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Archive for December 12th, 2010

Heatsink Thermal Coefficient

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

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.

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