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Wallacoloo / printipi

3d printing directly through the Raspberry Pi's GPIO pins
MIT License
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Reduce heater pwm frequency #38

Closed Wallacoloo closed 10 years ago

Wallacoloo commented 10 years ago

I'm told that the FETs often used for controlling a heater dissipate a significant amount of power when switching.

250kHz switching (current PCM over DMA rate) is way faster than is necessary for controlling a heater. I can't find any info on optimal frequencies, but I suspect there's very little discernible difference until you get below ~20Hz.

matthewSorensen commented 10 years ago

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 commented 10 years ago

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 commented 10 years ago

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 commented 10 years ago

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