Re: ballast resistor for 12v conversion

Posted by Tom Zander on July 04, 2001 at 18:08:48 from (152.163.207.84):

In Reply to: ballast resistor for 12v conversion posted by dan on July 04, 2001 at 09:25:23:

The voltage drop you are looking for will not occur unless there is current flow. The ignition points must be closed with the key in the on position to get a voltage drop. In a 12 volt negative ground system the feed from the key is connected to the plus terminal (positve). The minus terminal is connected to the distributor lead that is connected to the ignition points. These connections are reversed on positive grounded systems. The two small terminals are the primary winding of the coil. The ignition pionts connect the primary to ground causing a magnetic field to build up around the coil when current is flowing. When the points open the magnetic feild colapses around the secondary winding which is also in the coil. The secondary winding has current induced into it by the primary windings colapsing magnetic feild (The coil is actualy a step-up transformer). The large wire going from the coil to the distributor cap is connected to the secondary winding and provides a path for the high voltage induced into it to fire the spark plugs.
Ignition coils come in two types, internal and external resistors. Internal types do not use a ballast resistor since they have a resistor built in to the coil. Twelve volt internal coils will measure 3 to 5 ohms acrossed the plus and minus terminals. The external type will measure about 1.5 ohms and require a ballast resistor be used in series with the primary. Using the wrong coil and/or resistor combination will result in burned points, missfire, and plug fouling. Connecting the primary circuit improperly (polarity) will also cause problems.

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