Google ballast resistor

A Step Down Buck Converter might do the job, unlike a Ballast Resistor they're efficient and the output voltage is stabilised. This example is 18W (3A) – beefier versions available.

I assumed 'Starting Coil' to mean 'Ignition Coil' in which case 5 Amps should be plenty for a 12v coil. I suppose a 6V coil might need proportionally more amps to get the same sized spark.

But if Vintage means 'Starting Motor' I guess 60A plus would be needed to turn a small car engine.

+1 on that, Dave – I think it's the simplest and best way to get the current required.

You might call it old school and simple but it's also not a brilliant idea. There's no means for the lower 3 cells to recharge unless you grossly overcharge (dry out / gas) the top 3 cells. It'll work for a short while, then you are likely to find you've buggered the battery.

If you want to do it properly, you'd want to drop the 12V down to 6V (actually 14V down to 7V) using a converter, perhaps like the ones mentioned above. You can use a 12V battery with a centre tap this way without causing a charge imbalance.

Murray

Would it be possible to fit 2 x 6 volt batteries wired in series to feed the 12 system and take the 6 volt ignition side off both the batteries but in parallel…..hmmm dont know..? that was straight off the top of my head.

Ron

The starting system described by Vintageengineer using a magneto and trembler coil was used for starting large capacity but slow speed petrol engines such as large aircraft radial types. The trembler coil is similar to an electric bell/buzzer with coil, contact and battery in series. One side of the coil is common to the ground of the magneto and the other to the "live" contact of the magneto. The engine is primed with fuel/air called sucking in by turning the propeller over several times and then one cylinder is brought just past full compression where the magneto contact will be open. The guy turning the prop will stand clear and say to the pilot "contact" whereby he connects the battery to the trembler system. As it buzzes the large voltage spikes produced by the trembler coil will get boosted by the magneto producing a stream of sparks at the plug and the engine will chuff into life.

The modern method for aircraft is to have an impulse mechanism on one magneto that flips the magneto as it gets to TDC to produce a good spark. The trembler system is in effect an inductive version of capacitor discharge ignition.

To use a 6V trembler coil system on a 12V battery may only need a larger dropper resistor as the trembler is only used for a short time.

Clive

Now the clarification has confirmed my original assumption, I suggest again a buck converter like the one I linked to on Amazon; they sell a 10A unit if 3A is considered too small.

Comments on other ways of doing it:

• To use a separate 6v battery you have to find somewhere in the car to put it, install wiring to the trembler, and also arrange for the non-standard battery to be kept charged. As the duty cycle is low, a leisure battery should be used rather than an ordinary vehicle type. I suggest the combination is complicated and costly.
• Tapping 6v off an existing 12v battery requires the connections between cells to be exposed and most modern batteries are sealed. But as Muzzer explains, the arrangement is also thoroughly bad practice because it's going to damage the battery! Get you home bodge yes, permanent solution no.
• Ballast resistors may not be straightforward. As we don't know how much current the trembler draws we can't calculate either the electrical resistance required, or how much heat it will have to dissipate. I suggested a 5A peak current requirement and NDIY's figure of about 1A continuous seems reasonable. The numbers help a little – a resistor between 10 and 30w, and 1 and 6 ohms. More worrying to me is how well a 6V trembler will cope with having 12v whacked into it! At switch-on, the full 12v is applied via the resistor to the trembler. That causes extra stress on the low voltage side and outputs an extra high voltage pulse that risks puncturing the coils insulation, perhaps burning the trembler out after a few months. Ignition coils are often deliberately rated to run with a ballast resistor. I wouldn't assume the same is true of a trembler system – I've no idea how electrically robust they are. Can anyone comment?

Buck converters come as simple 4 wire modules available inexpensively for automotive application. You can view them as being a clever resistor where you don't have to calculate anything or worry much about overheating. Quite simple to install – two wires connect to the 12V system and 6V pops out on the other pair. Worth a try I feel: even the 10A unit on Amazon costs less than a tenner.

Dave

Edit : my typist is extremely careless.

Edited By SillyOldDuffer on 26/05/2018 18:41:34