Amount of W for 350x350 silicone heating bed

I am using a 24V 350W power supply. The size of the silicone heating bed is currently of 350mm x 350mm, what amount of Watts would you suggest?

Also, I have read on the Duet RepRap page: "Processor voltage is 3.3V and inputs are not protected, so using sensor with higher voltage need reliable protection. Optocoupler or diode protection may be more reliable than voltage dividers." any one would have a link with the procedure on how to do this?

Thank you.

I recommend a bed heater power density of 0.4W per square cm. Some people suggest 0.5, but that increases the chance of damage if uncontrolled heating occurs. Large beds are most easily heated using AC mains voltage bed heaters, but if course you need to take additional safety precautions.

The inputs of the later Duets - especially the Duet WiFi - are quite well protected against excessive voltage at the signal inputs. Nevertheless, if you use a 5V endstop, or even worse a 12V inductive sensor, you should design the interface between the two to limit the input to the Duet to 3.3V, even under fault conditions such as a broken ground connection.

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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].

To expand on what DC42 has said, 35cm x 35cm x 0.4W = 490 Watts which exceeds your 350 W power supply. At 24V, 490Watts is 20.1 Amps which exceeds the 15 Amp limit that the Duet WiFi can supply. So it rather looks like your best option is to use an AC mains heater controlled via a SSR (Sold State Relay) or similar.

You've already bought a silicon heated bed with 350x350mm? What is the resistance of the bed? That will probably limit the power you draw from the PSU. ( and increase heat up time )

I recenty purchased a 18 x18 heat bed the manufacturer recommended i use a sep PSU.
not sure if that helps you.