Ignition coils

[Dave21426]

I have read that all ignition coils are actually 6 volt coils. 12v cars just use a ballast resistor to step the voltage down. If they are all the same, why are some marked 12v and 6v, different part numbers, etc.

 

[TheBlacksmith]

I'm not sure that's exactly accurate. What I do know is that when cranking an engine, most 12 volt batteries drop down to about 10 volts, so the coils were designed to make full output voltage from 9-10 volts. Once the engine is running, the voltage rises up to 13 or more and to prevent overheating the coil, the ballast resister drops the volage. Note, the resistor is bypassed when the key is in the start position. A burned out resistor was infamous for a car that would fire up while cranking and immediately quit when the key was released.

I'm sure different cars, number of cylinders, etc. dictated differant part numbers. Most of today's cars have the ignition controlled by the ECM, so the dwell (charging) time can be adjusted as necessary for a hot spark without damaging the coil(s). Of course this level of interdependence probably increases the variations and amount of part numbers.

 

[dicer]

As long as the primary windings can take the extra current, it could be a 24 volt or more or less coil.

The original question answer. Because of the design of the primary windings, and the ampacity of them.

 

[patprimmer]

In my observation there are three traditional coil voltage ratings.

6 volt for cars with nominal 6 volt electrical systems. I think the VW Beetle was the last to use 6 volts and they changed to 12 in 1972 I think.

12 volt for cars with a nominal 12 volt electrical system. System voltage is delivered directly to the coil whenever the ignition is switched on

8 volts for cars with a 12 volt nominal electrical system. System voltage is delivered directly to the coil when the starter switch is engaged, however when the starter switch is released, power is provided to the coil via a resistor. This is the system outlined by TheBlacksmith and is quite common. It mainly helps with cold starts.

Regards
Pat
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[pontiacjack]

As noted, no, the coils are not all the same.
Most six-volt vehicles used ballast resistors.
To understand ballast resistor function/purpose you could refer to an old (forties or early fifties) Audel's manual. [Or, these days, maybe Wiki?]

 

[pontiacjack]

Hmm... Wiki authors need to consult a seventy-year-old Audel's book, since their brief description fails to state why it's labelled a BALLAST (!) resistor.

 

[Dave21426]

pontiacjack, you are backwards. No 6v coil has a ballast resistor and all coils are 6v, hence if you look at a 12v car like any 60's or 70's chrysler product you will notice a white resistor on the firewall. Its to reduce the running 12v voltage to 6v. Show me a picture or schematic of a 1940's 6v car with a ballast resistor.

 

[patprimmer]

All coils are not 6 volts.

Some run 12 volts with no resistor. Some run 6 volts with no resistor and some run 12 volts with a resistor. The ones that run 12 volts with a resistor are rated at 8 volts without a resistor. I never saw a 6 volt system with a resistor on the coil, but the only 6 volt system I have encountered was on old Beetles.

Regards
Pat
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[pontiacjack]

History lesson concluded for today; I won't do your homework for you...

 

[pontiacjack]

However, I'll offer a clue: the ballast resistor need not be a discreet component.

 

[Dave21426]

Here is why they are 6v:

http://www.weblube.com/12vOr6vCoil.html

(See attachment) for 6v electrical system wiring (ALL EARLY FORDS)

Pontiacjack show me some 'history' examples of a 6v system with a ballast resistor.

 

[dgallup]

Wow, that's the first 14 cylinder Ford I've ever heard of.

I've got a 6 volt '38 Ford tractor. At the moment it doesn't spark at all.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[Dave21426]

Put your glasses on read the schematic, but that is funny. Maybe it wont spark because someone put a ballast resistor on a 6V system.

 

[pontiacjack]

Under these forum rules I'm guilty of "leeching- benefitting from other contributors' posts while not contributing" since I joined. Therefore, I'll attempt to share my knowledge of this subject.
----
1) A ballast resistor (universal definition, not just automotive) has a high positive temperature coefficient; its resistance rises more steeply as a function of temperature than does the metal alloy of a 'normal' power resistor.
----
2) At some point (early 1930's?) in the advancement of automotive engines (with breaker point and coil ignitions) excess heating of coils became an issue at low engine speeds (the impedance of a coil's primary winding is predominantly inductive, so RMS current value falls off with increased engine speed).
----
3) See where this is going? 1) was applied to solve 2)!
----
Whew! This surely would be easier if I just had a copy of a 1940's-era Audel's Guide to photocopy and post!

----
Other pertinent facts:
----
Since a coil primary (in the manner of any electrical path) displays both resistive and reactive impedance, the problem was sometimes approached by manipulating these impedance components within the coil proper.
----
The move away from automotive ballast resistors gained momentum with the advent of transistorized ignitions, with their attendant reduction of turns-ratio of coils. The resulting lower inductance values of the new coils' primaries resulted in smaller primary current variation with engine speed.

 

[dicer]

So then Dicer had the best answer here, correct? LOL

 

[Fmangas]

I always thought the ballast resistor was in place to provide a resistive load on the points. The engine will run without the ballast resistor it will just burn the points real quick.

 

[patprimmer]

If the resistance in the windings is designed for a lower voltage supply, it might burn the points, although that is also influenced by the condenser and even so will take some considerable time. The first result will be an overheated coil that will leak oil or develop an internal short or open circuit.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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