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Dropping 12v dc to 6v dc

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vintagengineer25/05/2018 16:35:52
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468 forum posts
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I need to drop 12v dc to 6v dc on a vintage car. It's to run a Bosch Mag Starter coil ignition system. It will be quite high amps. Any ideas please?

Dave Halford25/05/2018 16:42:00
462 forum posts
4 photos

One of these maybe?

https://www.tayna.co.uk/car-batteries/classic-car/

Brian Rutherford25/05/2018 16:51:54
80 forum posts
3 photos

Google ballast resistor

Speedy Builder525/05/2018 17:49:40
1819 forum posts
128 photos

Dave H, I can't see a 12 to 6volt dropper on that site/link Am I missing something or did you mean buy 2 x 6volt batteries ?

BobH

SillyOldDuffer25/05/2018 18:05:31
4711 forum posts
1010 photos

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.

Grindstone Cowboy25/05/2018 18:27:36
124 forum posts
1 photos
Posted by Speedy Builder5 on 25/05/2018 17:49:40:

Dave H, I can't see a 12 to 6volt dropper on that site/link Am I missing something or did you mean buy 2 x 6volt batteries ?

BobH

Possibly Dave H is suggesting using one of their 12V batteries and tapping off the exposed connector to get 6V? But I'm not fully sure whether vintageengineer needs 12 AND 6 volts?

vintagengineer25/05/2018 18:33:32
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468 forum posts
6 photos

The car runs a 12v system but has a 6v Bosch Starting coil.So I need a way of dropping 12v to 6v but quite high amps.

Andrew Johnston25/05/2018 19:33:06
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4855 forum posts
544 photos

Before we can suggest a solution the problem needs to be properly specified. For a start how many amps? 10? 100? How stable does the 6V need to be, ie, what range can it vary over? A fraction of a volt or would a couple of volts be fine? Can the 6V unit withstand 12V if it isn't drawing any current, or momentarily, or will it go bang?

Andrew

SillyOldDuffer25/05/2018 19:58:57
4711 forum posts
1010 photos

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.

not done it yet25/05/2018 20:10:20
3358 forum posts
11 photos

12 volt ignition coils only take 5 amps max as a resistive component within a contact breaker system. As soon as the engine starts that will be reduced considerably - dwell angle and inductive nature of the coil will both reduce the current, the first during thewhole speed range and the second more so at high rpm.

The capacitive discharge system I made, back in the 70s could have used a 1A meter to virtually read rpm, IIRC.

vintagengineer25/05/2018 20:12:22
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468 forum posts
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No, it's a special Mag starter coil. It's a complicated ignition trembler coil and combined switch that starts the engine then you switch over to the mag to run the engine. The engine doesn't need a starter motor to start the engine.

Posted by SillyOldDuffer on 25/05/2018 19:58:57:

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.

Dave Halford25/05/2018 21:11:28
462 forum posts
4 photos

Sorry,

Posted by Rob Rimmer on 25/05/2018 18:27:36:

Posted by Speedy Builder5 on 25/05/2018 17:49:40:

Dave H, I can't see a 12 to 6volt dropper on that site/link Am I missing something or did you mean buy 2 x 6volt batteries ?

BobH

Possibly Dave H is suggesting using one of their 12V batteries and tapping off the exposed connector to get 6V? But I'm not fully sure whether vintageengineer needs 12 AND 6 volts?

Sorry, I was suggesting centre tapping the 12V battery, it's the simplest system, but possibly a bit 'old school'. For me Vintage and electronics don't mix

Grindstone Cowboy25/05/2018 21:19:23
124 forum posts
1 photos

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

Harry Wilkes25/05/2018 21:32:43
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719 forum posts
60 photos

Some years back to aid cold starting quite a few cars had 6v coils with a resistor in series but when starting the engine for a few seconds the resistor was bye passed this proved very useful in the really cold winter's

H

Edited By Harry Wilkes on 25/05/2018 21:33:07

Muzzer25/05/2018 22:05:15
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2904 forum posts
448 photos
Posted by Dave Halford on 25/05/2018 21:11:28:

Sorry,

Posted by Rob Rimmer on 25/05/2018 18:27:36:

Posted by Speedy Builder5 on 25/05/2018 17:49:40:

Dave H, I can't see a 12 to 6volt dropper on that site/link Am I missing something or did you mean buy 2 x 6volt batteries ?

BobH

Possibly Dave H is suggesting using one of their 12V batteries and tapping off the exposed connector to get 6V? But I'm not fully sure whether vintageengineer needs 12 AND 6 volts?

Sorry, I was suggesting centre tapping the 12V battery, it's the simplest system, but possibly a bit 'old school'. For me Vintage and electronics don't mix

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

J Hancock26/05/2018 12:18:08
314 forum posts

One wrong move and it's " Goodbye, 6V system".

Try to use a 6V battery to feed it and be worry free.

Ron Laden26/05/2018 14:09:47
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1362 forum posts
241 photos

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

Roger Hart26/05/2018 14:16:32
105 forum posts
22 photos

I would respectfully suggest this job is more difficult than it looks. To swing the 6 volt 'motor' requires high amperes of the order 200 to 300 and there will be some vile current waveforms and spikes as it turns. Really only a 6 volt battery can handle that and a fairly meaty one at that. Then comes the charging of the 6 volt battery from 12 volts. A simple dropper resistor is probably not so good here. Something is needed to regulate the 6 volt battery charge. Possibly the mag starter has the necessary to do that, maybe not.

vintagengineer26/05/2018 16:13:42
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468 forum posts
6 photos

There is no motor. It's a Bosch Dual Ignition Switch which uses a 6v trembler coil to start the engine. The engine is turned by hand to TDC then you switch on the ignition and the engine fires up.

Posted by Roger Hart on 26/05/2018 14:16:32:

I would respectfully suggest this job is more difficult than it looks. To swing the 6 volt 'motor' requires high amperes of the order 200 to 300 and there will be some vile current waveforms and spikes as it turns. Really only a 6 volt battery can handle that and a fairly meaty one at that. Then comes the charging of the 6 volt battery from 12 volts. A simple dropper resistor is probably not so good here. Something is needed to regulate the 6 volt battery charge. Possibly the mag starter has the necessary to do that, maybe not.

Russell Eberhardt26/05/2018 16:31:46
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2482 forum posts
85 photos

I presume that you have the system whereby the engine is started using a trembler coil and when running it is switched over to magneto. That type of system was often used with a non rechargeable dry battery as it was only used on the trembler coil for a short time. The battery was changed when it no longer produced enough power for the trembler coil. You could perhaps use a second small sealed lead acid 6 V battery and charge it when necessary. If you are not too worried about keeping things in period you could float charge it off the 12 V system with a small switching regulator.

Russell

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