The Ford Barn - View Single Post - 12volt conversion question ballast resistor

MikeK · 09-18-2013, 08:50 AM

There are advantages to running a coil with an external resistor. The white porcelain/ceramic external resistors are PTC, or Positive Temperature Coefficient resistors. When cold their resistance is lower and you get a hotter spark. In about 10 minutes of running they warm up and the resistance is higher, reducing coil average draw to about 3A. The average draw is a function of dwell.

Most 12V coils that are "internally ballasted" do NOT have a resistor inside. That would just generate useless, damaging heat. Instead, a true "ballast not needed" 12V coil has same number of turns in the primary winding, but a smaller wire (larger wire gauge #) to keep the amperage draw the same. The advantage of a PTC ballast resistor is lost.

If you do use an external resistor, keep in mind they get HOT and must be mounted with no wires touching them and in an airflow area. They will dissipate 30 -40 Watts of heat, as much as a small soldering pencil iron.

In anything electromagnetic, like transformers and motors, the magnetic field strength is a product of amperage x # of winding turns. There is no direct relationship to simple amperage, voltage, or wattage.

09-18-2013, 08:50 AM	#4
MikeK Senior Member Join Date: May 2010 Location: Windy City Posts: 2,919	Re: 12volt conversion question ballast resistor There are advantages to running a coil with an external resistor. The white porcelain/ceramic external resistors are PTC, or Positive Temperature Coefficient resistors. When cold their resistance is lower and you get a hotter spark. In about 10 minutes of running they warm up and the resistance is higher, reducing coil average draw to about 3A. The average draw is a function of dwell. Most 12V coils that are "internally ballasted" do NOT have a resistor inside. That would just generate useless, damaging heat. Instead, a true "ballast not needed" 12V coil has same number of turns in the primary winding, but a smaller wire (larger wire gauge #) to keep the amperage draw the same. The advantage of a PTC ballast resistor is lost. If you do use an external resistor, keep in mind they get HOT and must be mounted with no wires touching them and in an airflow area. They will dissipate 30 -40 Watts of heat, as much as a small soldering pencil iron. In anything electromagnetic, like transformers and motors, the magnetic field strength is a product of amperage x # of winding turns. There is no direct relationship to simple amperage, voltage, or wattage.