Dropping 12v dc to 6v dc

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

A dropper resistor is the simple and reliable way, you just need a reasonable idea of the coil's working amperage, divide six by the current in amps and select the nearest wirewound resistor.

For example, if it takes 0.5 amps, use 6/0.5 = 12 ohms. A 10 watt 12R resistor should be fine.

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

A trembler is basically a buzzer/shock coil, so not very choosy about voltage at all.

Once the engine has started, you can choose between Mag or coil so it needs 6v or it will burn out a very expensive switch!

A reasonably informative Wikipedia page : **LINK**

https://en.m.wikipedia.org/wiki/Trembler_coil

MichaelG.

It may not be that simple. The system is designed to be used with a battery. The battery has a low impedance. Changing the battery for a higher voltage one with a series resistor will likely change the operating frequency of the trembler. What effect that will have on the whole system I don't know.

In a vintage car it shouldn't be too difficult to find room for a small 6 V SLA battery.

Russell

A buck converter would certainly be the 21st Century way to do it. However, depending on its design it may not like feeding a trembler coil AT ALL! There may be nasty voltages getting back to the semiconductor devices that could blow them up. I read recently that certain stepper motor drivers are recommended to be used with ordinary transformer/rectifier power supplies rather than switched-mode supplies because of the inductive voltage spikes from the stepper coils.

I am now thinking of fitting a 6v battery and using a buck convertor just to charge the 6v battery from the 12v battery.

Beg pardon for assuming this was some kind of Dynastart system. To assume is of course to make an ass of u and me.

However I wondered what had led you to the 'high amps' part of the requirement. Is there something a bit odd about the trembler coil? I used to have an old model T coil and that was pretty low current (but fun). Just for scale 'high amps' to my mind is 20 amps or more but of course to some people 10 milliamps may seem a lot. Around 1 amp seems about right for the sort of trembler coil I think of.

I suppose a worry is over voltage on the trembler coil wrecking the high voltage insulation. Hence the 6 volt battery, OK but a bit of a nuisance. My only worry with a buck converter is the failure mode, does it go short circuit and hopefully blow a fuse in the 12 volt line or does it end up passing the 12 volts straight through.

Perhaps you might get rid of the battery and replace with a good sized capacitor - say 30,000 microfarads shunted by a 7 volt 5 watt zener diode, they tend to fail short circuit. Even quite respectable designers use zeners like this as a backstop against the failure of series regulators.

Anyway, good luck with the vintage motor.

If the series element or switch were to fail short, then the input voltage would appear as the output....Maybe a suitably rated zener connected across the output and a fuse on the input side may afford the protection...( or a crowbar circuit in place of the zener..)

Hallo vintageengineer,

the best way is to measure the current by aplying a 6 V Battery. Is the current about 2 Amps than use a simple bulb lamp with 6V 10 to 20 Watt und connect befor the trembler . The current flew then from the 12 V battery to the lamp and then to the trempler. Light bulb show you the curent flow. In this example you can use a 12 V 20 W lamp too.

You can measure the voltage drop on the lamp and at the trempler. No electronic no battery a very cheap system.

The only thing you have to do is measure the current of the trembler and choose a lamp.

Technical the lamp is a PTC resistor and have the possibility for some current regulation. And the lamp show you your systen is working.

Werner

Edited By Werner Schleidt on 27/05/2018 19:19:02

Edited By Werner Schleidt on 27/05/2018 19:22:28

It is a small world.

Currently I am researching the same subject for a friend. In regard to the resistance required I would suggest if you have a multimeter (preferably Digital ) you measure the resistance and using Ohm’s Law work out the resultant current. Again from Ohm’s law you would be then able to derive the resistance required and wattage of the ballast. My friends set up has 4Volt marked on the front switch plate, and 6V on the coil, perhaps parts from two units.

The from the following two sites you will be able to download information on Bosch ZU4, ZU6,ZF4 magnetos at www.oldcroak.com/zu4-zu6-zf4-zf6 and Bosch Dual Ignition For DU and ZR Magnetos at **LINK** For general magneto information try **LINK**

I have further info so will try to setup Public profile so that you can contact me directly.

Neil

I have used a similar arrangement to which Werner described many times. I charge my 6 volt batteries using my 12 volt charger with a 6 volt bulb in series,different wattage for more or less current.Simple and it works. John

The light bulb trick is a good one, though I can think of a few objections, none fatal:

• You need to measure the current taken by the coil to decide the size of the bulb.
• You need to find a bulb and holder that matches the current and voltage drop required.
• It's getting harder to find filament bulbs.
• A filament bulb may not be reliable in a hot vibrating engine bay.

Nimble's multimeter suggestion is a little too simple. The coil is an inductor, not a resistor, and a multimeter will only tell you its DC resistance, which will be much lower than the coils effective operating resistance.

In the absence of a ammeter a selection of light bulbs is quite a good way of guesstimating current; if a 6V 0.3A cycle lamp bulb glows dimly, normally, brightly or pops you know 'less than', 'approx equal', 'greater than', and 'much greater than'.

Dave

Dave , I must admit I have limited electrical knowledge, but your first point is that the current needs to be measured, by using my suggestion the initial currrent can be worked out with a value thar will protect the coil. from damage. Then if there needs to be any variation necessary maybe your method is the way to go. As you can see I am in contact with a user and his auto electrician and I have already asked him about the resistance of the coil. Another factor that may have to be considered is the capacitor OK.

Neil.

There seems to be some confusion between resistance and impedance. The impedance of an inductor is a complex variable. The real part is the resistance, and is equal to zero in an ideal inductor. Resistance is what is measured by a multimeter. The complex part, has a value dependent upon the inductance and frequency. The value is zero at DC, ie, zero frequency.

In the case of the trembler coil the impedance as such isn't important. Putting a voltage source across an inductor will result in a current that increases exponentially. In the limit the current will settle at the resistance of the inductor divided by the voltage. Unless an inductance meter is available the inductance will have to be measured by other means. Two ways are to look at the voltage risetime of a series R and L, or to use a frequency generator to determine the resonant frequency of an LC circuit, where the C is a known value.

On another matter, referring to the use of trembler coils on aircraft discussed by Clive. the Tiger Moths I've flown had impulse magnetos. So they were certainly in use in the 1930s. I have a vague recollection that early rotary engines, as opposed to radial, used trembler coils? Approach control in an aircraft with a rotary engine is aided by turning the ignition on and off with a switch, which may be easier with a trembler coil rather than a magneto.

Andrew

I'm not on firm ground with tremblers at all, but ignition coils are better documented. Have a read of this analysis - yes indeed capacity matters too. At the end he reveals the primary of his example coil has a DC resistance of 1.7Ω and that the AC resistance at 100Hz is 5 times larger at 9.78Ω

An ignition coil isn't meant to be fed DC continuously; it's a pulse device and works in that mode unless something breaks. One problem with protecting coils based on measuring only their DC resistance is that you're liable to cripple their operating performance : a DC measurement causes you to fit a higher resistance than is actually needed, and you weaken the spark.

An ignition coil sparks once per pulse, so the frequency varies proportionally with engine speed between say 8 and 500Hz. Not so a trembler. If I understand correctly (always in doubt!), tremblers buzz the coil to send a stream of sparks to the engine. Spark streams seem to be useful in the slow moving large capacity engines fed low quality fuel used in early cars. (Model T Ford had a 3 Litre engine!)

In the case of a trembler, I think the frequency of the buzzer matters. Even if I'm right, the opertating frequency is another mystery - I'd guess 100 to 200Hz because it's mechanical, but...

In practice I suspect that Vintage will get his engine to run however he puts approximately 6V across his trembler terminals. It's just that some ways of doing it are cheaper/more reliable/easier/interesting than the others!

Dave

Edited By SillyOldDuffer on 28/05/2018 13:55:31

Hallo vintageengineer,

the conclusion what Dave and I said is ,when you have nothing to measure you have to tried it.

Whit the lamp method you can not do so much wrong. Try at first from a car spare box of lamps with 12 volt

1. a 5 W Lamp ,if it burns bright the trembler does not work

2. a 10W lamp , this give you aprox 1 Amp the trembler could be work after a short time the lamp should getting darker and the trembler should work. If not you have to use step 3

3. a 20 W lamp, the trembler runs at the beginning with more Volts and high frequency if the lamp getting more temperatur it getting darker.

With this method for examble you can not normaly damage the trembler.

Werner

A trip to the car scrap yard will usually provide all the filament bulbs anyone could use. Why not simply connect an ammeter in series with the coil and then you will know just what current it requires. Never though such a simple thing would cover two pages. Where I once worked we had an arrangement using 110 and 240 volt bulbs and two copper rails/ busbars for battery charging 12 or 24 volt, from a 110 volt DC generator, using switches to increase the charging rate. Much the same as Werner described. John