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…
The handlebar-mounted PTT button for the amateur radio on my bike once again went toes-up, most like due to the accumlation of road dust and rainwater over the years. Rather than replace the switch, which would require peeling off a massive glob of hot melt glue and resoldering the wires, I just carved the tops off the rivets holding the cover in place, pried off the cover, and removed the button to reveal the top of the switch dome:
Handlebar PTT switch – corroded dome
Blech!
The dome flexes outward to contact the (rather crusty) terminals on either side, so all the action happens under the dome.
A lineup of the plastic button, the inverted dome, and the cover plate:
Handlebar PTT switch – components
The top and bottom of the dome show some grit: that’s where it contacted the switch terminals.
Wiping the crud out of the switch body, scrubulating everything with contact cleaner, and putting it all back together restored the switch to working order. There’s (once again) a snippet of Kapton tape over the cover holding it in place, but I don’t expect this to last very long:
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.
I didn’t bring the HDR-AS30V camera along on the Hudson River ride, simply because each battery lasts about 1.5 hr in 1920×1080 @ 60 fps mode and I wasn’t up to replacing batteries during the ride, then charging all three every evening. Obviously, the camera wasn’t intended for that use case.
Somewhat surprisingly, the Wasabi batteries deliver the same continuous run time as the Sony battery: 1:30 vs 1:33. I used 250 mA for those discharge curves, but I think something around 500 mA would better match the camera load.
I’m sorely tempted to drill a hole in the camera’s case and wire in a honkin’ big prismatic lithium cell.
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 Smell of Molten Projects in the Morning 