Category: Electronics Workbench

Having added a dummy load to pull a minimum current from both the +5 V and +12 V supplies, it seemed reasonable to connect the ATX +Power Good signal to the Arduino’s -Reset input. That ensures a power glitch will force a hard reset, rather than produce random crashes / instability / weirdness, and make the problem obvious.

Of course, that presumes the power supply notices the glitch and drops the +Power Good line. That’s probably an incorrect assumption, but the only way to test it requires hitting the power supply with a crowbar and I’m just not going to go there.

Anyhow.

This also ensures the Arduino gets a hard reset when I turn the power off by triggering the manual shutdown button on the thermal lockout control box. The Arduino draws power through the USB cable from the PC (at least on the Foxconn box I’m using) and sometimes starts up crazy; that will no longer happen.

The +Power Good signal arrives through the Gray wire on ATX pin 8 (it also drives an LED on the lockout box) and the -Reset signal is the outside pin on the Motherboard’s manual reset pushbutton. The new connection looks like this:

Being no fool somewhat cautious, however, I added a switch that can disconnect the two lines; if it turns out the +Power Good signal has any glitches, I can use the original mode while scratching my head.

A giant blob of hot melt glue holds the switch in position:

A wire burrows through one of the unused RS-485 connector mounting holes under the switch on its way to the ATX connector. It’s the Blue wire below the board in the previous picture, enters from the top right here, and terminates on the third pad over with the Black wire that joins the cable on the way to the Lockout box

Now I’ll see whether the Thing-O-Matic begins resetting at random moments. After doing all the various mods & improvements you’ve seen over the past few months the printer has been quite reliable, so I have some hope that this change will produce … no change.

As a quick test, I let the printer sit all day with the Thermal Core and HBP temperatures cycling around 50 °C and all four steppers enabled. Any reset would disable the steppers and make itself obvious, but after two days it’s all good.

One good sign: the LED ring around the extruder head just barely changes brightness as the heaters cycle.

Oh, and the steppers don’t overheat, either. This thing is starting to behave like a real 3D printer should!

If it does start resetting, however, I’ll add a latch inside the Thermal Lockout box that captures short +Power Good glitches and lights Yet Another LED.

The ATX power supplies commonly used in PCs generally require a minimum load of about an amp on the +5 V and +12 V lines to ensure good voltage regulation, but the Thing-O-Matic Motherboard has only a 30 Ω power resistor that draws 170 mA. They’re also designed for relatively constant-current loads, which the Thing-O-Matic is not, by any stretch of the imagination, so it’s no wonder the voltages jump all over the place.

I think, but cannot prove, that many of the random problems plaguing long-duration prints arise from power glitches. That load resistor on the +5 V line was, at best, a stop-gap measure, and this is what I had in mind from the start. There are additional constant loads here and there throughout the Thing-O-Matic, but it really does not apply a known minimum load to the power supply.

I got a stock of 6 Ω resistors for that heater project and used a trio here: one draws 800 mA from +5 V (about an amp, including the MB load) and the other two in series draw 1 A from +12 V. The little fan runs from +12 V, although I may connect it to +5 V to make it quieter; the 16 W total power dissipation is too high for convection cooling.

Note that these are 50 W resistors dissipating 6 W apiece while running at room temperature. Yes, they look like extruder heater resistors, but this is an application they’re designed for.

A bit of machining mated a junked CPU cooler to a half-inch slab of aluminum that serves as a heat spreader. The cooler had many tabs and protuberances on its bottom surface that I simply sliced off with an abrasive wheel. Two pieces of brass shim stock filled in a mysterious recess along the right edge in this picture. The cooler’s spring clamp engages a pair of wire tabs screwed to the spreader and the force smushes a layer of thermal compound into the air gaps.

The holes atop the cooler didn’t match up with any fans in my collection, but that’s why I have a Thing-O-Matic. I had to rotate the fan case to get the holes to fit, which was trivially easy in OpenSCAD.

And then it built about like you’d expect:

Yes, it really needs a finger guard, but that’s in the nature of fine tuning…

The OpenSCAD source code, with a rather fat ThreadWidth setting. IIRC, this was fallout from a random walk through the Skeinforge parameter space.

// Fan adapter plate for CPU cooler
//  Used for Thing-O-Matic minimum current loads
// Ed Nisley - KE4ZNU - Mar 2011

// Build with...
//	extrusion parameters matching the values below
//	+2 extra shells
//	3 solid surfaces at top + bottom

include </home/ed/Thing-O-Matic/lib/MCAD/units.scad>

// Extrusion parameters for successful building

ThreadWidth = 1.0;					// should match extrusion width
ThreadZ = 0.33;						// should match extrusion thickness

HoleWindage = ThreadWidth;			// enlarge hole dia by extrusion width

// Plate dimensions

PlateX = 70.0;
PlateY = 66.0;
PlateZ = 5.0;

FrameHoleSpace = 50.0;				// mounting holes in frame
FrameHoleXOffset = 10.0;			//  ... offset from front left
FrameHoleYOffset = 10.0;			//  ... which are *not* symmetrical!
FrameHoleDia = 3.0 + HoleWindage;	// from frame holes
FrameHoleRadius = FrameHoleDia/2;

FanHoleSpace = 40.0;				// fan hole separation in X & Y
FanHoleDia = FanHoleSpace * sqrt(2);	// diameter of hole circle
FanHoleRadius = FanHoleDia / 2;

FanAngle = acos(FrameHoleSpace / FanHoleDia) - 45;

FanDuctDia = 48.0;
FanDuctRadius = FanDuctDia/2;

FanCenterX = PlateX/2;
FanCenterY = PlateY/2;

FanScrewDia = 4.0 + HoleWindage;	// from fan frame holes
FanScrewRadius = FanScrewDia/2;

// Convenience settings

BuildOffsetX = 3.0 + PlateX/2;		// build X spacing between top & bottom plates
BuildOffsetY = 3.0 + PlateY/2;		//	... Y

Protrusion = 0.1;					// extend holes beyond surfaces for visibility
HoleZ = PlateZ + 2*Protrusion;

//-- Build it!

  difference() {
	cube([PlateX,PlateY,PlateZ],center=true);

	translate([(FrameHoleXOffset - FanCenterX),(FrameHoleYOffset + FrameHoleSpace - FanCenterY),0])
	  cylinder(r=FrameHoleRadius,h=HoleZ,center=true,$fn=6);
	translate([(FrameHoleXOffset + FrameHoleSpace - FanCenterX),(FrameHoleYOffset - FanCenterY),0])
	  cylinder(r=FrameHoleRadius,h=HoleZ,center=true,$fn=6);

	cylinder(r=FanDuctRadius,h=HoleZ,center=true,$fn=48);

	rotate(a=[0,0,FanAngle]) {
	  for(x=[-FanHoleSpace/2,FanHoleSpace/2]) {
		for(y=[-FanHoleSpace/2,FanHoleSpace/2]) {
		  translate([x,y,0])
			cylinder(r=FanScrewRadius,h=HoleZ,center=true,$fn=6);
		}
	  }
	}
  }

The original as-it-was-being-machined heat spreader dimensions: