could I have some help please??

Hi guys eventually I have manage to get my series 2a running, the promblem was the live wire feeding into the distributor points was stripped and was there fore shorting out. I let it run for a while the turned it off then as I started to fit the doors and bonnet then for some strange reason the ignition coil exploded is there any known reasons for doing this?

cheers guys

Scotty101

Is it a 6volt coil - some were?

Suicide? High voltage and water or lots of heat and water can make things go bang - although it shouldn't have happened with the ignition off. Worth a whizz round with a multimeter and a tidying of the wiring before fitting a new one.

Sounds silly I know but you have got the ignition switch on the coil power feed side not the coil/dizzy side ?
Only way a coil will blow like that is if it has power being supplied to it but it not doing anything
It will build up such a charge that they will split open and blow

pobby said:

Sounds silly I know but you have got the ignition switch on the coil power feed side not the coil/dizzy side ?
Only way a coil will blow like that is if it has power being supplied to it but it not doing anything
It will build up such a charge that they will split open and blow

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Nooooo! Basic physics - the coil can't "build up a charge" of any size. They don't work by "charge" at all - it's all to do with the conversion of current to magnetic field and back again. And anyway, the ignition was off when the coil exploded so neither terminal was connected to live.
My best guess would be water ingress which turned to steam as the coil heated up. Coils can run quite hot and would run even hotter if partially shorted by water. The pressure would continue to build when the ignition was switched off 'cos of the residual heat in the coil.

Pobs circumstance is correct, but the reason isnt so much 'build up'.
Whats happened is that the coil has had power to it continuously as mentioned.
The coil is sposed to be pulsed, a charge is built up in the coil, in the form of a magnetic field, and then dissapated as a spark when the points open (Lenzes back emf law if I recall correctly), if there is juice to the coil all the time the coil will saturate (once the magnetic field has reached maximum, which only takes a few thousandths of a second) and will draw excess power from the battery and get mad hot, most coils have liquid coolant in them, if you shake one you'll hear it, I suspect this coil got so hot with being powered up continuously that the liquid coolant expanded enough to burst the case.

dr pepper said:

Pobs circumstance is correct, but the reason isnt so much 'build up'.
Whats happened is that the coil has had power to it continuously as mentioned.
The coil is sposed to be pulsed, a charge is built up in the coil, in the form of a magnetic field, and then dissapated as a spark when the points open (Lenzes back emf law if I recall correctly), if there is juice to the coil all the time the coil will saturate (once the magnetic field has reached maximum, which only takes a few thousandths of a second) and will draw excess power from the battery and get mad hot, most coils have liquid coolant in them, if you shake one you'll hear it, I suspect this coil got so hot with being powered up continuously that the liquid coolant expanded enough to burst the case.

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If that were true you could never leave the ignition on with the engine stationary in case the points closed and powered the coil continuously. The current in the primary is limited by the resistance of the wire in the coil and should max out at a couple of amps - it will get hot but it won't be enough to burn the coil out or make it explode. The coils fitted to old landys aren't liquid filled anyway. Wiring the coil up back to front makes no difference to the primary windings but will stop the secondary operating as efficiently. More modern ignition systems use a slightly different principle and I'm not sure the coils would be as tolerant to straight DC - I'd have to try to blow one up and see!

Looks like we need an olde worlde ignition system primer.


The primary and secondary coils act as an autotransformer - changes in voltage applied to the primary causes current to flow which produces changes in the magnetic field in both the primary and secondary coils. This produces a higher induced voltage in the secondary coil, the ratio of the voltages equals the ratio of the turns in the coils. They only work with changing currents such as AC or switched DC.

Let's run through a cycle...

The points close, current grows exponentially (opposed by a back EMF) through the primary to earth via the coil and produces a magnetic field in both coils. This only produces a small induced voltage in the secondary coil.

Now the clever bit. Opening the points stops the DC current in the primary coil and the magnetic field collapses suddenly. This energy has to go somewhere and the primary coil sees a circuit back through the battery (which looks like a dead short to the primary) to earth and via the condensor to earth. The values of resistance and inductance of the coil and the capacitance of the condensor are carefully chosen to produce a resonant circuit. The energy bounces back and forth between the coil and the condensor, converting between magnetic field, electrostatic field and electricity. This generates a short burst of high frequency AC current in the primary which slowly decays away as the energy is turned into heat by he resistance of the coil. This AC current is then transformed by the auto transformer into a burst of high voltage AC in the secondary. The secondary coil sees a circuit to earth via the condensor (preventing the points from arcing across and burning out) and via the spark plug gap. The current jumps the spark plug gap producing an extremely hot plasma which ignites the fuel.

The points close and the cycle starts again.

Simples.

(I stole the drawing off a site on t'interweb - apologies if it's yours)

Thats all good, I'll just add, your comment of 'change in magnetic field', this marrys with my comment of charge build up, the magnetic field 'builds up' slowly due to inductance and is then converted back into an electrical charge (as well as other sources of energy you mentioned).
To expand on your first line of explanation, both windings have inductance, so the current in the primary doesnt increase exponentially, not to start with, it increases linearly due to the inductance, once the laminate iron core of the winding reaches saturation (about 1.5 Teslas for iron), the current slope then changes from linear to exponential, untill ohms law comes into effect and limits the current due to the resisitance of the winding.

The same radio frequency oscillator was used by marconi in the earliest days of radio, and its used nowadays in yer average tig welder as hf start, I made one from a scrap ignition coil.

Modern ignition systems use constant energy sytems, where the spark energy is measured by a microprocessor and the dwell time is adjusted so that the energy in the spark is constant with rpm, the system still operates as an rf oscillator as per the old system, the resistance of the coil is much lower, less than an ohm, so that small dwell times can be used to get a good spark at high revs.

The only thing I'm not sure of is that if you left a coil powered up all the time whether it would go pop or not, pobs was right though if a coil is left for ages eventually you'll knacker it even if it doesnt go boom.

Sorry to bore everyone else.

yep quickest way to knacker a coil is leave ignition on

jamesmartin said:

yep quickest way to knacker a coil is leave ignition on

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so when I ran out of fuel going down a hill, power brakes no longer worked and there was no shoulder on the road (nowhere to pull over) i stuck it in neutral and rolled, brakes felt useless lol and I left the ignition on so my indicators and brake lights worked.

when this happened I was knackering my coil?:doh: oh well couldn't be helped

Dont worry about it, it'll take ages probably an hour or so.