Dropping 12v dc to 6v dc

[vintagengineerParticipant @vintagengineer]

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!

Posted by Neil Wyatt on 26/05/2018 20:51:35:

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

[Michael GilliganParticipant @michaelgilligan61133]

A reasonably informative Wikipedia page : **LINK**

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

MichaelG.

[Russell EberhardtParticipant @russelleberhardt48058]

Posted by Neil Wyatt on 26/05/2018 18:22:09:

A dropper resistor is the simple and reliable way,

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

[John HaineParticipant @johnhaine32865]

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.

[vintagengineerParticipant @vintagengineer]

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

Posted by John Haine on 27/05/2018 12:22:01:

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.

[Roger HartParticipant @rogerhart88496]

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.

[John RuddParticipant @johnrudd16576]

Posted by Roger Hart on 27/05/2018 16:37:25:

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

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..)

[Werner SchleidtParticipant @wernerschleidt45161]

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

[NimbleParticipant @nimble]

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 http://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

[john fletcher 1Participant @johnfletcher1]

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

[SillyOldDufferModerator @sillyoldduffer]

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

[NimbleParticipant @nimble]

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.

[Anonymous]

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Nimble on 28/05/2018 11:09:09:

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.

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

[Werner SchleidtParticipant @wernerschleidt45161]

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

[john fletcher 1Participant @johnfletcher1]

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

[Russell EberhardtParticipant @russelleberhardt48058]

There seems to be a lot of mis-understanding here. As Andrew has said, the coil is not simply resistive and you can't treat it as such.

A trembler coil is an early type of transformer. It has a low voltage primary winding and a high voltage secondary. Ohms Law simply does not apply. You need to use calculus to determine the current at any given moment in time but can't do so unless you know the inductance value, the resistance, and the applied voltage. Adding a series resistor (or lamp) effectively adds to the coil resistance.

The operation is roughly as follows; A voltage is applied to the primary and the current in the primary starts to rise from zero at a rate determined by the inductance. It then follows an exponential function involving the resistance as well. The current results in energy being stored in an increasing magnetic field. When the magnetic field reaches a set level a contact opens and the current can no longer flow in the primary. The energy in the magnetic field has to go somewhere and that somewhere is the secondary winding. The secondary voltage rises rapidly until the spark occurs and the energy is dumped in the spark. The magnetic field collapses and the contact closes again starting the cycle over.

The current measured by an ammeter will be roughly half the current at which the contact opens. If a resistor is used to reduce the measured current to that value the contact will never open and the system will not work. What is needed is a constant voltage to drive the trembler coil.

Russell

[Neil WyattModerator @neilwyatt]

Posted by Russell Eberhardt on 30/05/2018 09:36:32:

What is needed is a constant voltage to drive the trembler coil.

Russell

No, a resistor will work fine, though you may need to experiment to get the right value.

The danger to the coil is overheating and as the thermal time constant of the coil will be vastly higher than the rate at which the unit 'trembles' the fact the voltage fluctuates over the cycle will be irrelevant.

[Ian S CParticipant @iansc]

In the early days 6 volts for a trembler coil was often got from 4 Everready no 6 cells, sometimes known as telephone batteries, I'v seen a power supply made up to resemble a box of no 6 cells. There is a site some where that you can down load a copy of the wrapper for a no 6, I think you can just squeeze it onto a sheet of A4 paper.

Ian S C

[SillyOldDufferModerator @sillyoldduffer]

An interesting video here about testing Ford Model T Trembler Coils.

I picked up:

• A Model T coil should be adjusted to draw about 1.3A at 6V. (The amount of current is determined by the size of the gap on the primary contact, which means it is frequency dependent.)
• The Model T has 4 trembler coils, one for each cylinder, so a trembler box would need 5 or 6A. (This might be where Vintage's lots of amps comes from.)
• Judging by the tone of the buzz, I guess the trembler is running at about 200Hz. As I have a poor ear, could a musical member please confirm.
• Adjusting the trembler requires a test rig fitted with an ammeter

Someone said the coil should be run from a low-impedance current source. That's dead easy if you have a 6V accumulator. The problem with a resistor is Neil's "you may need to experiment to get the right value". I'm not comfortable with that because the 'right value' isn't a constant – it depends on the internal resistance of the battery, the size of the contact gap, the value of the condenser, and the AC resistance of the coil, which also depends on the tremble frequency, and engine rpm. No doubt these errors tend to balance out, but you now have an awkward problem if you wire the thing up and find the engine doesn't work properly. Perhaps the spark is too weak, or a double spark is firing the mixture early. It could be the trembler is out of adjustment, or that the resistor is throwing the system out over part of its working range.

John Fletcher said 'why don't you just measure it with an ammeter'. Very good point! I'm of an age where I assume decent ammeters to be expensive and hard to obtain. I expect to spend a month's salary plus on an Avo. Wrong again! For less than £10 you can have an accurate digital ammeter. Have a look at ebay. At those prices it's not worth messing with light bulbs.

If a resistor is used, then a lamp might be the best way of providing one. Although filaments are sensitive to vibration, in this application their resistance changes helpfully with temperature.

Dave

Edited By SillyOldDuffer on 30/05/2018 11:28:42

[Russell EberhardtParticipant @russelleberhardt48058]

Yes, a resistor will allow it to work but the value will need to be somewhat lower than that calculated from the measured current.

Not much danger of overheating the coil unless the contact is faulty and remains closed. In operation the current is a sawtooth waveform and the average current will be determined by the magnetic flux required to operate the contact not by the resistance. A fuse would be advisable to protect the coil in the event of a stuck contact.

Russell

[duncan webster 1Participant @duncanwebster1]

Posted by SillyOldDuffer on 30/05/2018 11:26:42:John Fletcher said 'why don't you just measure it with an ammeter'. Very good point! I'm of an age where I assume decent ammeters to be expensive and hard to obtain. I expect to spend a month's salary plus on an Avo. Wrong again! For less than £10 you can have an accurate digital ammeter. Have a look at ebay. At those prices it's not worth messing with light bulbs.

If a resistor is used, then a lamp might be the best way of providing one. Although filaments are sensitive to vibration, in this application their resistance changes helpfully with temperature.

Dave

Edited By SillyOldDuffer on 30/05/2018 11:28:42

A trembler coil will draw a very spiky current, it's not certain that a cheapo digital ammeter will accurately measure it. An old moving coil would be more suitable.

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