gr8flyer55

Ustra, at low rpm, off time for transistor is long enough to not allow it to get hot. At high rpm 12,000 - 20,000 , on time is longer and makes the transistor run hotter. A heatsink will help on the transistor to run cooler. Also higher amperage rating of the 2sc5707 may help. Higher rpm draws more current, and makes more heat.

John