Using Duet wifi or ethernet with 27V supply
I am planning to move to Duet, but my heatbed is slightly underpowered at 24V so I have been driving it with 27V.
Duet is rated for 12-24V, will it stand the 27V supply? I would not like to use neither two PSU's nor an external MOSFET board.
With 24V supply the hotbed takes ages to heat up, and will not go above 85C.
The other option is ripping off the current 300x300mm 24v/270W silicone mat heater and replacing it with 24V/400W one, that seems
now to be easily available. But that would mean almost 17 amps at 24V, while the Duet is rated for 18, which is perilously close.
The question is, is that current draw still safe for Duet?
The stepper drivers are rated at 30V. If you move the stepper motors by hand or command them to decelerate rapidly then they act as generators and feed power back into the supply rail, increasing the voltage. That's why we specify 25V maximum VIN input.
One option for you is to use a 27V supply but connect 3 silicon rectifier diodes (e.g. 1N5400) between the PSU V+ output and the VIN+ terminal of the Duet, to reduce the voltage at the Duet to about 24V. Then connect your bed heater between V+ from your power supply and the Bed- output of the Duet, leaving Bed+ not connected, so that the bed heater gets the full 27V.
If you change the bed heater to a 400W one then you can safely run it at 17A through the Duet, but in that case I suggest you use a fan to cool the Duet PCB.