Duet 3 32v on 3D printer?

[TypQxQ]

@Danal You say bang-bang on OUT_0, but you can run it on PID just as the Duet2 right? I can't see anything in the documentation about it not being so. Like I said I'd run the T0 from it and run bed from OUT_1 with SSR to 220v.

And just for clarification, you don't need a common ground anymore?

[TypQxQ]

@gfisher Good question, I just saw that there are no cheap 32v power suplies like Meanwell. Cosel has one for 300€.

[Danal]

@gfisher said in Duet 3 32v on 3D printer?:

What about the Meanwell LRS-350-36 turned all the way down to 32.4v? Would that extra ~.4v hurt the Duet 3?

Probably not. I'm sure the is some margin. Were it me, I'd go for it. YMMV.

[Danal]

@TypQxQ said in Duet 3 32v on 3D printer?:

@Danal You say bang-bang on OUT_0, but you can run it on PID just as the Duet2 right? I can't see anything in the documentation about it not being so. Like I said I'd run the T0 from it and run bed from OUT_1 with SSR to 220v.

You may have missed the key words that already answered your question (PWM and PID are different aspects of the same thing, for our purposes):

@Danal said in Duet 3 32v on 3D printer?:

@TypQxQ said in Duet 3 32v on 3D printer?:

Both the 6HC and the 3HC accept 32V in, with nothing special. Just connect it to VIN, and it is used for all motion, etc. In addition, the 6HC has a separate IN that is bang-bang or PWM controlled OUT to the bed. Specifically, you could run VIN at 32V for all steppers, and BED-IN at 24V for a 24V heater on the bed.

@TypQxQ said in Duet 3 32v on 3D printer?:

And just for clarification, you don't need a common ground anymore?

Correct, AFAIK. I don't have access to schematics or gerbers.

Having said that, I personally would insist that any CNC/Printer/motion controller machine have ONE ground, physically wired as a "star" (no point-point-ground runs). I've seen many machines that weren't built that way and were glitchy, seen them suddenly clear up and be perfectly stable when re-wired with a physical star.

[dc42]

The limiting factor is the 5V regulator chip, which has an operating range of up to 36V and absolute maximum 40V. The reason we suggest a 32V upper limit is that when stepper motors decelerate, they pump energy back into the suppy, which may cause it to overshoot. Similarly, when you send M18 mto turn all the motors off, energy is pumped back into the supply. Duet 3 has one 220uF capacitor per driver to help absorb this, plus an extra one on the 3HC.

The voltage including overshoot when all the motors are turned off is a little less than given by this formula:

sqrt(V^2 + n * (L/C) * I^2)

where V is the supply voltage, L is the motor inductance per phase, I is the peak current as set in M906, C is the total capacitance on the VIN rail, and n is the number of motors.

Example: MB6HC board, 6 of these Nema 34 motors https://www.omc-stepperonline.com/nema-34-stepper-motor/Nema-34-CNC-Stepper-Motor-34Nm481576ozin-40A-858x858x68mm-4-Wires.html (4.4mH phase inductance) set to 3.5A, 32V supply:

sqrt(32*32 + 6 * (0.0044/0.00132) * 3.5 * 3.5) = 35.62V

[A Former User]

Suppose one could add a clamping diode to the supply if pushing past the recommended limit, but given they usually fail short its a pain to have on board with heavy ground planes if they fail. However an axial version on the screw terminals might be a way to stay slightly safer if going "off piste"

[dc42]

@bearer said in Duet 3 32v on 3D printer?:

Suppose one could add a clamping diode to the supply if pushing past the recommended limit, but given they usually fail short its a pain to have on board with heavy ground planes if they fail. However an axial version on the screw terminals might be a way to stay slightly safer if going "off piste"

Or a 10000uF capacitor across the PSU output, if your PSU doesn't object to that.

[A Former User]

Just thinking out loud, even if the PSU were to object, one could add a series resistor to limit the inrush current and use a spare IO to control a relay to short out the series resistor when charged. G4 and M42 in config.g ought do it.

[TypQxQ]

@dc42 said in Duet 3 32v on 3D printer?:

Example: MB6HC board, 6 of these Nema 34 motors https://www.omc-stepperonline.com/nema-34-stepper-motor/Nema-34-CNC-Stepper-Motor-34Nm481576ozin-40A-858x858x68mm-4-Wires.html (4.4mH phase inductance) set to 3.5A, 32V supply:
sqrt(32*32 + 6 * (0.0044/0.00132) * 3.5 * 3.5) = 35.62V

So for 6 NEMA17 set to 2A each and a suply voltage of 34V would give a overshoot to just under 35.16V
At 33v in it gives me 34.19v and at 32.4v it gives me 33.6v.

Am I right to assume that it would work without problems with a Meanwell 36v supply turned all the way down to 34.4v (+/-1%, according to datasheet) ?

[A Former User]

(you can also replace a resistor in the PSU to change the range a little more, thats what I ended up doing with my 36v supply, i needed to go down to 30v. efficiency may suffer a little, but I'm not powering a bed so not a big issue)

[DaBit]

@dc42 said in Duet 3 32v on 3D printer?:

Or a 10000uF capacitor across the PSU output, if your PSU doesn't object to that.

The average Chinese PSU objects heavily to that. In fact, most PSU's do.
Which makes sense: inside the PSU the power rail for the switcher is powered by two means: one is a bias winding on the transformer, the second is a high value resistor that charges the capacitance on the switcher power rail.
During startup the capacitance charged until a threshold is reached, the switcher starts, and the bias winding must be generating power before the energy in the switcher rail capacitance is depleted.

If there is a short or high capacitance on the output of the supply, the bias is not quick enough, which gives the probably familiar 'tik....tik...tik...' and not starting behaviour.

32V input and 40V absolute maximum rating is too little difference for a zener or transient suppressor to work well. An active circuit using something like a TL431, a power transistor and 3 resistors would work.

But as DC42 mentioned: even with 6x big 3.4Nm NEMA34 motors the overshoot is less than 34V, so there is actually no problem.

[TypQxQ]

So, If using a 5v external power supply like a Meanwell LRS-75-5 to externally power the Duet 3, Raspberry Pi, etc. one would bypass this limitation.

What would the maximum allowable voltage be then?

3 power supplies on a single printer? Now that is ! And SSR to them for control....

[A Former User]

@TypQxQ said in Duet 3 32v on 3D printer?:

one would bypass this limitation.

no, the tmc driver have an internal 12v supply that isn't much higher rated than the 5v supply; so unless you separate the Vmotor for the drivers then you're essentially getting a "return to go and do not collect 200" card

(edit: might not be a problem with the tmc5160 just the 5161, but check the datasheet to be sure in any case)

[TypQxQ]

@dc42 said in Duet 3 32v on 3D printer?:

Duet 3 has one 220uF capacitor per driver to help absorb this, plus an extra one on the 3HC.

So does the 3HC handle voltage overshoots better with an extra capacitor or worse with only one capacitor?

(English isn't my first language if it's obvious from the statement above)

[A Former User]

it is not the number of capacitors as such, but the combined value of them. removing one to add a bigger one is more work than just adding a bigger one, not to mention if its significantly larger it will not fit the pcb, ergo adding one to the screw terminals is easier and gets the job done.

[TypQxQ]

@bearer So the same formula can be used for the expansion but change from 6 steppers to 3? That gives
sqrt(32.4*32.4 + 3 * (0.0044/0.00132) * 2 * 2) = 33.012V

And same for 6 steppers:
sqrt(32.4*32.4 + 6 * (0.0044/0.00132) * 2 * 2) = 33.612V

So , am I right to draw the conclusion that it is marginally better to put 32.4v on a 3HC expansion than on a 6HC main board?

[A Former User]

@TypQxQ said in Duet 3 32v on 3D printer?:

@bearer So the same formula can be used for the expansion but change from 6 steppers to 3? That gives

i think the 0.00132 value is the total capacitance for the main board. you'd need to calculate that for the expansion as it also has fewer capacitors. (but yes, i should expect it to be marginally better due to the additional cap, but not by much)

and this is analouge stuff, I'm not known for practicing this dark art:)

[dc42]

@bearer said in Duet 3 32v on 3D printer?:

i think the 0.00132 value is the total capacitance for the main board. you'd need to calculate that for the expansion as it also has fewer capacitors

Correct.

[TypQxQ]

@dc42 Does the firmware support diffrent voltage on Duet 3 6HC and connected 3HC expansion?

[dc42]

@TypQxQ said in Duet 3 32v on 3D printer?:

@dc42 Does the firmware support diffrent voltage on Duet 3 6HC and connected 3HC expansion?

Yes. And a different input voltage for OUT0 on the MB6HC too.