12 VOLT ELECTRICS

FOR YOUR AUSTIN 7

(Feb 95)

Those of you with long memories may recall that I wrote an article on this subject several years ago. When I was restoring my latest Austin 71 assumed it would be po~gble to have 12 volt electrics and went straight for it by fitting a 12 volt coil, 12 volt bulbs throughout and found that a lawnmower battery would go nicely into the standard battery box under the seat. I was then committed and it had to work.

The other electrical goodies, namely the starter and horn, are quite happy working on 12 volts. The engine turns over briskly and the horn would wake the dead. The secret is in being able to charge the battery.

Like George Jolley I have spent many years employed for my "skills’ with things electrical but with a different emphasis. I worked in a research establishment where the idea was to push forward the frontiers of science. It was second nature for me to investigate why the books tell you never to run an A7 dynamo with the battery disconnected. The answer is that it reaches almost 20 volts which would make an awful mess of 6 volt bulbs and probably the coil. I also investigated how much current could be obtained and discovered that with the third brush set for maximum output a 12 volt battery could be charged at 19 amps (a remarkable figure which just happens to be the rating of post war 12 volt dynamos). George has been telling us in his admirable articles about dynamos that it is not wise to take too much current unless you are good at repairs. I felt that 19 amps from a dynamo officially rated at 9 amps was asking for trouble and came to the same conclusion as George that the way to set the third brush was to ensure there was a small charge with all the lights on.

It is possible (the books tell you it wont work) to use a 12 volt combined cut-out and mechanical voltage regulator with plenty of spark suppression across the regulator contacts but the points need fairly frequent attention with a file. It would be unwise to set about modifying your car without having access to a decent voltmeter or if you are exceptionally clueless about electricity. The simplest conversion is merely to ensure that the cut-out operates at 12 volts, either by obtaining a 12v replacement or by measuring the coil resistance of the 6v unit and adding a resistor of equal value. My cut-out has a 30 ohm coil and an additional 30 ohm resistor did the trick. The third brush in the dynamo can be adjusted to ensure that there is a small charge with headlights on and the engine running at reasonably high revs. Final adjustment is best achieved with the help of a road run because the current reduces when the dynamo achieves its working temperature, and it can be ensured that you don’t have to drive too fast to stop the battery discharging. With my upgraded headlamps this setting corresponded to an initial charge rate of 14A with the ignition switch set for high charge and lights off. George Jolley nearly had apoplexy when I told him this but my dynamo hasn’t melted! The low charge current will probably now need attention and there is nothing for it but to attack the field resistor found under the terminal cover on the dynamo. This resistor should be increased to reduce the current and vice versa.

I wasn’t content with the original uncontrolled charging system and soon tired of filing the contacts in my mechanical voltage regulator. The reason for this premature contact failure appears to be a combination of the high field current required to get 12v from a 6v dynamo (twice the design figure) and the peculiarities of the 3-brush system which doesn’t like having the field current switched on and off rapidly. "Modern" voltage regulators are electronic and give a controlled field current which does not pulse on and off. Regulators of the 4TR type used with early alternators can be found at autojumbles and are reasonably cheap. I was determined to make one of these work with my dynamo.

Alternators with separate regulators normally have the field connected internally to the +1 2v point and the regulator completes the field circuit to earth. This is exactly the opposite to the 3-brush dynamo where any control has to be between 12v and the field coils. At first glance it appears that you have wasted your money on the regulator. There are, however, two possible solutions, the simple one and the one I have used. If the car is wired for positive earth everything is "upside down" in the circuit diagram and the regulator will work. Connect the regulator terminals as marked, - to the dynamo output, + to earth, and F to the field terminal. You can, if you like, leave the high charge connection on the lighting/ignition switch connected to the dynamo field, at least until you are sure the voltage regulator is going to survive. The field resistor on the dynamo should be removed.

My car is negative earth which presents further problems. It is easy to reverse the polarity and perhaps that is what I should have done if I wasn’t so stubborn. I dismantled the voltage regulator and reversed the polarity of all the electronic components, replacing the transistors with suitable complimentary types. I then had to file the + and - marks off the case of the regulator and wondered why I had been so daft.

The result is that I can happily drive my car at night with decent lights and no fear of flattening the battery. I also get a high charge rate when first setting off without lights but this drops to a trickle charge which is much kinder to the battery.

Anyone interested in modifying their car is welcome to phone or write for advice.

John Ashbridge, 01946-67640