Heatsink Thermal Coefficients: Convection

Having plugged the previous holes, I screwed down a pair of power resistors atop some heat sink compound: 6 Ω and 1 Ω. The general idea is that a stock PC power supply will dump about 50 W into the heatsink: 2 A @ 12 V = 24 W and 5 A @ 5 V = 25 W.

Thermocouples atop the 6 Ω resistor (R), on the opposite side of the heatsink just below that resistor (Bot), and at the far end of the heatsink (HS). They’re all held in place with foam blocks, in the hope that the steady-state air temperature between the foam and the resistor / heatsink will be pretty close to the temperature of the source.

Ambient in the Basement Laboratory is hovering around 60 °F = 15.6 °C, which is pretty mumble chilly. The numbers use the actual ambient for each test.

The first test powered just the 6 Ω resistor, because I wanted to find the natural convection capability of the heatsinks, which will be pretty low. Reaching steady state required a bit over an hour in each case; I recorded temperatures every ten minutes, which really chops up the day, but prevents forest fires.

Heatsink flat on its back in the worst possible orientation, atop a pair of wood blocks 35 mm off the bench:

• R = 64.3 °C
• Bot = 60.9 °C
• HS = 133 °F = 56.1 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.14 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.7 °C/W

Heatsink on edge, fins horizontal, clamped in a vise a few inches off the bench:

• R = 57.0 °C
• Bot = 52.4 °C
• HS = 121 °F = 49.4 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.19 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.4 °C/W

Heatsink on end, fins vertical (best orientation), same vise:

• R = 51.6 °C
• Bot = 46.4 °C
• HS = 114 °F = 45.6 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.22 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.2 °C/W

All those numbers are suspect, of course, but the general trend is comforting. The heatsink temperature might be better figured as the average of the Bot and HS values, but they’re pretty close.

Figuring ΘRB = 0.2 °C/W says the resistor will be 5 °C above the heatsink, which means we’re not dealing with insanely high temperature differentials. This is a Good Thing and shows that thermal compound helps.

Figuring ΘHA = 1.5 °C/W means convection really isn’t going to work, because at 50 W the heatsink will be 75 °C above ambient. That’s much too hot, but I need forced air flow to circulate hot air inside the box, anyway, so this is something of a worst-case situation.

The heatsinks will probably be in the second configuration, with fans blowing along the horizontal fins. If the fans fail, things will get downright toasty. We need a mechanical thermal cutout switch, heatsink temperature monitoring, and fan status feedback.

The raw data: