The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

ET227 Transistor: Monster Heatsink Mounting

Back in the day, heatsinks like this sat atop Moah Powah Pentium CPUs:

ET227 transistor on heatsink

I picked it because the hulking ET227 transistor fit neatly on its backside, it seemed capable of handling 30 to 50 W of power, and I have several of them in the Big Box o’ Heatsinks. No careful thermal analysis was involved…

Mounting it on the polycarbonate sheet inside the repurposed GX270 case involved drilling & tapping a pair of 6-32 holes in one side:

ET227 Heatsink – tapping

That’s not rigid tapping on a Sherline, it’s aligning a hand-turned tap in the spindle bore. Sorry.

And, yeah, you’re not supposed to leave the semiconductors mounted when you’re drilling the heatsink. I figure there’s nothing I can possibly do without using a hammer that will bother that transistor in the slightest. What, me worry?

The transistor collector runs at line voltage, which means the entire heatsink will pose a lethal shock hazard. I thought about isolating the collector and failed to come up with anything I’d trust to be both thermally conductive and electrically insulating over the long term; the screw heads must be isolated from the collector plate, too.

The screws stick out below the polycarbonate sheet, just above the grounded EMI shell lining the case, so I flattened them a bit:

ET227 Heatsink – mounting screws

The simple rectangular strip to the rear of the chassis mounting clips is just slightly thicker than the screw heads, so they can’t possibly contact the case:

Chassis Clips

It gets glued to the underside of the nearly invisible sheet:

ET227 heatsink – gluing screw shield

With Kapton tape over the heads, Just In Case:

ET227 Heatsink – mounted

It makes a nice linear counterpoint to the jumble of AC interface wiring:

AC Interface Chassis

The insulating sheet on the case lid came from the bottom of the original GX270 system board, where I think it served much the same purpose. It’s surely not rated for AC line voltages, but the thought must count for something:

AC Interface Chassis

More of the parts are flying in formation…

2014-09-29
Rectifier Mounting Screw Tweakage

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.

That was easy…

2014-09-26
If You Can Get To BNC, You Can Get To Anything
That’s what Mad Phil taught me, back in the day, and it’s still true:

15 W Dummy Load – Stacked Adapters

From the top:
- 15 W dummy load with N female
- N male to BNC female
- BNC male to UHF female
- UHF male to UHF male
- UHF female on homebrew antenna mount
Obviously, I don’t have enough adapters: I need one with N male to UHF male.

I actually spent money to get from the reverse-polarity SMA connector on the Wouxun radios directly to UHF female, matching the cable to the antenna mount in one step.

Sometimes an unsteady ziggurat of adapters isn’t appropriate.
2014-09-23

Panel-mount Fuseholder Holder

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 — 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:

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);
}

2014-09-22

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
FC1002 Frequency Counter Faceplate: BLAM!

So I picked up the frequency counter and found this:

FC1002 Frequency Counter – split shattered faceplate

The outer, previously cracked pieces of the faceplate split parallel to the front panel, separating into two layers, and popped free of their mount. The layer closest to the panel remains intact.

The fragments were flexible and the bottom layer was rigid, suggesting the faceplate consisted of two parts, perhaps an acrylic (?) base with a soft silicone (?) poured atop it for armor and scratch protection.

It still works fine and the acrylic (?) layer will suffice for my simple needs, despite being slightly marred by the cyanoacrylate glue I slobbered into the cracks.

I definitely didn’t see that coming…

2014-09-17
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