@oliof Thanks, good to know! Yes, I'm looking to build a CNC lathe / 4th axis carving machine, possibly based on duet3, just trying to iron out uncertainties before making final a decision on which system to use as control.

@droftarts Understood, optoisolation still a good idea, I'll see about building it in!

@droftarts Oh, thats a much better idea! I am sure I could free up a fan header. So if I am reading the m-codes correctly:
M950 H0 C"bed_heat" T0
just becomes
M950 H0 C"fan2" T0
That is easy enough, thanks for your help.

@dc42 @droftarts thanks guys, appreciate the fast replies!
So it sounds like it should be OK, at least the first option, but the LED might fail - not a big deal to me, I can barely see it anyway. I could try to swap the resistor but board level mods are not something I have really done before. I note you've both stayed away from answering the second question though! Perhaps it is less predictable?

Using the output to drive another switching device would be safer true, but I'd rather not go buying a SSR unless I have to, not something I am familiar with and sourcing looks difficult or expensive. Alternative is driving another mosfet, but Vgs of 24V would cause problems, perhaps a simple voltage divider would work if the gate current can be kept within spec. Or I could buy a 24v to 5v optocoupler on ebay, that would be safer, I have some STP40NF10 mosfets around which should work, but then I also need to add a 5v rail. Running it all direct and from a single supply with a dc-dc 30 -> 24V for the board would be easier, but perhaps I am being lazy

Hi, couple of quick questions with my maestro board. I'm using a hot bed thats underpowered, poor manufacturing tolerances, so I'd like to boost it a little by connecting perhaps 28-30V. Would it be OK to run this as a second power supply, routing the higher voltage through the bed and control mosfet, common ground between supplies, obviously not running the maestro itself at that voltage. I guess it will be fine but just want to double check!

A second option, rather than running two supplies, I wonder about running a 30V supply with dc-dc step down to the board's 24V. Same setup, but there could be a tiny lag between the 30V turning on, and the 24V being available. My guess is that it will be fine as the mosfet will be held low, but want to double check.... I'm mechanical first, not electrical

Understood issues with power scaling to the square of voltage etc.

@oliof Thanks, good to know! Yes, I'm looking to build a CNC lathe / 4th axis carving machine, possibly based on duet3, just trying to iron out uncertainties before making final a decision on which system to use as control.

@dc42 Thanks, although I dont have a duet 3 to test with at the moment :). Best I have is an old maestro running 2.03 for my 3d printer. Perhaps I could update, and then create a high details file to get a feel for it, if there are no known benchmarks.

Realising this may be a difficult (or impossible) question to definitively answer, and likely depends on the complexity of each line, but is there any ballpark figure for the number of gcode lines / second that the duet3 can handle? I run a rotary axis carving machine, also building a lathe in the next few weeks, just wondering how much to future proof it in case RR gets G93 moves in the future! I probably reach 200-300 lines / sec at the moment, 4 axis (but mostly A and Z moving for these runs).

@dc42 +1 would love to see this!! Was discussing it with T3P3Tony in November actually.

@t3p3tony Ah OK, so could be around 6 months then, I'll go UCCNC in the meanwhile.

@t3p3tony That sounds great!! Although it does mean I need to ask when 3.5 final release is expected :).