Category: Electronics Workbench

Just got a stack of surplus fans for the heatsinks I’ll probably use in the disinsector: Cofan FP925HH12B. Either these are really old or there’s a typo in the model number printed on the fan label. I’d expect it to be …9225… to indicate a 92 mm frame x 25 mm thickness. It’s close enough to get to their specs page.

They draw 370 mA (rated 450 mA) in free air = 4.4 W. The “HH” designation means “Super High Speed” and wow are they noisy despite their 37.6 dbA spec. Fortunately, that won’t matter inside a closed box.

The label is downstream, which means the hub and bare blades are upstream. They’ll be under the shelves, so I think I can get away without guards.

The model number breakdown says that they have a “protection chip” which seems to be the thing that produces the status output. That output (yellow wire) is a square wave with a frequency twice the fan’s actual speed: 107 Hz → 6420 cycles / min → 3210 RPM. The open collector output requires a pullup resistor to +5 V. It take a while to spin up the fan enough to get a decent output waveform; don’t sample the status for maybe a second after power-on.

Putting a fan on that crude air flow straightener produces about 3.2 m/s across the exit end, which works out to 10.5 ft/s = 630 ft/min. There’s a distinct flow dent near the middle of the air column, so I suspect the baffles are too far upstream for this fan.

Making a number of barely justifiable assumptions, the output is around 1.4 m³/min = 50 ft³/min, which is close enough to the rated free air output of 1.8 m³/min = 64 ft³/min.

They will start and run at 5 V (below the 8 V minimum rating), draw 110 mA, spin at 1470 RPM, and push 1.1 m/s. For whatever that’s worth.

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?

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.

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.

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.