Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The sewing machine motor runs from 120 V AC or DC, drawing a few amps with the rotor locked, so a hulking 300 V 10 A bridge rectifier (Motorola MDA962-4, if you’re keeping score) seems grossly overrated. On the other paw, I have one, so why not?
The mounting holes pass 6-32 machine screws, but the recesses in the top seem meant for fillister head screws that I don’t have. Fortunately, I do have a lathe:
MDA962-4 rectifier – screw head adjustment
And then they just drop into place:
MDA962-4 Bridge Rectifier – installed
You can see why recessing the screw head below the top of the rectifier is a Good Thing.
The Dell GX270 system board mounts on a tray, latching into small tabs, with a single screw locking it in place. The tray then slides into the metal EMI shield / case, latching onto more tabs, with a spring-loaded pair of tabs snapping into a slot under the green latch:
Optiplex GX270 – system board tray
All that is well and good for a mass-production PC system board, but poses a problem for mounting anything else: there’s no room for screw heads below the tray, adhesives really don’t bond to slightly flexible aluminum sheets, and I definitely can’t do large-scale precision metal bending.
So a cheat seems in order. The general idea is to support a 6 mm polycarbonate sheet on clips that slide under the small tabs along the front, support the sheet on the rear tabs, and secure it with the screw. That’s thick enough to allow tapping holes for mounting screws, so everything else can mount to the sheet.
The sheet fits around the power supply on the right, protrudes over the rear of the tray to the back of the case (with a recess around the green latch), and clears the hinge assembly on the left. There are no dimensions, as it’s all done by eye with the Joggy Thing.
AC Chassis Shaping
A drive bay EMI plug from a long-discarded PC provided some nice springy steel strips that slide neatly under those tray tabs:
Drive EMI shield
That actually took a bit of trial-and-error:
AC Chassis mounting brackets – practice makes perfect
My first attempts used slightly thicker steel that didn’t fit nearly as well, plus I wasn’t quite sure how wide they should be.
As with nearly all plastic doodads around here, the white plastic mounting clips / brackets come from the M2:
Chassis Clips
The two brackets in the middle of the solid model slide around the tabs at the rear corners of the tray and capture the bent-over top section below the polycarbonate sheet.
The strip in the rear goes around the screws holding the heatsink to the sheet; more on that later.
The PLA brackets get themselves glued to the sheet with IPS #4 solvent adhesive, a hellish mixture of chlorinated hydrocarbons that attacks most plastics with gleeful enthusiasm. I positioned the brackets on the tray, slobbered adhesive on their tops, slapped the polycarbonate sheet in place, and applied clamps:
AC Chassis – gluing bracket blocks
The final bonds weren’t as uniform as I’d like, but they seem rugged enough. The lip along the rear of the tray was slightly higher on the left edge, which may have interfered with the clamping pressure; it’s obviously not a controlled dimension.
The tapped holes in the sheet accommodate screws for various bits & pieces.
The original Kenmore Model 158 sewing machine used a two-wire line cord:
Kenmore 158 – terminal block
In light of my modifications, grounding the frame seems prudent. The heap produced a long IEC extension cord with screw-mounting ears on the socket end that fit neatly into the GX270’s rear panel area occupied by two PCI cover plates, so a bit of Quality Shop Time seemed in order.
The GX270’s carcass yielded a complex bit of sheet metal that held the hard drive and a few other odds & ends, with some clean right-angle bends in exactly the right places:
Dell drive bracket – intact
Some bandsaw work removed the gimcrackery:
Dell drive bracket – first bandsaw pass
More bandsawing produced a rough outline:
Dell drive bracket – second bandsaw pass
Sawing to length, removing the small flanges, and squaring the sides:
Dell drive bracket – squaring edges
I traced the existing PCI cover tabs, bandsawed the outlines, and filed to suit:
Dell drive bracket – basic outline
They look a bit ragged, but fit well enough:
Dell drive bracket – trial fit – interior
From the outside, it looks like it grew there:
Dell drive bracket – trial fit – exterior
The divider between the PCI slots succumbed to tin snips and a bit of filing. The tabs climbing over the bottom edge come from the internal EMI shield covering the entire back surface.
A bit of coordinate drilling and manual milling produced the IEC socket outline
Dell drive bracket – drilling and milling
Which looks pretty good from the inside:
Dell drive bracket – IEC socket – interior
And great from the outside, if I do say so myself:
Dell drive bracket – IEC socket – exterior
Match-drilling a #6 clearance hole below the hole in the clamp arm, then ramming a self-tapping case screw into it, provides a convenient grounding point for the sewing machine cord:
IEC Socket Mount – ground screw
The chassis lid has two matching holes for those screw heads, which would ordinarily hold the two PCI cards in place. I could remove the clamp arm, but it doesn’t get in the way of anything.
Under ordinary circumstances, a fuseholder mounts in a square-ish panel cutout, but there’s no convenient panel to be found in the repurposed GX270 case. So now there’s a holder for the fuseholder stuck to the side of the power supply inside the case:
Fuseholder – installed
The square tube covers the entire fuseholder, with the quick-connect tabs protruding from the back, to provide enough surface area for the double-stick foam tape.
Looking down into the solid model, you can see the reduced width near the back end:
Fuseholder Holder
The black fuseholder contains a 5 A fast blow fuse, which should be entirely adequate for normal operation. In the event that a wire breaks loose and contacts the metal shell surrounding the whole chassis, it will pop instantly. That won’t disable the power supply, but it will remove line voltage from the entire motor controller chassis.
Remember that the source power line goes to the center QC tab, thus burying the always-hot contact deep in the fuseholder.
The OpenSCAD source code:
// Fuseholder mount
// Ed Nisley - KE4ZNU - August 2014
//- Extrusion parameters must match reality!
ThreadThick = 0.20;
ThreadWidth = 0.40;
HoleWindage = 0.2; // extra clearance
Protrusion = 0.1; // make holes end cleanly
AlignPinOD = 1.70; // assembly alignment pins: filament dia
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//----------------------
// Dimensions
Shell = [25.0,25]; // outside = bezel size + some stiffening
Mount = [17.3,15.7,21.0]; // mount section = slight compression in X
Base = [13.5,15.7,17.0]; // clearance over crimped contact
OAL = Mount[2] + Base[2];
//----------------------
// Useful routines
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);
}
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() {
translate([0,0,OAL/2])
cube([Shell[0],Shell[1],OAL],center=true);
translate([0,0,Base[2] + Mount[2]/2])
cube(Mount + [0,0,2*Protrusion],center=true);
translate([0,0,Base[2]/2])
cube(Base + [0,0,2*Protrusion],center=true);
}
This angled ring fits under a repurposed CPU cooler:
Blower Mount – solid model
Viewed perpendicular to the angled surface, it’s a circle, so what looks like a vertical cylinder is actually slightly oval to make the top come out right. That way, the walls are vertical, not angled, and it doesn’t stand crooked on the base plate.
Such a shape is trivially easy for a 3D printer:
Blower mount – on build platform
And looks about like you’d expect on the blower, which is why that surface must be a circle:
Blower Mount – bottom view
A trial fit in the case, along with a bunch of parts I haven’t written up yet:
Blower Mount – installed
Under normal circumstances, you’d want the blower a bit higher and level, but there just wasn’t anywhere else to fit the fuseholder. Besides, this way the airflow goes slightly upward toward the clearance over the top of that monster heatsink. Some air flows along the side of the heatsink to cool the isolated power supply you can’t quite see in the far corner of the chassis beyond that tangle of wires.
The angle seems pretty close to right, although I must get the rest of the circuitry running to know if the airflow can actually transfer the heat from the heatsink out of the case.
It doesn’t take much OpenSCAD source code to define the shape:
// Blower mount
// Ed Nisley - KE4ZNU - August 2014
//- Extrusion parameters must match reality!
ThreadThick = 0.20;
ThreadWidth = 0.40;
HoleWindage = 0.2; // extra clearance
Protrusion = 0.1; // make holes end cleanly
AlignPinOD = 1.70; // assembly alignment pins: filament dia
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
//----------------------
// Dimensions
MountOD = 85.0; // a bit smaller than the housing OD
MountID = 60.0; // carve out to reduce printing time
Base = 5.0; // minimum thickness (allowing for some overhang)
ElevationAngle = atan(20/90); // net tilt across fan base
ElevationDelta = MountOD * tan(ElevationAngle);
echo(str("Elevation angle: ",ElevationAngle," delta: ",ElevationDelta));
//----------------------
// Useful routines
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);
}
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() {
scale([1,cos(ElevationAngle),1])
cylinder(d=MountOD,h=Base + ElevationDelta);
translate([-MountOD,-MountOD/2,Base])
rotate([ElevationAngle,0,0])
cube([2*MountOD,2*MountOD,ElevationDelta],center=false);
translate([0,0,-Protrusion])
cylinder(d=MountID,h=Base + 3*ElevationDelta);
}
Dell built the GX270 I’m repurposing back in 2004, early on in the capacitor plague years, but only one of the system board caps showed signs of leakage:
Capacitor plague – 2004 Dell Edition
While I was harvesting some of the connectors, it occurred to me that those powdered iron inductors might make good current sensors, as they’re already wound with heavy gauge copper wires.
I picked an inductor with enough turns and, although slitting didn’t pose much of a problem, the saw did make a mess of the turns adjacent to the cut:
Powdered iron toroid – slitting
Iron powder has more magnetic remnance than ferrite, to the extent that iron swarf clogged the gap. After the first pass, I ran the slit toroid through the degausser to shake it clean and see what damage had been done. It looked OK, so I realigned it on the saw blade and continued the mission, with all the dust vanishing into the vacuum cleaner’s snout.
Removing the damaged sections left 22 turns. For comparison, I converted the 56 turn ferrite toroid into a 25 turn model by paralleling two 25 turn sections:
Slit toroids – iron – ferrite
The enamel wire on the iron toroid measures 40 mil diameter, close enough to 18 AWG.
Paralleling two 24 AWG windings on the ferrite toroid produces twice the copper area of a single winding, so the resistance is the same as a single 21 AWG winding (3 AWG steps = factor of two area change). That’s three steps smaller than the 18 AWG on the iron toroid, so the resistance is a factor of two larger than the heavier wire.
The paralleled winding has the advantage of reducing the power dissipation required to produce the same magnetic flux density, without the difficulty of winding heavier wire. That may not actually matter, given the relatively low currents required by the motor in normal operation.
Wedging a Hall sensor into the gaps and stepping the current produced two useful graphs:
Iron and ferrite toroids – Hall sensor output
The iron toroid has lower permittivity (less flux density for a given magnetizing force), which means the full-scale range exceeds 3 A and the useful range up to 1 A covers only 300 mV.
The last point on the ferrite curve shows the Hall sensor output saturating just over 4 V, with 1.5 V of range.
The slope, in mV/A
Powdered iron: 340
Ferrite: 540
Boosting the slope of the powdered iron by 25/22 gives 386 mV/A, so the iron permeability really is 70% of the ferrite. That’s modulo the gap size, of course, which surely differs by enough to throw out all the significant digits.
Obviously, an op amp circuit to remove the offset and rescale the output to 0-5 V will be in order.
The previous graph for the ferrite toroid with the complete 56 turn winding shows, as expected, about twice the output of this 25 turn version:
FT82-43 – 56 turns – 24 AWG
The linear part of that line is 1375 mV/A, although I can’t vouch that the data came from the same Hall effect sensor. Scaling it by 25/56 gives 613 mV/A, suggesting it’s not the same sensor.
Having developed an emotional attachment to the ferrite toroid, I’ll use it in the first pass of the current feedback circuit. If the motor need a bit less sensitivity or lower resistance, the powdered iron toroid looks like a winner.
Memo to self: Always degauss iron toroids before slitting!
The general idea is to gut an old Dell Optiplex GX270 and stuff the high-voltage parts of the sewing machine controller inside a well constructed and solidly grounded metal shield inside a not-too-ugly plastic box. It’d be nice to reuse the power control button and status LEDs on the front panel…
The few parts on the front of the through-hole board:
Dell Power Button PCB – component
The copper side, with annotations:
Dell Power Button PCB – copper
The red tracer on the ribbon cable goes to Pin 1, which is a blind key on the PCB.
The LEDs do not have ballast resistors, so those must go on a circuit board somewhere else.
The Smell of Molten Projects in the Morning 