Re: It Could be Your Battery
 

If I must :rolleyes:

Everybody would say: It reduces the spark across the contact breakers so making them last longer. Which is true, but this was just a bi-product.

The real reason is as follows: The ignition coil only produces high voltage when the magnetic field collapses when the power is cut off to the coil. The faster the magnetic field collapses the higher the voltage that is produced from the coil.

When the power is shut off to the coil the capacitor releases its charge back through the coil causing the magnetic field to collapse faster as the polarity of the voltage has been reversed. Without the the capacitor you only get a spark of about 6,000 volts. With you get about 12,000 to 15,000 volts.

Modern electronic ignition systems have bossted this to between 30,000 to 40,000 volts these days.

Tim.

Re: It Could be Your Battery
 

This I agree with.

This I don't. Sorry, I think your electrical theory is flawed!

Here's why: The condensor is connected in parallel with the points. When the points are closed, there is no stored charge in the condensor (it's effectively short circuited by the points, therefore must be fully discharged).

And this is what I think happens: When the points begin to open, the condensor gives the inductive current (that the coil is beginning to generate) somewhere to flow other than striking an arc across the points. As the points continue to open, the condensor capacitor continues to charge itself up, allowing the current to flow for just enough time (before saturating) to allow the points to open far enough that an arc cannot be sparked.

Obviously this all happens v. quick in mechanical terms but it makes a difference electrically. Progressively drawing an arc across opening points is a relatively slow way to increase resistance, so it leads to a reduced rate of change (reduction) of current, thus a lower sparking voltage. By including the condensor, the effective resistance is rapidly increased as the condensor charges, leading to maximised rate of change of current in the low voltage circuit causing higher induced voltage in the high voltage coil.

So the aim of the condensor is to eliminate sparking of the points. I'd agree that reducing spark erosion on the points is a secondary reason but the primary reason why we don't want arcing across the points is that it reduces the electrical time constant. You are correct that a better spark is produced with a working condensor, and the points will also last much longer!

Re: It Could be Your Battery
 

In simple terms, all of the above. Plus any remaining charge that the cap has will assist in coil charge, effectively adding to dwell time - the system won't be back at zero potential as the cap will have some charge, (small, but every little helps), when the points close again.

Re: It Could be Your Battery
 

You said it yourself, my battery was £25.

Re: It Could be Your Battery
 

Have a look at this link

http://www.mgaguru.com/mgtech/ignition/ig108.htm

You will see the voltage at the points chart shows that the polarity does reverse as it goes to a negative voltage. This was the best example I could find, all the others only seem to bother with what happens with the secondary circuit.

I agree that this is to do with the charging of the condenser in effect we are both right.

Tim.