Category: Electronics Workbench

Electrical & Electronic gadgets

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
NP-BX1 Lithium Batteries: 6 Month Status

The battery capacities after six months are, of course, lower:

Sony NP-BX1 – OEM Wasabi – 2014-08-17

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.

2014-08-25

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
Bike Helmet Earbud Iteration

Based on having to seal the rear vent hole of the previous earbud, I did the same for the new one:

Earbud – blocked vent

The audio quality was terrible, so I tried another bud with a foam windscreen over the hole and a hole punched in the middle of the double-sided white foam tape:

Earbud – foam over vent

The audio remained unintelligible, so I tried an upscale (but still cheap, because surplus) Koss earbud, first without blocking the vents and then with snippets of Kapton tape:

Koss earbud – tape over vent

The earphone has three slits on each side, but only the middle slit has a hole penetrating the case; it must be a stylin’ thing.

That sounded better, so I’ll roll with it. There’s supposed to be a foam cover over the housing, but those things always get grody and fall off; there’s not much point.

As nearly as I can tell, contemporary earbud designs optimize for volume (dBm/mV) and thumpin’ bass, all to the detriment of actual audio quality. Based on numerous samples over the years, there is zero correlation between price (admittedly, on the low end) and audio quality (admittedly, with my crappy hearing).

I own a pair of very nice (and thoroughly obsolete) Shure E2c sound-isolating ear beetles that sound great (even with my crappy hearing), but I’m unwilling to chop them up for the bike headset …

2014-08-18
Gapped Ferrite Toroid: 5 A Calculations
Using a Hall effect sensor to report on the Kenmore 158’s universal motor current puts different limits on the ferrite toroid than the LED current sensor: higher current, bigger wires, and mandatory galvanic isolation. One could, of course, just buy an Allegro ACS713/4/5 (or whatever) sensor from, say, Digikey, but, for a one-off project, it’s more interesting to run the numbers and build the thing.

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 V_CC/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.

FT50-61 toroid:
- µ = 125
- Saturation B = 2350 G
- MPL = 3.02 cm
- Effective MPL = (3.02 – 0.15) + (125 · 0.15) = 21.6 cm
- N = 28 turns
A somewhat larger FT82-43 toroid:
- µ = 850
- Saturation B = 2750 G
- MPL = 5.26 cm
- Effective MPL = (5.26 – 0.15) + (850 · 0.15) = 133 cm
- N = 25 turns
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.

A table of magnet wire sizes with varying insulation from Cooner Wire.

Some general notes about building & measuring inductors from the University of Denver.

Doodles for the FT82-43:

FT82-43 Doodles

Doodles for the FT50-61:

FT50-61 Doodles

Running the numbers using the Magnetics Inc equations:

Ferrite Gap Doodles
2014-08-14