Ignition Coil Question

To the other poster ( richard? ).. your capacitor / condensor is going to be fine going from 6v to 12v.. the breakdown point on those ignition caps is probably something like 120v or so.

Soundguy

In the electrical world, its known as the contact "area rule" of resistance. Bigger the contact, less the voltage drop ...or... the greater the amps capacity. Ohms Law STILL RULES.

You need a refresher course in "transformer theory". Learn about turns ratio, ironcore magnetic fields, and why transformers don"t work on DIRECT CURRENT but ignition coil transformers do..... ...respectfully, Dell

Heres the deal, ElectroMagnetic Theory 101: ALL coils are made from wire. ALL wire have a natural resistance depending upon its length and its diameter. Long wire, more resistance. Skinny wire, more resistance. Simple, eh? ...but wait... ignition coils are electromagnetic devices. What make an electromagnet? electrical current (amps) thru a wound coil of wire. More turns, stronger magnet. More turns needs longer wire ...oops!... longer wire, more resistance, more resistance less current, less current, weaker magnet; see where we're going with this problem? ...but wait... bigger wire has less resistance, less resistance more amps, more amps stronger magnet; see where we're going with this?

But what does this really mean? Resistors don't make magnetic fields needed to make ignition coils work. All resistors do is REDUCE the current (amps). Reduced current, weaker magnetic field. Weaker magnetic field weaker sparkies. CONCLUSION: extra internal resistor is BAD. This is TRUE for any electrical device. All a resistor does is waste power in the form of extra heat. Law of thermodynamics still rules.

Now then there are "12 volt coils" that require external resistance ...BUT... they are NOT really 12 volt coils, they really are 8-9 volt coils that have an external resistor BY-PASSED during starting thereby giving the 8 volt coil a 12 volt jolt for hotter sparkies when starting. Its a crutch for weaker smaller 12 volt batterys that the starter motor drags down. Thats a battery theory thing.

You could probably use your 74 points style coil in a 12 volt tractor conversion. ...BUT NO-WAY... could you use your 79 transistorized coil in a points triggered application.

You can't shadetree test a sparkcoil except on a running engine. ...but... you can always take your ignition coil into a real automotive tune-up shop and pay $25 to have them check your $15 coil on their testing machine. Your call..... ....Dell, the self-appointed sparkie-meister

The resistor came about in the early 6 to 12v conversion days when coils were wound with enough resistance (small wire) to provide ~4-5 amperes of point contact current with 6 volts across them.

With the advent of 12 volts, somebody somewhere decided that they would use the same coil design, just add an external resistor to drop the extra 6 volts. Well 6 volts and 4 amperes = a 1.5 ohm coil primary and a 1.5 ohm resistor.

This is not all bad as the 6 volt coil was already designed into things, people had them, the thermal profile was known so reliability of a new design was not a problem and all the good stuff.

Then all they had to do is find a remote place to locate the resistor.

Some coils are marked 12v. If you measure them with an ohm meter, you should measure 3 ohms in keeping with the 4-5 ampere point current. (Just this past weekend was brousing thru my MF35 Shop manual and the numbers I am speaking where there.)

A coil can be tested with a sawtooth waveform (preferably; slow rise and abrupt fall) but it takes a generator and an oscilloscope to view the results. Operational on an engine is best.

The problem with mounting the resistor inside the coil, or reducing the primary wire area to get twice the resistance is reliability. Instead of 6 watts dissipated in the coil (plus other losse) you now have 12 watts. Coil gets hotter. Heat kills sparkeys so reliability suffers.

Only difference between point and elect ign would be voltage. Most elect ign's use higher voltage (like 40kv) whereas most older point units put out more like 18k..... as I recall.

HTH

Mark