Why 19V ?

Hi, is it possible to know why the choice for 19V for the fan and the heater cartridge ?
If i want to change the hot end with an hexagon, by example, the heater cartridge is 12V or 24V. How does it work with the Duet ?

I believe 19V was chosen because low cost 19V laptop PSUs are readily available. The Duet works fine on both 12V and 24V. It's better to use 24V if you have 0.9deg/step motors, or a large heated bed.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your answer.

So, for changing the hotend, which voltage is better ? 12V or 24V ? Do I need to change the PSU to 24V ? Is there parameters to change in the configs files ?

In fact my problem is this one :
I live in Switzerland. For a 30£ purchase at RRP , there is an expensive shipping and again 20£ for customs via UPS.
So, I would like to find other resellers - on France maybe - so I can buy parts with less shipping taxes and without customs.
But the Fisher seems to diverge from standards.

24V is more future-proof than 12V. You don't need to change the config files to adjust to a different voltage, although you may need to adjust the hot end heater parameters to adapt to a different hot end.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].