AC / DC ignition coil question

[Geordie]

I think I already know the answer to this but I was baffled by Ducati Elite wiring in 1968 and think I am only starting to understand it now I approach fitting the battery to this one.

'62 Mototrans Elite. The coil that was in place when I bought the bike as non-running.

AC coil.jpg

The loom that came with the bike is as

WireDiag.png

which means the coil input is connected to battery +ve via the key switch.

I have read that some Ducatis could be run without a battery, presumably fed by AC.

Is this an AC coil on a DC bike? If so, the bike never could have run with it connected as it is?

I would be grateful for any elucidation

Ian

[Jordan]

It looks to be a DC ignition system in the wiring diagram.
The coil has "Alterna" printed on it?

Who knows what has happened to the bike, several decades after it was made.
As it was not a runner when you acquired it, there is nothing reassuring about the current arrangement.

A new 6V DC coil could be tried?
https://www.classicbikeshop.co.uk/6-volt-coil.html

[Geordie]

Thanks Jordan, and for the link. The previous owner told me the bike had run, after he acquired it, by connection of a 6v battery and remote fuel tank. I had no reason to doubt him but the carnage I uncovered as work progressed didn't bode well.

My battery arrived today and I connected up to check out the wiring. As all was successful and the coil had not protested, I checked for a spark at the plug - and kicking over there was one!

That leads me to wonder what the difference is between a DC and an AC coil.

Anyroadup, when the bridge rectifier eventually arrives, I'll try my first engine start with this coil and the aim of correct operation of engine and electrics.

Always a tense moment after a full rebuild.

[Jordan]

I could be wrong, but an AC coil needs to saturate (build up magnetic charge) in a much smaller time frame than a DC coil.
That's because it relies on just one pulse of a sine wave, whereas a battery can provide current for much longer.
There may be 4 or 6 pulses per revolution from an alternator, and then it's not at a nice steady level anyway.
So the AC coil is made with thicker wires to quickly saturate before the points open.
It could be that an AC coil will work with a battery, but current would be very high, cause excessive battery drain and overheat the coil.
Corrections are welcome!

[Geordie]

Jordan, that sounds entirely plausible as unless there is some other component in the casing then primary and secondary windings are the logical difference. I have been looking for a SCR wiring diagram but not come across one yet - I am just curious to know if the feed to the coil is direct from AC or via a rectifier on bikes that run without a battery.

Am still waiting on delivery of bridge rectifier so to complete the loom and move to testing.
Ian.

[Ventodue]

Following on from what Jordan said, this is my understanding. Again, any corrections welcome!

AC and DC coils don’t operate differently. The difference is in their construction.

All coils are step-up transformers. They comprise two windings of wire around a common iron core.

You apply an electrical supply to one winding. This creates a magnetic field around the iron core. You remove the electrical supply, the magnetic field collapses and as it does, a voltage is induced in the second winding. What determines the amount of voltage induced in this second winding is the ratio of the numbers of turns of wire of each winding - the winding ratio.

For example, with a winding ratio of 50:1 between the primary (aka low tension) and the secondary (aka high tension) windings, 12V supplied to the primary winding is stepped-up to 600V on the secondary.

Now that 50:1 ratio can be had from having 10 turns on the primary winding and 500 on the secondary. Or from having 20 on the primary and 1000 on the secondary. And so on. Same winding ratio, same step-up voltage.

BUT the more turns, the more current the coil will draw.

Now:
In a DC system, the battery provides a relatively low voltage + relatively high power (amps). And it does so constantly.

In an AC system, the source coil provides a relatively high voltage + relatively low power. And it does so only as brief ‘peak voltage’.

So the windings of an AC coil must have a relatively low resistance. That means fewer turns (or thicker wires as Jordan points out). But this low resistance also means that, when volts from the source coil are applied to it, the coil builds the magnetic field quickly.

So:
Yes, you can use DC with an AC coil. BUT it being a low resistance coil, the high battery power might burn its windings out.

Conversely, if you try AC with a DC coil, the low power from the source coil may not allow the magnetic field to grow sufficiently so preventing the required step-up in voltage in the secondary winding that is needed to jump the gap in the spark plug.

[Ventodue]

<snip> I have read that some Ducatis could be run without a battery, presumably fed by AC.

True. But not, as I gather you know , the Barcelona Elite.

Snippet from the parts book. "CC" = corriente continua = DC.

Coil.jpg

[Jordan]

with a winding ratio of 50:1 between the primary (aka low tension) and the secondary (aka high tension) windings, 12V supplied to the primary winding is stepped-up to 600V on the secondary.

An interesting and useful phenomenon is that a points type ignition system actually makes hundreds of volts for an instant, when the points open. It's these bigger volts that then get subjected to the turns ratio to create the yet much higher voltage for the spark plug.
Ignition coils would be physically much bigger if that were not the case.

[Ventodue]

An interesting and useful phenomenon is that a points type ignition system actually makes hundreds of volts for an instant, when the points open. It's these bigger volts that then get subjected to the turns ratio to create the yet much higher voltage for the spark plug. Ignition coils would be physically much bigger if that were not the case.

Now that, Jordan, I did not know. Thanks.

[Geordie]

Thank you all for the time put into this subject. I will be in the UK soon and will obtain a DC coil as per Jordan's link as the price is good relative to here.

The bridge rectifier didn't turn up on the last day it was promised so I rode to the electronics shop and bought one (at twice the price).

Getting close to combining fuel, compression, timing and spark.

Anything could happen!

Ian.