Tag: Sewing

Fabric arts and machines

Current Sensing: Powered Iron Toroid
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!
2014-09-18
Dell Optiplex GX270 Power Control PCB Connections

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 connections:

16 14 12 10 8 6 4 2

Gnd nc nc nc nc HD+ HD- Button+

Gnd nc Gnd Pwr Y+ Gnd Pwr G+ Gnd Key

15 13 11 9 7 5 3 1

2014-09-12
Dell Power Supply: Extracting Some AC
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:
- White – neutral
- Black – hot
The nice thing about standard color codes: everybody can have one!

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…
2014-09-04
ET227 Transistor DC Current Gain Variation

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 I_B 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 I_B = 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 I_B = 30 A, V_BE 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 I_B = 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 I_B = 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…

2014-08-29
FT82-43 Slit Toroid: Calibration

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.

2014-08-26

FT82-43 Slit Toroid: Armor

Given the fragility of ferrite toroids in general and slit toroids in particular, a touch of up-armoring seems sensible:

FT82-43 toroid - mounted — FT82-43 toroid – mounted

The solid model includes a toroid shell with roughly the right curves:

Toroid Mount - Show layout — 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 — 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();
}

2014-08-22

FT82-43 Slit Toroid: Construction

The FT82-43 toroid slit easily enough, using the same diamond-wheel Sherline setup as for the smaller toroids:

FT82-43 toroid – slit

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.

Now, for a bit of armor…

2014-08-21

16	14	12	10	8	6	4	2
Gnd	nc	nc	nc	nc	HD+	HD-	Button+
Gnd	nc	Gnd	Pwr Y+	Gnd	Pwr G+	Gnd	Key
15	13	11	9	7	5	3	1

	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

	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

Tag: Sewing

Current Sensing: Powered Iron Toroid

Dell Optiplex GX270 Power Control PCB Connections

Dell Power Supply: Extracting Some AC

ET227 Transistor DC Current Gain Variation

FT82-43 Slit Toroid: Calibration

FT82-43 Slit Toroid: Armor

FT82-43 Slit Toroid: Construction