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Archive for December 10th, 2015

Hard Drive Platter Mood Light: Thermal (Mis)Management

So another knockoff Neopixel started flickering and its blue LED went dark:

Hard Drive Mood Light - blue failure

Hard Drive Mood Light – blue failure

Squirting it with circuit cooler brought it back to life, albeit briefly, so it’s a real thermal failure. OK, after I get smacked upside the head twice, I can recognize a problem when I see it.

I removed the top cover and jammed a themocouple into the screw hole in the middle of the pillar:

Mood Light - thermocouple location

Mood Light – thermocouple location

A folded tissue weighted down with random desktop junk kept the breeze out of the interior:

Mood Light - PWM 128 temperature measurement

Mood Light – PWM 128 temperature measurement

If the middle of the column hits 50 °C, what’s it like inside the 5050 packages with all those LEDs blazing away? Looks like I’ve been cooking those poor knockoff Neopixels to death.

The temperature is 50 °C with the LEDs running at maximum PWM = 128. Reducing the maximum PWM to 64 reduces the core to 30 °C and that dead blue LED springs back to life.

Figuring each LED package dissipate 250-ish mW at full throttle, that’s 120 mW at PWM 128 / 60 mW at PWM 64. The set of 12 packages dissipates 1.4 W / 750 mW, so, in a 22 °C room, the thermal coefficient is up around 10 to 20 °C/W, which is somewhere between bad and awful. Running the LEDs at full throttle obviously isn’t an option and even half-throttle really doesn’t work.

So, OK, mounting LED strips on a clever 3D printed plastic column with zero air circulation isn’t nearly as smart an idea as I thought: barely more than a watt burns right through the redline.

The Neopixel specs have nothing to say about the thermal coefficient from the LED junctions to the package leads, but cooling the copper conductors in the flex PCB can’t possibly hurt.

No, I do not want to CNC machine an aluminum pillar with little tabs on the platter for better heatsinking. It would be an interesting design project, though.

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