## Heatsink Thermal Coefficients: Forced Air

Continuing the experiment with forced air, I added some of those fans. Same thermocouple arrangement as before, heatsink sitting on the bench with the fins horizontal and the fan blowing across the bench. This obviously leaves a bit to be desired as far as air flow control goes, but it’s close enough to get a general idea of what’s going on.

With the fan in the air flow straightener, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:

• R = 27.7 °C
• Bot = 23.4 °C
• HS = 66 °F = 18.9 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.16 °C/W

That’s more like it!

With the bare fan sitting on the bench, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:

• R = 26.7 °C
• Bot = 22.4 °C
• HS = 64 °F = 17.8 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.12 °C/W

The bare fan actually does a better job than the flow straightener. Just from the general feel of the breeze, I think the fan’s air flow entrains a bunch of ambient air and slams it across the entire heatsink, rather than hitting just the central area.

Encouraged by that, I doubled the power to 50 W: 2.6 A in the 6 Ω resistor = 41 W and 3 A (the limit of my bench supplies) in the 1 Ω resistor = 9 W. Because the heatsink is now getting energy from two sources, the heatsink temperatures won’t be directly comparable to the previous ones.

With the bare fan in the same position as before, 50 W dissipation:

• R = 36.8 °C
• Bot = 29.4 °C
• HS = 72 °F = 22.2 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.14 °C/W

The heatsink will be 7 °C above ambient at 50 W and the resistors 4.5 °C at 25 W each above that. The resistors will be 11.5 °C = 21 °F over ambient.

Again, the average of the Bot and HS temperatures might be more meaningful.

The heatsink has fins on both side, but so far I’ve been using only one fan. Putting another bare fan on the opposite side, also 1 diameter away, so the heatsink gets ambient air from sides, thusly:

That produces even better results:

• R = 33.9 °C
• Bot = 24.9 °C
• HS = 66 °F = 18.9 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.22 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.067 °C/W

The 6 Ω resistor is dissipating 41 W, rather than the 24 W I plan to use: figuring the resistor-to-heatsink thermal coefficient at 0.2 °C/W seems OK. The heatsink-to-ambient coefficient is breathtakingly good: cooling both sides seems like it ought to cut the thermal resistance in half and it does! Call it 0.1 °C/W.

So, bottom line: with two fans and 50 W, the heatsink will be 5 °C over ambient. Dissipating 25 W in each resistor will raise them 5 °C over the heatsink and 10 °C = 18 °F above ambient.

With the box ambient at 140 °F and two fans per heatsink, the resistors should tick along under 160 °F. That’s plenty toasty, but only slightly above my rule of thumb: If you can’t hold your thumb on it, it’s too damn hot. And, heck, we’re building a heater here, right?

On the other paw: six fans?

In reality, that layer of thermal goop between the case and heatsink determines much of the resistor temperature. One fan per heatsink should be entirely adequate. I should try this with one fan blowing parallel to the fins, with the notion of putting a fan directly upstream of each heatsink or between each pair of heatsinks.

The raw data: