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UM2+ Power Splitter: An Attempt at Improving Wall Consistency

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Posted · UM2+ Power Splitter: An Attempt at Improving Wall Consistency

Many moons ago, forum member [torgeir] reported a phenomenon on his UM2+, where he noticed wall inconsistencies on his prints that were related to effects from the hysteretic ("bang-bang") regulator of the bed heater.
at that time , i was looking into that issue as well and came to the conclusion that,opposed to other's believes, the wall incosistencies did not arise from temperature fluctuations of the heated bed, but were actually caused by voltage fluctualtions of the main 24V power supply. to prove that, i altered the bed temp regulator in the firmware from hysteretic to PWM PID control. this made the wall incosistencies immediately disappear, but at the cost of (and again, pointing at a power supply issue) an ever so slightly and annoyingly flickering LED illumination. i tried to ignore the flickering, but alas, after a couple of months, i could no longer bear it and decided to do something about it.
the idea was to split the main 24VDC bus from the main power supply into two branches: one branch would feed three MOSFET switches that would actuate BED, EXT1 and EXT2 by using the respective ultimaker main board's outputs as mere control signals, rather than as power-delivering outputs.
the other branch would go into a  beefy step-up regulator that would provide a super-tight regulated 24VDC power output to supply just the guts of the ultimaker main board itself, i.e., cpu, display, stepper drivers and all other paraphernalia.

you can see the result of my efforts in the pictures below. the power supply is a fanless UHP-350-24, connected to an AC switch box that is mounted on the side of the printer, so that i have the main power switch accessible from the front side of the printer.
the green-yellow PE connection is also connected to the 24VDC minus, to ensure that 24V-side electronics are not building up charge that could cause a destructive discharge when, say, connect the USB cable to it.
the 24VDC output is connected to the "power splitter" board through the main entry fuse (2 polyfuses in parallel). to the left of the 24VDC entry point is the regulated 24VDC power output for the ultimaker board, also fused with a single polyfuse. on the right hand half of the power splitter you can spot three identical power trains BED, EXT1, EXT2 (EXT2 partially populated, not used), each consisting of a "real" gate-driving opto-coupler, a power MOSFET, a free-wheeling diode, an indicator LED and a polyfuse. BED doesn't have an extra fuse, it is using the 24VDC entry fuse, while EXT1, EXT2 do have their own fuses.
the opto-couplers serve two purposes: they allow for noise isolation (not used here, but can be configured via jumpers), and drive the MOSFETs with sufficiently high gate voltages. in this case 12 VDC, which is generated on-board via the chunky device on the top left of the board. this in turn allows the MOSFETs to operate at 10 A continuously, while getting just warm to the touch.

the two test cubes side-by-side tell the whole story. on the left cube, the center area is printed while bed heater is on, operating in the original hysteretic mode. top area is printed with the bed heater turned off. the right cube was printed with the power splitter installed and bed heater turned on with PID PWM control.

both cubes were printed with Ultimaker Silver PLA, 60 °C bed, 210 °C nozzle 0.4mm, 0.15 mm Z, spiral mode, 40 mm/sec.


bottom line and TL;DR:  mind your power supply!



- you have seen a similar post from me before, discussing a different main board installation on the UM2+. well, it took me a while to figure out that the board would never live up to my expectations, so it had to leave the building,  r.i.p..

- no, i'm not going to make this commercially available, as the boat for all UM2+ mods has set sail long time ago already. this mod was meant to be a proof of concept only.




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