Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The case from a Dell Optiplex GX270 will hold the Kenmore 158 sewing machine’s motor control electronics, because it has a well-grounded metal box inside the plastic shell that will protect fragile humans from line voltages. The GX270 power supply will suffice for the usual stuff, but the bridge rectifier, power transistor, and suchlike require a direct connection to the AC line.
Rather than add another plug, I soldered a nice two-wire line cord to the IEC socket terminals inside the GX270’s power supply:
Modified Dell power supply – interior
The cord follows the IEC/EU standard color code:
Blue – neutral
Brown – hot
The power supply follows the US standard color code:
The yellow cable tie anchors the cord to a metal tab that, when bent at right angles, provides a convenient exit from the power supply at exactly the right location:
Modified Dell power supply – AC cord exit
The power supply mounts with the label facing inward, directly adjacent to the PCI slot covers. The new cord emerges near the bottom, inside the recess that formerly accommodated the board.
Definitely not UL approved, but we’re well beyond that stage anyway…
A Squidwrench Weekly Doings being useful for short-attention-span projects, I measured the DC current gain for all five ET227 transistors. The test conditions fall far below the ET227’s 1 kV / 100 A ratings, but they’re roughly what the sewing machine motor controller calls for.
The transistors don’t even begin to turn on until IB gets over about 50 mA, because there’s a 13 Ω shunt resistor (as measured, for either polarity) between the base and emitter terminal:
Fuji ET227 – equivalent circuit
In the ET227’s normal use, that resistor dumps the Miller effect charge injected from the collector (with the intent of improving the switching time), but you must ram nearly 70 mA into the resistor to get 900 mV at the base, so the actual transistor base current isn’t all that high for low collector currents. But you measure gain by dividing goes-outa by goes-inta, so that’s what I’ll do.
The ET227 needs something like IB = 30 A to switch 100 A at the collector, so a few dozen mA into that resistor rounds off to zilch for its usual driver circuit. FWIW, with IB = 30 A, VBE tops out at 2 V: the resistor carries 150 mA and dissipates 300 mW.
Anyhow, randomly labeling the transistors from A (on the heatsink) through E, then hitching them up to a 1.8 A bench supply with a 33 Ω resistor to the base terminal provided some readings at single-digit collector voltages.
For IB = 72 mA:
IB
IC
hFE
A
72
490
6.8
B
73
540
7.4
C
74
480
6.5
D
75
440
5.9
E
76
520
6.8
For IB = 108 mA, with one bumped-knob outlier:
IB
IC
hFE
A
108
1220
11.3
B
101
1190
11.8
C
108
1280
11.9
D
108
1170
10.8
E
108
1320
12.2
Although the gain around 1 A comes out slightly higher than while running the motor, it’s in the same ballpark. This is not a high-gain device: it’ll need a driver after the optoisolator to squeeze enough current through the collector.
Eks tried to unload a huge old Tek transistor curve tracer on me that would be ideal for this sort of thing. I’m still not tempted…
I’d have trouble faking this with a straight face:
FT82-43 – 56 turns – 24 AWG
That’s measured with the 56 turn winding connected directly to a bench power supply, cranking up the current, taking the reading, and turning the current back down again, so as to avoid cooking the poor thing inside its PLA armor:
FT82-43 toroid – mounted
The “49E” sensor came from one of the bags of eBay fallout. They saturate around 4.25 V; the outputs above 4 V lose their linearity due to the sensor, not ferrite saturation.
The original calculations guesstimates suggested 25 turns would produce full scale at 5 A, so 56 turns should top out at 2.2 A. Frankly, given all the imponderables in this lashup, a factor of two seems pretty close.
Offsetting the output by -1 A would yield a 2 A range that’s just about exactly right. Unfortunately, some fiddling about with neodymium magnets suggests that you (well, I) can’t stuff enough opposing field into the slit without saturating (some part of) the ferrite core, reducing the permeability, and blowing all the assumptions.
So that suggests a buck winding, obviously with more turns to allow less current for the same magnetizing force. Wrapping 110 turns reduces the buck current to 500 mA and assuming a bit over an inch/turn requires 10 feet, which is nearly 1 Ω of 30 AWG wire: the buck current dumps another 250 mW into (a somewhat larger version of) that PLA armor.
Or just throw away half of the Hall effect sensor range and use an op amp along the lines of the LED current sensor.
Given the fragility of ferrite toroids in general and slit toroids in particular, a touch of up-armoring seems sensible:
FT82-43 toroid – mounted
The solid model includes a toroid shell with roughly the right curves:
Toroid Mount – Show layout
That puts a nice rounded shape on the bottom of the armor, not that that makes much difference:
Toroid Mount – Build layout
The central hole passes a 4-40 brass, nylon, or stainless steel screw. Most of the magnetic field stays within the ferrite and, heck, this isn’t a crazy-sensitive analog application, so even an ordinary steel screw shouldn’t cause any particular problems.
The rectangular (not pie-wedge) slit barely passes the Hall effect sensor.
I’ll pour some clear epoxy over the toroid, with tape masking the ferrite core and sealing the ends, to immobilize the windings. That sounds like a good idea after calibration and suchlike.
The OpenSCAD source code, which should be sufficiently parametric that I can crank ’em out for all the other toroids large enough to accept a screw:
// Toroid coil mounting bracket
// Ed Nisley - KE4ZNU - August 2014
Layout = "Mount"; // Coil Mount Build Show
//- Extrusion parameters must match reality!
// Print with 4 shells and 3 solid layers
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
ID = 0; // subscripts for cylindrical objects
OD = 1;
LEN = 2;
Coil = [10.25,23.50,8.3]; // wound toroid core
SensorThick = 2.0;
BaseThick = IntegerMultiple(1.0,ThreadThick); // baseplate under coil
WallThick = IntegerMultiple(1.0,ThreadWidth); // walls beside coil
ScrewHoleDia = 4.0; // allow alignment slop around 3 mm / #4 screws
//----------------------
// 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,
$fn=Sides);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
RangeX = floor(100 / Space);
RangeY = floor(125 / Space);
for (x=[-RangeX:RangeX])
for (y=[-RangeY:RangeY])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//----------------------
// Basic coil shape
module CoilShape() {
CornerRadius = min((Coil[LEN] / 2),((Coil[OD] - Coil[ID]) / 2)) / 3;
MidRadius = (Coil[ID] + Coil[OD]) / 4;
HalfX = (Coil[OD] - Coil[ID]) / 4 - CornerRadius;
HalfY = (Coil[LEN] / 2) - CornerRadius;
echo(CornerRadius,MidRadius,HalfX,HalfY);
color("Goldenrod")
render(convexity = 2)
rotate(180/20)
rotate_extrude(convexity=3,$fn=20)
translate([MidRadius,0])
hull()
for (i=[-1,1],j=[-1,1])
translate([i*HalfX,j*HalfY])
circle(r=CornerRadius,$fn=24);
}
//----------------------
// Mount
module Mount() {
difference() {
rotate(180/20)
cylinder(h=(BaseThick + Coil[LEN]),d=(Coil[OD] + 2*WallThick),$fn=20);
translate([0,0,-Coil[LEN]]) // make screw hole
rotate(180/6)
PolyCyl(ScrewHoleDia,3*Coil[LEN],$fn=6);
translate([0,0,BaseThick + Coil[LEN]/2]) // set bottom curve
CoilShape();
translate([0,0,BaseThick + Coil[LEN]]) // clear out top
CoilShape();
translate([(Coil[ID]/2 + Coil[OD]/2),0,0])
cube([Coil[OD],SensorThick,3*Coil[LEN]],center=true);
}
}
ShowPegGrid();
if (Layout == "Coil") {
CoilShape();
}
if (Layout == "Mount")
Mount();
if (Layout == "Show") {
Mount();
translate([0,0,(BaseThick + Coil[LEN]/2)])
CoilShape();
}
if (Layout == "Build") {
Mount();
}
I’m pretty sure that chip at 1 o’clock happened while it was clamped in the vise between two cardboard sheets, but I haven’t a clue as how it got that much force. In any event, that shouldn’t affect the results very much, right up until it snaps in two.
Although the current will come from a (rectified) 120 VAC source, the winding will support only as much voltage as comes from the IR drop and inductive reactance, which shouldn’t be more than a fraction of a volt. Nevertheless, I wound the core with transformer tape:
FT82-43 toroid – wrapped
That’s 3M 4161-11 electrical tape (apparently out of production, but perhaps equivalent to 3M’s Super 10 tape) cut into half-foot lengths, slit to 100 mils, and wrapped ever so gently.
The thickest offering from the Big Box o’ Specialty Wire was 24 AWG, so that’s what I wound on it:
FT82-43 toroid – wound
That’s 56 turns, which should convert 2.2 A into 1000 G (enough to max out the Hall effect sensor) and is more in keeping with 24 AWG wire’s 3.5 A current rating.
The insulated core requires just under 1 inch/turn, so figure the length at 56 inch. The wire tables show 26.2 Ω/1000 ft, so the DC winding resistance should be 120 mΩ. My desk meter has 0.1 Ω resolution, which is exactly the difference between shorted probes and probes across the coil: close enough.
The inductance is 170 µH, so the inductive reactance at 120 Hz = 128 mΩ.
The motor winding resistance limits the peak current to about 200 V / 40 Ω = 5 A, in the absence of the transistor current limiter, and, if it gets above that, things have gone very, very wrong. Mostly, I expect currents under 1 A and it may be useful to reduce the full scale appropriately.
The cheap eBay “SS49” Hall effect sensors I’m using produce anywhere between 0.9 and 1.8 mV/G; I’ll use 1.4 mV/G, which is at least close to the original Honeywell spec. That allows a bit over ±1000 G around the sensor’s VCC/2 bias within its output voltage range (the original datasheet says minimum ±650 G), so I’ll use B = 1000 G as the maximum magnetic flux density. The overall calibration will be output voltage / input current and I’m not above doing a one-off calibration run and baking the constant into the firmware.
The effective mean path length turns out to be a useful value for a slit toroid:
effective MPL = (toroid MPL - air gap length) + (µ · air gap length)
The SS49 style sensor spec says they’re 1.6 mm thick, and the saw-cut gaps run a bit more, but 1.5 mm will be close enough for now.
The relation between all those values:
B = 0.4 π µ NI / (effective MPL)
Solving for NI:
NI = B · (eff MPL) / (0.4 π µ)
Solving for N:
N = B · (eff MPL) / (0.4 π µ I)
You always round up the result for N, because fractional turns aren’t a thing you can do with a toroid.
The saturation flux density seems to be measured at H = 10 Oe, but that applies to the intact toroids. The air gap dramatically reduces the effective µ, so you must apply a higher H to get the same B in the ferrite at saturation. At least, I think that’s the way it should work.
H = 0.4 π NI / (geometric MPL)
Then:
FT50-61: H = 58 Oe
FT82-43: H = 30 Oe
I’m surely missing some second-order effect that invalidates all those numbers.
Figuring the wire size for the windings:
FT50:
ID = 0.281 inch
Circumference = 0.882 inch
28 turns → wire OD = 0.882/28 = 31 mil
20 AWG without insulation
FT82:
ID = 0.520 inch
Circumference = 1.63 inch
25 turns → wire OD = 1.63/25 = 65 mil
14 AWG without insulation
Of course, the wire needs insulation, but, even so, the FT82 allows a more rational wire size.
Page 4.12 of the writeup from Magnetics Inc has equations and a helpful chart. They suggest water cooling a diamond-bonded wheel during the slitting operation; my slapdash technique worked only because I took candy-ass cuts.
Mulling over how to add a 1/rev sensor to the sewing machine motor, it occurred to me that simply drilling a hole through the pulley would provide a clean optical path and a convenient 2/rev output signal.
However, the OEM pulley doesn’t extend beyond the end of the shaft:
Kenmore 158 – AC drive motor – overview
Rather than drill a hole in the shaft or (attempt to) affix something onto a pulley that spins at 10 kRPM, I figured I should make another pulley and mutilate that.
Because this will surely call for more than one new pulley before I get everything right, a lathe form tool seemed in order. Introducing a suitable blank from the Bin o’ 1/4 Inch Bits to Mr. Bench Grinder produced a likely looking candidate, with an included angle of about 35° (a skosh over 17° on each side) and sized just a wee bit narrower than the pulley groove.
From the top:
Pulley form tool – top
From the side:
Pulley form tool – side
Skim the surface of a 5/8 inch rod to match the pulley OD, plunge a cutoff tool to make most of the cut, insert bit in holder, align perpendicular to workpiece, line up to center of cut, slobber on more cutting lube:
Pulley form tool – prepared blank
Plunging the tool slowly into the cut produces … no chips … nothing … smoke?
Come to find out that the Bin o’ 1/4 Inch Bits contained not just lathe tool bits & blanks made from tool steel, but one length of 1/4 inch square key stock made from ordinary soft steel:
Pulley form tool – damage
I should have known that from the type of sparks flying off the grinding wheel, right?
You knew that just from looking at the first picture, because a real lathe bit blank wouldn’t be all beat to shit…
The Smell of Molten Projects in the Morning 