Category: Machine Shop

The basic problem with the heater on the Heated Build Platform is that the SMD pads must both make electrical contact to the Molex-style connector and withstand mechanical stress from the dangling wires & cables as the platform moves along the X and Y axes. Rather than replace the entire heater, I attached pigtail leads to the PCB, anchored those leads to the wood platform under the heater, and routed the cables through the deck under the Y axis stage a bit differently.

However, attaching pigtail leads to the PCB poses a problem, because ordinary electronic hookup wire has thermoplastic insulation that melts or deforms at temperatures well under my usual 110 °C platform heat setting; shorting the heater wires would be a Very Bad Thing.

Some concerted rummaging in the Big Box o’ Multiconductor Cable turned up a hank of Teflon-insulated shielded two-wire cable that, as nearly as I can tell, has pure silver conductors and shield braid: the ends were tarnished like silver and there’s nary a trace of copper in the fresh cuts. It must be military surplus and, based on a vague recollection, was most likely cough salvaged by my father, who worked as an avionics tech at Olmstead AFB in the mid-60s. Ya gotta have stuff, right?

[Update: Alas, it’s not pure silver, as shown in the comments.]

The general idea is to scuff up the shiny PCB surface enough to anchor blobs of JB Industro Weld epoxy that surround brass tubes holding the cables. A pair of tubes secure each cable and provide strain relief; the cable is free to move, but not by very much. The thermistor cable has a long arch that will, I hope, keep the cable at the platform temperature and reduce its cooling effect on the thermistor:

The alligator clips connect to a bench power supply that delivered 4 V @ 2 A = 8 W that heated the PCB to about 40 °C in the rather chilly Basement Laboratory and encouraged the epoxy to cure in less time than forever.

The final result looked like this, with Anderson Powerpoles now attached to the heater cable:

The 24 AWG conductors in the cable may seem scanty for 6 A of heater current, but, hey, they’re silver.

The three-pin connector on the end of the thermistor cable is a pure kludge, built from a 4-pin header to match the CD-ROM audio pinout on the new cable from the Extruder Controller. I kept the default pinout on this end to provide some protection against plugging it in backwards:

With all that in hand, I screwed the PCB to the aluminum sub-plate, bolted it to the plywood platform, and stuck the cables onto the platform with adhesive clamps:

Reaming out the hole between the red and black Powerpole shells provided just enough room for an M3 screw to anchor them to the HBP: they won’t flop around under acceleration.

The thermistor cable exits to the left, the rest to the right, and I’m unhappy with the overall routing. I added a small bumper (made from bent steel shim stock) to keep the thermistor cable out of the gap between the Y axis stage and the left side wall:

So, yes, it works, but it sure ain’t elegant.

The first object was the revised platform level test pattern:

The platform is holding level within ±0.05 mm across build plates 1 and 2, somewhat better than before. On the other paw, the whole thing doesn’t have many hours on it…

Given that the SMD pads fell off the HBP circuit board and I must replace the connector, I figured I may as well also replace the remarkably stiff MBI thermistor cable with a much more flexible CD-ROM audio cable. Although the EC end of the MBI cable looks like a standard CD-ROM audio connector, it’s been rewired. No problem: this is not an audio application and I’m going to do exactly the same thing.

The Extruder Controller, however, doesn’t have a matching connector and the recommended attachment involves simply jamming the connector onto the pin header, per this detail cropped from that photo in the MBI assembly instructions:

Here’s a better closeup of my EC, taken from the other side:

The header block breaks out the Arduino’s Analog Input pins, with A6 in the front of that photo. From left to right, the pins under the HBP connector are A6 / +5 V / Gnd. Unfortunately, the connector wiring and alignment puts the thermistor signal on the cable shield, with the Gnd and +5 V wires safely tucked inside. This is, shall we say, suboptimal.

The Gnd connection provides a low-impedance connection to the least-noisy part of the circuit, so putting it on the shield tends to prevent the relatively high-impedance signals within from picking up noise. This isn’t always successful, for a number of reasons, but it’s a Good Idea.

Although probably doesn’t make much difference (it’d just add a bit of noise to the HBP temperature signal), but if I’m going to be rewiring it anyway, the cable shield will be at ground potential with the signal  wire inside. Here’s my cable & connector, rearranged to make that so:

The analog audio connector on the back of old-school CD-ROM drives, back before digital audio output from the drives actually worked, had four pins:

• Left (white) and Right (red) audio channels on the outer pair
• Ground (black) on at least one of the central pair

So the red wire will be in the far right-hand socket of the connector shell; depress its locking tab, slide it out of the shell, poke it into the socket between the other two wires, push to click, and you’re set. Conveniently, this puts the +5 V supply on the red wire, which is sorta-kinda standard. Your cable colors may vary; pay attention to the actual wiring and ignore the color code!

Tape the connector in place (with the empty socket now toward the board edge) to prevent the tangle of wires in the Thing-O-Matic’s electronics bay from dislodging it at an inopportune moment:

Admittedly, that arrangement still tucks the +5V wire right next to the signal wire inside the shield, but it’s a step in the right direction.

You could flip the MBI cable around, too, as long as you also rearranged the pins at the HBP end to match.