Following on from the electronic ignition thread my brain has started ticking!

I currently run 12V with an electronic solid state regulator. At the moment all is very well, shows between 0 and half amp charge with headlights on when motoring, so should be OK. With lights off the unit "controls" charging to whatever is required depending on battery voltage.

BUT..........

As someone who likes old mechanical things I have my reservations about my conversion. If it should go wrong it is not really possible to fix the potted device by the roadside if at all! I could "bodge" the wiring a bit to get a charge into the battery to get me home, albeit a very high charge rate, and those in the know tell me that a diode would act as the cut out. Thinking about the various regulators that have been much discussed on this and other forums and also the standard 3rd brush set up I have had a thought

If I convert back to a simple cut out and third brush, could I rig up a mechanical linkage to the third brush adjustment to allow tweaking of the charge rate from the cockpit? This would allow more control over the rather coarse summer and winter settings as standard, but still be simple enough to fix or repair at the roadside? . I could back it off for normal use , and at night perhaps tweak it up to a sensible limit which could be gauged by the ammeter reading to prevent frying the dynamo?

Any thoughts?

Steve.

Hello Steve,

I personally think that you are on the right track.

But I am an advocate of the KISS principles.

I really don't think a "remote" mechanical means of varying the charge rate has much going for it unless you are a follower of Mr. Heath Robinson.

Quite apart from the mechanical implications that you would have to get around, I think you would run the risk of demanding more out of the poor long suffering dynamo than could reasonably be expected.

Far better, methinks, to set up your maximum charge,(theoretically 8amps according to Mr Austin and Mr Lucas for continuous duty) to what you consider to be your maximum charge point.

Then, instead of having just one value of field resistance (normally a nominal 3 ohms to provide half charge) you could arrange for a combination of resistors to give a range of other charge rates) These resistors, which would need to be of sufficience current carrying capacity to handle the field current could conceivably be mounted somewhere in handy reach, and if you had a sufficiently robust switch, could be "adjusted" at will.

I suppose that you could even try a variable rheostat if you could find maybe a 5 ohm one which was of sufficient current carrying capacity.

I have a modern car which has all sorts of inbuilt electronic gismos built in by the factory.

THE ONLY TROUBLE I EVER HAVE HAD WITH THE CAR ARE THE ELECTRONIC GISMOS -IMAGINE USING A COMPUTER CONTROL TO OPERATE THE REAR LAMPS!

Best regards

Barry R

Hi Barry

Thanks for advice.

The extra resistors sound like an idea that could be workable. This would probably be kinder on the battery and give me a slightly finer control than the normal set up. I am seriously thinking of going back to this sort of set up even though the electronics are fine at the moment.
I note your reference to Heath Robinson.... Perhaps I should send you a pic of the car!!! A home made mechanical linkage would not look out of place!! .

regards

Steve

p.s A friends car has the lights operated by the ECU?? WHY??? The dear old on off switch's days are numbered I fear!!!

I converted my Ruby to 12 volts some years ago and ran happily on a solid state cutout and regulator which gave the sort of reading that Steve quotes for around 3 years until it burnt out. Being reluctant to fork out another £50+ I replaced the third brush, and rather that purchase a 12v cutout, I settle for replacing it with 4 x P600 diodes - each rated for 5 amps - but if you go down this street, as I found out the hard way, you need to fit a battery isolation switch - good security anyway - as the diodes permit a small back leak and will slowly drain the battery over weeks of inactivity). This action also removed the old “Summer - Half” 1.5 ohm wire resistance.

With a conversion to 12v it is necessary to limit the voltage going to the field coils and a 1 ohm (25 watt wirewound)resistor needed to be inserted in the wire to the field coils ('F' terminal on the dynamo).

It is now generally accepted that the original 1.5 ohm wire resistance permits too much charge on its ‘Summer/Half’ setting. I considered that I didn't want to achieve more than around 1-2 amps max at cruise when ‘Summer/Half’ is selected, so fitted a 2 ohm (I used a 50watt wirewound) resistance between the ‘D’ and ‘F’ terminals.

Nevertheless, although the max charge I now had was around 9amps at 12 volts, this would not balance the discharge when I used the headlights (2 x 60/55watt bulbs). To remedy this, I mounted a simple flick, on/off switch that bypassed the 1 ohm resistor in the field circuit, so feeding the whole output of the dynamo to the field circuit. This did the trick and I have run for hours this way. Too much! I hear the shout - but think, the solid-state regulator balanced the whole output for 3+ years and then it was the regulator not the good old Lucas 35 dynamo that gave in!
This account and my concerns as well as an expert response to it, can be seen on the Forum of the Austin Seven Owners Club (London) website, under "Electrics" of course!

I have now had a little bit more time to have a further think about Steve's quest for "finer" control of the dynamo output.

Now the normal value of the electrical resistance of the field coils in a standard A7 dynamo is about 3 ohms, and the field field "control" resistor is nominally 3 ohms, and when this is switched into circuit by the Ignition/Charge switch in the half charge position, the field current is halved, thus reducing the charge current by half.

So, what might be the most elegant way of achieving this "finer" control would be to try and obtain a rheostat, let us say of 5 ohms maximum resistance, and wire it via a simple switch (of a construction suited to carry a current of two amps or preferably a bit more for safety) across the Dynamo D and F (SH) terminals.

Then when a dynamo output greater than half charge, but less than full charge was required, it should be a simple matter to adjust the rheostat for a suitable output. For example, let us assume a "standard" field control resistance of 3 ohms, with the resistance of the rheostat adjusted to 3 ohms, the nett value of field control resistance would be 1.5 ohms and a higher output than half charge, but less than full charge would be available. Similarly, at other settings of the rheostat a variable range of current output between half and full charge would be available.

What about charging currents of less than half charge I hear you ask!

Well that is a little bit more involved.

One method might be to change the original "standard" field control resistance for one of , say, 5 ohms.

That would mean that the so called "half charge" setting would be quite a bit lower than half charge, but, with the rheostat switched across it as above a wider range of adjustment, from full charge to a quite small charge should be available.

This method might also have some appeal to people like Brian, as it could probably be arranged to obviate the need for the "bypass" switch for the extra series resitor in the field circuit and give a relatively smooth adjustment over quite a wide range to meet individual requirements.

The other method would be to arrange some further switching to insert the rheostat in series with the existing field resistor, but in my opinion that is getting a little messy.

I think that "Radio Spares" (RS Parts I think) or Maplins should be able to provide a sutable rheostat, but you might just get lucky with a unit from a dimmer switch of the type which used to be used to control the interior lights on some posher cars from the 1960's forward.

An yes, I have no doubt you could come up with a solid state way of doing it.


Whatever you do, make certain that the components you use are man enough for the task.

I would be interested in hearing from anyone who actually tries such a modification.

Regards,

Barry R.

Thanks everyone!

I will definitely try something on the lines of what has been suggested, and report back

May I just remind everyone of the original reason....

I have NO CHARGING problems at the moment, but have concerns about the possibility of a breakdown involving the sealed/potted electronic control unit. To carry a spare solid state unit is expensive and in the event of failure I dont realy want to spend another £50 on one!
So I want to change back to a more original..ish set up both to aid roadside repair and cut future costs. However because I am now used to the charge current being welll controlled by the electronics I will be trying a set up like that suggested just to allow a bit more control. I DONT NEED OR WANT TO GET MORE CURRENT FROM THE DYNAMO!!! With my 36W headlights I can balance the demands just nicely without needing to overstretch the dynamo! The need for control is more to do with not cooking the battery than anything else.

Back to the point...the final set up must be reasonably cheap and fixable within reason by the roadside with some cheap to carry spare parts or else it just defeats the whole object of the exercise!!!


I will report back with results

Thanks

Steve

Hi Barry and Steve,

The nearest to your specification Dial Rheostat listed by RS Components is as follows:
Power @ 25ºC (W) 25
Resistance 4·7 Ω
Operating Voltage (V) 10·8
Wiper Current Max. (A) 2·3
Part No. 235-6923
Price £22·85p + VAT

The mounting surface must be earthed
A knob and dial would also be required for the unit

The units are said to be designed for heavy duty applications such as repeated over-loads, transients and conditions where shock and vibrations occur - they're made from vitreous enamel.

Jeff

Thank you Jeff.

I'm scribbling down some diagrams at the moment, just getting my head around the issue! Might be experimenting next week

Steve

p.s Another point in favour of going back to 3 brush dynamo is that if I have a dynamo failure I can "borrow" a spare from a travelling companion! (my dynamo is converted to 2 brush).

Been looking at diodes on the Maplins site after reading the stuff on Austin Seven owners club forum (London). The P600 style diodes that have been quoted as workable seem to be very cheap? i.e cheap enought to carry spares, and a lot cheaper than the purchase cost of the cut out I need? Schottky diodes are also mentioned and these seem to be available in quite high current carrying spec, and again seem to be inexpensive devices?

I know there are two schools of thought with respect to diodes versus cut out, but given the fact that I have no cut out at the moment and diodes are cheap, plus its a special so were not talking original spec here! I also should mention that I run a different ignition switch from standard and a stand alone ammemter with excellent resolution reads -12 to +12 A. My terminals and connections will differ from a normal car but if it works the principle will be the same.

One thought ref the field fuse? The solid state unit is protected via a 7.5A fuse. If I wire a set up as we are discussing i.e 3 brush with rheostat control I will need to fit a field fuse. What value ought I be using (12V).

Regards

Steve.

Can I just say.... I think it is correct to be looking for a quick and cheap way for 'roadside' repairs.

In the past one of the main principles of A7 ownership was the ease with which repairs (especially temporary 'get-you-home' repairs) could be effected, also the ready availability of spares - not much chance these days from your AA/RAC man - but if you are two or more A7's are gathered together, then the interchangeability (that's a long word after a busy day in 'The Legion') of parts is always usefull, especially for us lads and lass's in Cornwall who travel that bit more than the average.

I like this Forum, some excellent information, I just hope some of it reaches my cars.

Sandy .....

Hello Steve,

On the subject of diodes, I do think that Schottky diodes are the way to go. They have the advantage of very low forward resistance, and therefore a lower voltage drop, and therfore less heat dissipation.

If you do opt to use any sort of diodes though, you will need to have a battery isolation switch as they will pass minute current in the reverse direction. This may not be a bother when the engine is running, but when the car is out of use, can eventually discharge the battery through the dynamo, and can also apparently depolarise the dynamo.

You probably should have a field fuse.If you are using a 12v mode, and haven't had the field coils rewound then the maximum current condition in the field would be the result of 12 volts across 3 ohms. That is 4 amps. So a fuse of around 7.5 amps or thereabouts should do the trick.

The 4 amp figure, which represents a 100% overload on what Mr. Lucas intended, is not really likely to actually occur in operation, as the voltage applied to the field is "picked off" by the third brush at something less than the full 12 volts, so the overload when operating a 12 volt system would not be quite that severe.

If using diode/s you will need to provide good heat sinking, and you will also need to be careful about making certain that you don't fit the battery in reverse polarity, otherwise you may well require your spare diodes.

Regards,

Barry R.

The diodes I referred to I got from Maplin - but don't forget the battery isolation switch to stop battery drain.

The P600 diodes I have used for the last 18 months or so were, as they say, "cheap as chips". As each is rated at 5 amps, I used 4 in parallel and have no trouble whatsoever.

Hi Steve and Barry,

Looking at the RS catalogue, they list the following P600 diodes (Standard Rectifiers) made by General Semiconductor amongst a huge number of other diodes.
They also list an equally large number of Schottky Barrier Rectifiers under the same specification descriptions but including VF max(V)@
The Schottky prices range from a few pence each to £50·00p each

P600B
IF(A) 6·00
VRRM(V) 100·00
IFSM(A) 400·00

P600D
IF(A) 6·00
VRRM(V) 200·00
IFSM(A) 400·00

P600M
IF(A) 6·00
VRRM(V) 1000·00
IFSM(A) 400·00

Jeff

P.S. I did set out the above spec descriptions in sub and superscript as they should be, but the site won't accept them in that format and has turned them into single height same size caps.

I too recommend a battery isolation switch. The type with a red removable key are popular. I think the original design was by Hella, but there are a number of look-alikes around too. I found that one of these cheap ones caused serious voltage drop problems with the high current of a 6V starter. The cure was to use a meatier design designed for big boat diesels and obtained from a chandler. I mounted this in the base of the spare side of the battery box, with the key inside the car near the passengers feet. This saves opening the bonnet to operate the switch