Tim4 ignition

[dsage]

BTW. I don't have any circuit boards left for my IGBT driver. But I gave the circuit board files to the Editor of Model Engine Builder Magazine. He was going to look into getting some made. I know he has built many of them for the Bay Area Engine Modelers Club (in California). He might have some to sell. They're pretty popular out there in California.
I think that is mentioned in one of the links I gave above.

 

[bluejets]

A standard coil will have a resistance of less than an ohm. So on 12 volts you will have over 12 amps of current. Basically a dead short. (assuming the battery can keep up). That's why the transistor is getting hot.

This is only at standstill, you forget inductive reactance when operational.
Ballast resistors are conventionally used on 8v coils on 12v systems.
In that instance, the ballast is switched out to allow higher start-up spark.

 

[dsage]

All true. But some model engines - like hit miss engines - run slow enough and with more than required dwell (point on time) so the coil has more than enough time to saturate. So you should always do your calculations based on the resistance of the coil. This is especially true for the TIM4 circuit because IF the engine ever stops with the points closed (or whatever) then history has proven that the coil and usually the transistor goes up in smoke.
My IGBT driver mentioned earlier has a time out circuit so if the ignition stays activated for more than a predetermined time it cuts off the drive to the coil. Also the transistor is capable of high currents.