greencardigan wrote:

I managed to get the inverted pwm working on OT1 and with a resistor across the OT1 pins I get a nice 0V to 5V.

However, I still think there is an issue with the open collector and resistor setup. When the Arduino is powering up or resetting, the pull up resistor is going to pull the output to 5V and turn the connected heater on full power! Or am I missing something?

Maybe it would be best to do non inverting output and connect directly to D09.