Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
I decided to replace the sawed-off flanges on that salvaged heatsink to make all three use the same mounting arrangement, whatever that might turn out to be.
Nothing particularly fancy about it: two random chunks of aluminum sheet and two thinner strips, sanded to roughen their surfaces, and epoxied into place.
The repaired heatsink is marginally taller than its siblings, but not so anybody will ever notice, and it’s no more off-kilter than they are, either.
A quartet of 5/16-inch lathe bits provided the right spacing to hold the heatsink over its new flanges while the epoxy filled in all the gaps and irregularities. I probably should have paid a bit more attention to squaring things up, but it’s good enough for what it’ll need to do.
Found this inside a friend’s dead USB memory stick:
Cold solder joint in USB memory
The leads come from a teeny 12 MHz crystal. The solder blob on the other side looked just fine, but you simply can’t tell by looking.
As it turned out, the stick was dead for some other reason: the Flash memory controller chip got hot when the stick was drawing power. Resoldering all the joints had no effect, which wasn’t surprising.
I suspect a killer static discharge or some such calamity.
The big one is rated 50 W @ 25 °C ambient. Use two, derated by 50%, times three air-cooled heatsinks for 150 W of low-temperature heating. The little one is 25 W @ 25 °C.
The derating curve is linear from 100% @ 25 °C down to 10% @ 250 °C, when mounted to a square foot of flat aluminum plate: -0.40% / °C.
Assuming a max heater ambient of 150 °F = 65 °C, you can use 84% of full power. Derating by 50% isn’t all that unreasonable.
The relevant hole locations:
50 W: X=1.562 inch / 39.67 mm Y=0.844 inch / 21.44 mm
25 W: X=0.719 inch / 18.26 mm Y=0.781 inch / 19.84 mm
10 W: X=0.562 inch / 14.27 mm Y=0.625 inch / 15.88 mm
Divide those by 2.0 for from-the-center offsets, which may be more useful for manual CNC operations: zero at the resistor mounting center, then back-and-forth from there.
The mounting hole size for 25 & 50 W resistors: 0.125 inch / 3.18 mm diameter, just exactly what you want for a 4-40 mounting screw. Tap drill #43, clearance drill #32 (close fit) or #30 (loose fit).
The mounting hole size for 10 W resistors: 0.094 inch / 2.39 mm to fit a 2-56 screw. Tap drill #50 (better: #49 for 50% threads), clearance drill #43 (close) or #41 (loose).
To get an idea of how those recycled heatsinks performed, I soldered a pair of 8 Ω 25 W power resistors in series, clamped them to the first heatsink out of the dishwasher, fired up a bench power supply, and took some quick data.
Ambient is about 63 °F with more-or-less still air. Temperature measured with an IR non-contact thermometer aimed at a strip of masking tape on the edge of the heatsink. The resistors (and the center of the heatsink) are somewhat hotter than that, as you’d expect. The numbers include the resistor case-to-heatsink thermal coefficient, too.
Held edgewise in a vise with the fins horizontal (like this: ===, the second-worst possible orientation), a few inches above the bench, the temperature stabilizes in about an hour:
16 W -> 101 °F: 2.4 °F/W
32 W -> 132 °F: 2.2 °F/W
64 W -> 188 °F: 2.0 °F/W
The alert reader will note that 64 W is somewhat excessive, given that the resistors are 25 W each. The temptation to run the supply at constant currents of 1.0, 1.4, and 2.0 amps was just impossible to resist, OK?
Heatsink – vertical
Held edgewise with the fins vertical (like this: |||), also with a few inches of clearance to the bench, the temperature stabilized in a matter of 10-20 minutes. I didn’t bother with the lower power tests:
64 W -> 166 °F: 1.6 °F/W
Putting a bare CPU case fan 2 inches from one side of the heatsink, aimed directly at the middle, with no attention whatsoever to ducting or air flow rates, produced a stable temperature in a few minutes:
64 W -> 85 °F: 0.3 °F/W
That’s under 0.2 °C/W with airflow on only one side. Zowie!
While I must run these tests again with the resistors & fans I intend to use (and better control over the air flow), things are looking good.
Some diligent rummaging turned up a trio of heatsinks that I think will work nicely for the Hot Box Disinsector. As nearly as I can tell from the date codes, they started life at IBM in the early 70s and are built to take a direct hit; the bent fins show they’ve taken a few shots along the way. Those clips applied direct clamping pressure to the transistor cases: much better than screws that can will eventually deform the aluminum and stop forcing the case against the heatsink.
Heatsink with TO-3 transistors
To judge from the crud built up on the fins and the fact that some of the transistors are now completely open, these puppies were run hard and put up wet.
I wiped off the heatsink grease, cleaned off the bigger chunks of crud, and popped them in the dishwasher for a good scrubbing.
Heatsinks ready for reuse
Lovely! The web across the middle is 1/4-inch thick; they don’t make ’em like that any more. The rear heatsink lost its mounting flanges along the way; I have no idea if I sawed those off or somebody else got there first.
I’ll probably plug the holes, just because it’s a nice idea; the sinks are certainly overqualified for their next job as they stand. TO-3 transistors have power ratings over 100 W: these are at least 300 W heatsinks, albeit with an unknown operating temperature. Egad & similar remarks.
In round numbers, I need maybe 150 W of heat at 140 °F ambient. Each heatsink will dissipate 50 W, which is certainly well under its original rating. Figuring 0.5 °C/W sink-to-ambient (call it 1 °F/W) with decent air flow, dissipating 50 W will raise the heatsinks to 190 °F.
That’s higher than I want, so more doodling & measurements are in order.
This Peltier cooler just emerged from a pile o’ stuff on the Electronics Workbench, so I combined it with a scrap CPU heatsink (using plain old water as “thermal grease”) and fired it up to get some quick numbers for future reference.
Peltier cooler test lashup
It draws 3 A (the bench supply’s current limit) at 5 V. The cold side got down to 19 °F with the hot side at 75 °F: ΔT = 56 °F.
That’s with zero thermal load, other than whatever arrives from plain old air and those two plastic clamps. It looks like a nice one, so it’s maybe 10% efficient and could pump a watt, barely enough to cool a simple circuit.
Freezes a drop of water just fine, though.
The I-V curve is nearly bar-straight over the first five volts: call it 620 mΩ. The thing would draw 7.5 A at 12 V, call it 90 W, and could pump maybe a whopping 9 W from the cold side.
Actually getting good numbers would require some serious work that I’m not up for. In particular, everything has a serious temperature coefficient, so nothing would be the way it looks. I have doubts about the efficiency guesstimate; I’d like to actually measure that sometime.
But it confirms my opinion of Peltier coolers between hundred-watt CPUs and water-cooled heatsinks: pure delusion.
While excavating the top of my workbench and putting things away, I managed to drop my favorite needle-nose tweezers… which, of course, landed point-down on the concrete floor:
Mismatched tweezer jaw
Well, that gave me an excuse to match up the jaws. If you take a close look at most of your tweezers, they’ll have jaws that don’t quite come together evenly, so you’re trying to grab things with a single point instead of between two flat surfaces.
A brief session with coarse and medium diamond files produced this pleasing result (with a mm scale for size):
Matching tweezer jaws
Much better!
Another trick that works well: grab a piece of fine sandpaper in the tweezers, scrub sideways, and repeat for the other jaw. That’ll flatten out the jaws, make them reasonably parallel, and put the scratches in the direction that helps the most when you’re pulling something. Works best if the jaws are already pretty well aligned.
