КТ815, КТ817, КТ819 или другие аналоги.
A constant resistor R2 is specially introduced into the circuit, the purpose of which is to limit the minimum fan speed in order to ensure its reliable start even at the lowest speed. Otherwise, the user may put too low a voltage on the fan, at which it will continue to spin, but which will not be enough to start it when turned on.
The higher the resistance, the higher the minimum fan speed
