Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I’d originally planned to drive the new HBP with a boost converter from the 24 V supply brick, but that didn’t quite work out. The arrival of a 36 V brick from halfway around the planet solved that problem, but the RAMBo platform heater’s 15 A ATO fuse restricts it to 24 V and I don’t quite trust that MOSFET for high current applications, either.
Sooo, I went full-frontal Cupcake with a solid state relay screwed to a pair of existing holes (!!!) in the M2’s frame:
M2 – SSR for Improved HBP
Note: that’s a DC-to-DC SSR, not the more common DC-to-AC SSR. Basically, it’s an up-armored optically isolated MOSFET, not a triac, and, yes, capitalizing acronyms and initalisms can be contentious.
Because the RAMBo’s MOSFET now drives the piddly current required to activate the SSR, I rewired the power to apply the M2’s 19.5 V brick to all three inputs by adding two red jumpers on the right side of the Phoenix plugs:
M2 HBP SSR Wiring
The M2’s hulking 12 V brick now resides in the Basement Warehouse’s Power Supply Annex.
The HBP cable comes in from the right side and the 36 V supply arrives through the Powerpole connectors in the lower right. Tucking the ferrite slug on the 19.5 V supply behind the wire loom prevents the cable from pulling the Phoenix connectors out of the RAMBo board at an inopportune moment.
The original M2 HBP wiring got uncomfortably warm carrying the 10+ A for that platform. It would probably work OK at a lower current, but I’d already put Powerpole connectors on the new HBP. So I ran that cable outside the loom and abandoned the original pair inside.
The SSR switches the +36 V wire, leaving the HBP at 0 V when it’s not activated: supply hot → SSR → HBP → supply common. That makes no practical difference, but it feels like a Good Idea. Also, the Kapton tape across the SSR terminals should be barely adequate to prevent contact with random conductive clutter; I’m channeling the true spirit of DIY 3D printing…
I don’t have a connector matching the M2’s 100 kΩ platform thermistor, either, so I just ran the new cable down the outside of the loom and conjured up a two-pin socket for the header on the RAMBo board:
Those are thinwall open boxes that came out 5±0.03 mm tall across the array, so the platform is just about as level / aligned as necessary for my simple needs.
The Z-min switch will move to get rid of that stupid block epoxied to the platform, so I didn’t record the G-Code tweakage…
I applied a tiny rat-tail file to the holes until they became a free sliding fit for the screws.
The wave springs are mostly decoration, as the silicone rubber disks now take the compression load from the screws, and the platform is quite rigidly mounted.
The new platform eliminates the M2’s original aluminum support spider, the aluminum heater & heat spreader, and the corner supports & clips, all of which add up to about 780 g. I didn’t bother changing the Y axis acceleration to match, as all those numbers seem rubbery.
Minus the support spider, the platform rides much lower on the Z axis stage than the M2’s platform. Unfortunately, the Z-min switch clamped to the top of the rear Z-axis guide rod can’t get any higher, even after rearranging the cable and fiddling with the LED:
M2 – Z min limit switch
As a first-pass hack, I moved the switch to the rear of the X gantry and applied Gorilla Tape to hold it in place:
M2 – Z-min switch at rear X gantry
That required a small block to raise the platform enough to activate the switch before hitting the nozzle. I epoxied a snippet of brass rectangle tube to the left edge of the platform, directly under the switch lever:
M2 – Improved HBP – rear switch tab
The awkward position activates the switch with the platform as far to the rear as possible, so that you can’t inadvertently drag the dangling switch lever across the block in the wrong direction.
I think it’s stupid, too, but it let me bring up the printer and make sure all the electronics kept working. The next step was to relocate the switch to a more rational place
The improved platform for the M2 runs at 30 V, but the RAMBo board specs limit the max HBP voltage to 24 V, presumably because the 15 A ATO fuse won’t clear a high-voltage, high amperage DC short. While setting up the SSR that drives the new platform, I looked up the specs for the PSMN7R0-60YS MOSFET controlling the bed heater and … it doesn’t have a logic level gate.
The rDS spec is an impressive 6.4 mΩ max, but that’s at VGS = 10 V. The 1 mA threshold voltage VGS(th) = 4 V max, which means there’s only 1 V of headroom to turn the transistor on enough to pass upwards of 10 A.
The typical ID vs. VGS curve (Fig 6) shows 20 A at maybe 4.2 V, but the typical RDSon curve (Fig 8) shows the resistance skyrocketing for VGS under maybe 4.8 V; sliding that curve a wee bit to the right would cause a Very Bad Thing to take place.
A 20 mΩ resistance dissipates 4.5 W at 15 A, which seems rather aggressive for the small PCB copper-pour heatsink on the RAMBo board. It’s a somewhat more bearable 2 W at 10 A, but I think that’s still too high. Of course, the typical dissipation will should be much lower…
A good engineering rule of thumb is to ignore the datasheet’s “Typical” column and design using the “Minimum” or “Maximum” columns, as appropriate. When you depend on typical specs, getting “the same part” from a different supplier can provide a real education in supply-chain management.
I suspect tolerance stacking works well enough that nearly all the MOSFETs on nearly all the RAMBo boards run cool enough to survive, but I’d rather see logic-level MOSFETs in Arduino circuits where the maximum gate voltage won’t ever get above 5 V.
If I were doing it, I’d add 1.75 mm alignment holes in each part, but clamping each phalange in both directions came out close enough:
Anthromod Finger – clamping
The tolerances were a bit tight and it required some trimming before all the joints flexed freely. I used short segments of 3 mm orange filament for the knuckle hinges and heat-staked the ends, rather than having to trim a trio of 3 mm screws:
Anthromod Finger – detail
After making three short rubber bands by tying and trimming loops from a longer band, the finger curled up just like yours:
Anthromod Finger – curled
The overall quality isn’t as good as I’d like: there’s a bit of uplift on the edges and corners. If I print another one, I want to try less than 0.2 infill and less cooling.
The brassboard PCB for the Hall effect blinky light is too bendy for the SMD parts to survive much debugging, particularly with all the wires hanging off the edges, so I whipped up a stiff mounting bracket that captures the whole thing, with a flange that fits in the work stand arms:
PCB Test Frame – solid model
I ran some self-tapping 4-40 hex-head screws into the holes while the plastic was still warm on the M2’s platform:
PCB stiffener with screws on M2 platform
Six screws seem excessive and I’ll probably wind up using just the middle two, but there’s no harm in having more holes and fittings than you really need.
The flange fits neatly into the board holder on the Electronics Workbench, above all the construction clutter:
PCB stiffener in board holder
The nice thing about having a 3D printer: when you need an object like this, a couple of hours later you have one!
The OpenSCAD source code, slightly improved based the results you see above:
// Test support frame for Hall Effect LED Blinky Light
// Ed Nisley KE4ZNU - Sept 2013
ClampFlange = true;
//- Extrusion parameters - must match reality!
ThreadThick = 0.25;
ThreadWidth = 0.40;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
Protrusion = 0.1;
HoleWindage = 0.2;
//- Screw sizes
inch = 25.4;
Tap4_40 = 0.089 * inch;
Clear4_40 = 0.110 * inch;
Head4_40 = 0.211 * inch;
Head4_40Thick = 0.065 * inch;
Nut4_40Dia = 0.228 * inch;
Nut4_40Thick = 0.086 * inch;
Washer4_40OD = 0.270 * inch;
Washer4_40ID = 0.123 * inch;
//- PCB sizes
PCBSize = [46.5,84.0,1.0];
PCBShelf = 2.0;
Clearance = 4*[ThreadWidth,ThreadWidth,0];
WallThick = IntegerMultiple(4.0,ThreadWidth);
FrameHeight = 5.0;
ScrewOffset = 0.0 + Clear4_40/2;
OAHeight = FrameHeight + Clearance[2] + PCBSize[2];
FlangeExtension = 3.0;
FlangeThick = IntegerMultiple(1.5,ThreadThick);
Flange = PCBSize
+ 2*[ScrewOffset,ScrewOffset,0]
+ 2*[Washer4_40OD,Washer4_40OD,0]
+ [2*FlangeExtension,2*FlangeExtension,(FlangeThick - PCBSize[2])]
;
echo("Flange: ",Flange);
NumSides = 4*5;
//- Adjust hole diameter to make the size come out right
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,h=Height,$fn=Sides);
}
//- Put peg grid on build surface
module ShowPegGrid(Space = 10.0,Size = 1.0) {
RangeX = floor(100 / Space);
RangeY = floor(125 / Space);
for (x=[-RangeX:RangeX])
for (y=[-RangeY:RangeY])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//- Build it
ShowPegGrid();
difference() {
union() { // body block and screw bosses
translate([0,0,OAHeight/2])
color("LightBlue")
cube(PCBSize + Clearance + [2*WallThick,2*WallThick,FrameHeight],center=true);
for (x=[-1,1], y=[-1,0,1]) {
translate([x*(PCBSize[0]/2 + ScrewOffset),
y*(PCBSize[1]/2 + ScrewOffset),
0])
color("Orchid") cylinder(r=Washer4_40OD,h=OAHeight,$fn=NumSides);
}
if (ClampFlange)
translate([0,0,Flange[2]/2])
color("SeaGreen") cube(Flange,center=true);
}
for (x=[-1,1], y=[-1,0,1]) { // screw holes and washer recesses
translate([x*(PCBSize[0]/2 + ScrewOffset),
y*(PCBSize[1]/2 + ScrewOffset),
-Protrusion])
rotate((x-1)*90)
PolyCyl(Tap4_40,(OAHeight + 2*Protrusion));
translate([x*(PCBSize[0]/2 + ScrewOffset),
y*(PCBSize[1]/2 + ScrewOffset),
OAHeight - PCBSize[2]])
PolyCyl(1.2*Washer4_40OD,(PCBSize[2] + Protrusion),NumSides);
}
translate([0,0,OAHeight/2]) // through hole below PCB
cube(PCBSize - 2*[PCBShelf,PCBShelf,0] + [0,0,2*OAHeight],center=true);
translate([0,0,(OAHeight - (PCBSize[2] + Clearance[2])/2 + Protrusion/2)]) // PCB pocket on top
cube(PCBSize + Clearance + [0,0,Protrusion],center=true);
}
The 36 V supply has the same M4 mount points as the 24 V supply, so I cut up another pair of those long 6-32 threaded standoffs to make four mounts. This time, instead of meticulously drilling-and-tapping the M4 holes, I just poked a clearance hole in the end of each stud with a #23 drill (0.154 = 3.9 mm) that came out a nice slip fit, cut the heads off another quartet of M4 screws (actually, a quintet, as there’s now one stud lost in the lathe swarf), dabbed some JB Quik epoxy in the holes, and rammed the studs in place:
Power supply brick – M4 stud standoffs
Pause for a while and it’s all good. If the epoxy loses traction with the supply hanging from the mounts, it’ll be pretty obvious…
For what it’s worth, the studs come from an M4 hex-and-Philips screws used in some PC cases (The more common M3 screw doesn’t work here, but I think I bought ’em from the same source) . Cheap and readily available, but chrome plated and murder on saw blades; I use an abrasive cutoff wheel. A quartet of equally standard 6-32 PC case screws hold the mounts to the top of the PC case…
[Update: The talk went well and took a bit under three hours, although by mutual agreement we didn’t fire up the M2 at the end. I’ll work on a short talk about Design for Printability and we’ll run that with a separate printing session. A good time was had by all!]
The Smell of Molten Projects in the Morning 