Reduce heater pwm frequency about printipi

[matthewSorensen]

250kHz is definitely far too fast. 25kHz is reasonable, and has the advantage of being about most hearing thresholds, in case there are magnetic effects/weird PSU things happening....

FET choice can reduce switching heat (low threshold, small gate charge), and a proper FET driver should basically eliminate it...

[Wallacoloo]

That's a fair point about audible effects.

This may be getting nitpicky, but I imagine with a large current going through the hotend resistor, and with the thermistor being < 1 cm away, they may be coupled in some way (E/M fields inducing voltages). Because of things like inductance, I imagine that the faster the switching frequency, the lesser the interference. But is there any detectable interference between a typical hotend and thermistor for this to even be a consideration?

[matthewSorensen]

I would worry far more about cross-talk along the nice long unshielded wire run... We could very easily figure out if that is something to reduce - a matter of asking Ben if you can use the 'scope for a few minutes.

[Wallacoloo]

Added support for specifying a desired frequency for PWM. Hotends default to 25kHz.