The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

Makergear M2 Improved Platform: Solid State Relay and Thermistor

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:

Improvised 2-pin header socket

Plug it all together, tweak the startup G-Code to properly trip the Z-min switch at its new location, and it Just Works:

M2 Improved HBP – alignment test

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…

2013-10-11
Makergear M2 Improved Platform: Mechanical Adjustments

The PCB under the improved flat-top glass platform has three soldered-in-place M3 screws that fit the M2’s Y stage support:

M2 – Improved HBP – bottom view

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

2013-10-10
RAMBo Platform Heater MOSFET: VGS Specs

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 r_DS spec is an impressive 6.4 mΩ max, but that’s at V_GS = 10 V. The 1 mA threshold voltage V_GS(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 I_D vs. V_GS curve (Fig 6) shows 20 A at maybe 4.2 V, but the typical R_DSon curve (Fig 8) shows the resistance skyrocketing for V_GS 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.

2013-10-09
Tour Easy: Handlebar Wrap

After more than a few years, the handlebar grips on my Tour Easy are rather worn, so I recently wrapped them with cheerful red and yellow silicone tape.

Back in the day, you wrapped with cork tape and had to worry about the direction on each side. Silicone tape fuses into a solid mass and the orientation shouldn’t matter; that’s a Good Thing, because I’m not sure what direction would be correct in this situation.

The yellow section covers the SRAM twist grip, which means it has a moving joint at each end. I suspect the tape will pull back from the larger part of the grip and form an unsightly lump just behind it.

It’s certainly much grippier than I expected…

(The small pushbutton switch is the PTT for the amateur radio HT that does voice and APRS/GPS.)

2013-10-08
Humanoid Finger

Just for fun, I printed out Anthromod’s Kickstarter Hand finger:

Anthromod Finger – parts on M2 platform

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.

2013-10-07
Monster in the Mist

We biked to Saugerties for the Hudson Valley Garlic Festival and spotted this monster looming in the morning mist during the ride home:

Excavator on CSX gondola car – side

The end view shows it’s not an optical illusion:

Excavator on CSX gondola car – end

Some Google Maps fiddling reveals the plant, with the excavator atop the first car on the siding, down in the lower-left corner of the image:

Google Maps – Kings Highway at Tissal Rd

A zoomed view, rotated a quarter-turn CCW so it’s not quite so vertiginous:

Google Maps – Kings Highway at Tissal Rd – detail

My search-fu isn’t strong enough to uncover the plant’s name. They’ve obviously been doing something involving gravel and either asphalt or concrete for many years, so it’s not a prank…

2013-10-06
Getting More Clearance While Bicycling: Fluids Division

Based on recent experience, this “Baby, Think It Over” rig works even better than a propane tank:

Tour Easy BOB Yak – Gasoline can

I was going to take a picture with it posed next to the gas pump, but the whole affair isn’t all that stable: it’s tough to look cool when your fancy faired Tour Easy ‘bent flops over like dead possum…

2013-10-05