There used to be a guy here that got into making parts like the rotor for the Harmon Collins dual coil set up. Dzusfast is his handle but he hasn't posted in a long time. Ballast resistors for 1.5 Ohm coils are generally 1 to 1.5 Ohms. A person can use 3.0-Ohm coils without a ballast on 12-volt systems.