Re: Electrical question

Posted by John T (Ballast Basics) on July 25, 2006 at 15:59:01 from (66.244.90.5):

In Reply to: Electrical question posted by Guff on July 25, 2006 at 10:13:25:

Guff, Bob and Bob M and myself n others have covered this in great detail often before but I will lay it all out once again for future reference.

BALLAST RESISTOR BASICS

QUESTION: My question, if I replace with an automotive 12 volt coil do I get one with a built in resistor or without?

ANSWER: If you replace it with a coil AND DONT WANT TO USE ANY EXTERNAL BALLAST RESISTOR it must be a full true 12 volt rated unit. They are often labeled "12 Volts" or "12 Volts No Ballast Required" CAUTION DO NOT use one labeled "12 Volts for use with an external ballast resistor" cuz its in effect a 6 volt coil and will overheat UNLESS it has the added external ballast.
ANSWER: Most 12 volt coils DO NOT actually have a "built in" resistor. They are 12 volt coils cuz the resistance of their primary winding is in the 3 ohms range versus say 1.5 or so ohms range found in 6 volt coils. They have more primary winding resistance cuz they have more wire length and/or the wire is higher in resistance itself.
NOTE: 12 Volt coils are often referred to as "Internally Ballasted" for the reasons described above

QUESTION: Or is there resisitance in the wiring?

ANSWER: Its true some auto manufacturers used resistive wire links but thats NOT a common thing on tractors, if they required external ballast, they generally used the regular old ceramic/porcelain bathtub style wirewound units. So, the answer is theres likely NOT resistance in the wiring.

QUESTION: One guy says there has to be resistance or the points will burn up?

ANSWER: Its indeed true that ifffffff you use a 6 volt coil on a 12 volt tractor and dont use any external voltage dropping (12 to 6) current limiting ballast resistor, the points will burn up quickly PLUS the coil will overheat n be damaged also. Sooooo He's right, if you use a 6 volt coil on a 12 volt tractor YOU MUST ADD SERIES BALLAST

QUESTION

My local old tractor guy has a John Deer 12 volt replacement coil for $14 bucks, from the same time period. He thinks it will work fine?

ANSWER: Assuming its a true full 12 volt rated (internally ballasted) coil and labeled "12 volts" or "12 volts NOT for use with an external ballast" etc or "12 volts NO ballast required" YES IT WILL WORK

KETTERING COIL IGNITION THEORY:

A coil is more of a current device then a voltage device. When current passes through its primary winding energy is stored in the magnetic field and when that current is interrupted (points open) the field collapses and voltage gets induced over into the high voltage secondary winding. THE REASON FOR THE USE OF CURRENT LIMITING BALLAST when you use a 6 volt coil on a 12 volt tractor is to limit BOTH the coil and points current to around the 3 to 4 amp or so range OTHERWISE the points burn up quickly (thats the weak link) and if the coils current is greater then it was designed for (like if you apply 12 volts to a 6 volt coil) it will overheat and go bad. THEREFORE in order to limit coil and points current to say 4 amps if you use 12 volts and have a 6 volt coil YOU MUST ADD EXTERNAL BALLAST so the total resistance (coil plus ballast) is around 3 ohms 12/3 = 4 amps BUTTTTTTTTTTT IF THE total resistance of the 12 volt coil alone is in the 3 ohms range, then current is 12/3 = 4 amps again so all is well

THE DIFFERENCE in a 6 and 12 volt coil is the primary resistance, a 6 is around 1.5 ohms while a 12 (internally ballasted) is around 3 ohms, so the points current is in the 4 amps range so they dont burn up prematurely.

As far as stored energy n how hot the spark is, since the coil is more a current then any voltage device (Its an inductor and they store energy in the magnetic field produced when you pass current through a coil of wire) and since theres still around 4 amps passed through EITHER a 6 volt or a 12 volt coil with an external ballast, either set up can in theory develop approximately the same spark gap energy as the other.

NOTE: The points n condensor would be the same REGARDLESS if used on a 6 or 12 volt cuz their voltage withstand rating is wayyyyyyy higher then 6 or 12 volts anyway.

BOTTOM LINE: On a 12 volt tractor you must EITHER use a full true 12 volt rated (internally ballsted has around 3 ohms resistance) coil OR ELSE if you use a 6 volt coil (around 1.5 ohms resistance) YOU MUST ADD SERIES VOLTAGE DROPPING (12 TO 6) CURRENT LIMITING EXTERNAL BALLAST RESISTANCE (around 1.5 ohms) or else the coil overheats PLUS the points burn up quickly. The choice is yours and the spark gap energy wont be all that much different as explained above.

If you wish I can dig out an artice on Ballast Resistors I wrote for the Green Magazine and e mail it to you, let me know.

Hope this explains the whole Ballast Resistor thing so post back any questions or questions about the ballast by pass staryt systems on tractors that used it.

Best wishes n God Bless all here

John T retired Electrical Engineer

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