[rcheshire (Rowland)]

Thanks Jay. I think it can be simplified further. The more I play with it, I realise it morphed from one thing to another. A tweak here and there should make it switch a bit cleaner.

EDIT: The solution is a mosfet driver. Alternatively, and I tried this last night, a LM317T can be used as a switch. PWM on the input pin and drive the gate with the output. This gets 3.89 volts to the gate from 5v pwm. Depending on the mosfet, it works. I've had the mosfet gate switching at <3v and still getting a full 24C differential - mosfet is warm to touch. I recall this was suggested in an earlier thread.