Duet maestro with >24V heated bed

MRob

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.

dc42

@MRob:

The bed heater mosfet on the Maestro was specified as IPD036N04L which is rated at 40V. We didn't manufacture the Maestro, so I suggest you check the marking on it.
The 10u ceramic capacitor in parallel with the mosfet is rated at 35V.
The resistor in series with the LED is value 4K7 size 0603, power rating not specified so likely to be either 0.1W or 0.125W. At 30V VIN this will dissipate about 0.167W when the bed heater is on at full power, so it may burn out. If you have the means to replace it, you could replace it by a 10K resistor.

HTH David

droftarts

@MRob The first option sounds okay, but I'd be wary of mixing voltages on the Duet, because mistakes happen! The voltage limit on a Maestro is 25V.

In addition to @dc42's reply, you also need to check that the current the bed will draw doesn't exceed 18A, and ideally lower than that; there's no fuse protection on a Maestro board, and the MOSFET and terminal connections can still heat up and fail.

A better way to control the bed at a different voltage would be using an SSD, and you could then use any voltage you want. See: https://docs.duet3d.com/User_manual/Connecting_hardware/Heaters_bed#bed-heater-driven-using-a-solid-state-relay

Ian

MRob

@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

droftarts

@MRob The second option is a bit less predictable; I'd worry if there was a failure in the step down converter, you'd fry your Duet. It just feels there's more to go wrong.

You don't have to use the bed heater output to drive an external MOSFET or SSR, you could use an I/O pin to control it. The only problem is that the Maestro is a bit short on I/O pins, and even shorter on PWM pins; see https://docs.duet3d.com/Duet3D_hardware/Duet_2_family/Duet_2_Maestro#inputoutput

Basically, it's either a heater or fan output, or the Z probe MOD output (3.3V signalling). However, all of these switch on the GND side, so you can take 5V from anywhere on the board, wire it to the MOSFET/SSR, and connect it to the GND pin of the chosen header (eg bedheat), and the signalling won't be at VIN. If you use FAN 2, it conveniently has a jumper next to it to set the fan positive pin to 5V, so that would keep things neat.

Ian

MRob

@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.

droftarts

@MRob said in Duet maestro with >24V heated bed:

M950 H0 C"fan2" T0

Yes, that should work to control an external MOSFET or SSR. A MOSFET still has the chance of failing open, and potentially feeding the supply voltage back through the control pin, of course!

Ian

MRob

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

MRob

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