John Williams and 12 volts

[cardiffrob]

Typical. Just been given a couple of regulators CJF3 and CFL, both with 12VER written on them. Any use?

[john williams]

Rob, firstly you were correct in questioning my arithmatic regarding extra field resistance. 6 ohms is the standard field resistance on Lucas C40 dynamos.  Ideally on the conversion you need to add a resistor that will bring your modified total field resistance to 6 ohms. I now add the additional resistor between the third brush box and the earth brush box, instead of at the dynamo terminal block, so at first glance you cannot see the dynamo modification. This system will only work with a Lucas RB109 or equivilent. John

[cardiffrob]

Thanks for the info, John. I've still got the length of heater wire you gave me for the job and will stuff it in today. By the sounds of it I can do away with the little bridging wire and instead run the resistance wire between the 2 cables via some extensions so the hot wire lives outside the casing. The RB is already in place and wired up from several years ago.
I measured 1.9 ohms once I had subtracted the resistance in the AVO meter so will use 4.1 ohms of heater wire (about 6 inches or so)

On the home stretch!

[Duncan Grimmond]

As far as I know, my pal who did the sparks fitted a 12v regulator and used the standard 6v dynamo which was on the engine. He may have made other adjustments but I don't know what they may have been. The only way to get a quart out of a pint pot is to keep topping it up as you go...

[Zetomagneto]

Rob, firstly you were correct in questioning my arithmatic regarding extra field resistance. 6 ohms is the standard field resistance on Lucas C40 dynamos.  Ideally on the conversion you need to add a resistor that will bring your modified total field resistance to 6 ohms. I now add the additional resistor between the third brush box and the earth brush box, instead of at the dynamo terminal block, so at first glance you cannot see the dynamo modification. This system will only work with a Lucas RB109 or equivilent. John

Hi John, thanks for generously sharing information here. I have googled RB109 regulator, nothing comes up, plenty of 106.What is an equivalent of an RB109 please.
Also Could I use a wirewound resister of the type sold by CPC or Rapid. As well as being the correct ohms requirement I image the wattage would also need to be quite high, any suggestions?

Best Wishes Gene

[Bob Culver]

AUSTIN SEVEN 12v DYNAMO
The topic of 12v conversion without any rewind or resort to electronic regulator was done to death on the old site. To summarise all in one place again...
Many of the past descriptions omitted key points so, exactly as described, many were dubious and not sustainable (at least theoretically!)
The armature current is the normal limiting factor. Corresponding to about 8 to 10 amps charge max on the ammeter day operation is OK, sufficient to balance the original 18w lamps at night, but not higher wattage lamp substitutes. The total armature current is somewhat higher as also includes the field current and ignition current, not shown as a charge on the ammeter.
The field current at max 3rd brush setting is about 5 volts divided by the field resistance of about 1.5 ohms, about 3 amps. With no other changes the heating effect of this current increases as the square of voltage so, as operated as 2 wire, at full 12v the field heating is increased over 5 times! For a dynamo that runs very hot anyway this is pushing luck. (The extra field current also adds to armature load, so the ammeter output needs corresponding reduction)
Hence the ideal 12v conversion involves rewinding of the field.
With the field connected to the output and no load (ie not connected to battery or any lamps etc) the dynamo will produce much more than 6v, the basis of 12v operation. With the same field current, watts output (volts x amps) on 12v can be vaguely doubled. (As also is the load on timing gears!)
Some adopt 12v just for the convenience, maintain dynamo output at a low level, and rely on a home charger, but the car is then limited for night use whereas completely originally was not!
12v conversion does not necessarily require a voltage regulator. On 12v the field current needs to be limited somehow. Some achieve by simply extreme retarding the 3rd brush. This is the ideal as there is no power loss. Not foolproof as the field current is uncertain (unless measured by one method or another) and anyone can innocently alter. A resistor (as for summer charge) can be permanently connected as well/instead.
Electromechanical regulators need to be distinguished from simple cutouts, as original. Any Lucas pattern 12v e.m regulator (ie feeding voltage to the Field) should suffice. All have some current related regulating feature but Seven dynamo will barely produce enough to utilise.
Some have claimed that the buzzer switching action of electromechanical regulators rattles the timing gears. Nevertheless several have used electromechanical regulators, most without a 3rd brush and some apparently without any field current limitation device! Switching more than double normal field current, regulator contact life was short. Mysteriously the dynamo fields were claimed to survive although much of the time, as when lights on, would be fed constant 12v
Retention of 3rd brush as the means of moderating max field current would seem ideal but I dunno if the brush and commutator are affected by the switching action. (I suspect not as brush contact in never broken. The regulator contact would need to be bridged to set the peak charge rate. Has anyone applied an e.m regulator and retained 3rd brush?)
(incidentally, if timing gears do not rattle and there is no commutator problem I cannot see why an e.m regulator cannot be used with 6v.)
Particularly if operated 2 wire, some extra means of limiting field current is necessary, such as the resistance in previous post above. For the limited use associated Sevens a regulator can presumably handle more than the normal 2 amps. An added field resistance operating like the summer resistor, losing say 7v at 3 amps (about 2 1/2 ohms) is 21 watts wasted, rather a lot when the useable output is 12v x 8amps, 96 watts or less.
The dynamo output terminal D is apparent as wired direct to the live insulated brush. The wire from the Field terminal disappears into the field winding.
Digital meters do not behave near running Seven engines, although a resistance of a few hundred or more ohms and/or an ignition sized capacitor across the input may assist.