

That old Zener was an ultra primitive way to turn excessive voltage into heat - of the diode. I don´t think it matters to the alternator if power is used on quartz bulbs or on heating the Zener. The alternator is working on its maximum all the time : When it is hooked to a 20 W bulb with 100 max output it will heat the Zener with 80 W . When you got a 100 W quartz it will put all into the bulb and the diode will stay cool. No different to the alternator.
In our case of BTH the Zener is not suitable as it is 14 V max for Nortons and all , and costs more than the clever Ali converters. Also, when the BTH in great condition can reach more than 7 V at high revs the converter can use that bit of extra power for your 7 V system as it does not turn any overvoltage of more than 7 V into waisted heat but converts it into 7 V for real use - unlike lots of motorbike regulators that earth one phase in overvoltage for heating up the stator.

Vic

 

[delboy]

Hi Chris.S,
A simple wiring diagram for "A" electrical duffers, with what you propose in post 75, would be most helpful to people.
Regards,
Delboy.

 

[Marvel]

Has anyone used a Wassell regulator with a Miller magdyno to replace the cut-out and 3rd brush? Is it any good? Is there a wiring diagram for it?