Installed Vortex II fan today

Lynnmor wrote:

Putting silicon under resisters might be a bad idea, as they produce heat.

That seems like a valid point. I assumed that since silicone can handle temperatures above about 500F that there would be no problems, but lets investigate further.

So I took the fan out and measured it.

The measured currents in the forward mode were:
0.48 amps (low)
0.64 amps (med)
0.99 amps (high)

There are three 5-ohm, 20 watt resistors (each measure about 5.5 ohms). The forward mode uses zero, one, or two resistors. The reverse mode uses zero or one resistor.

For the low-speed case, we have 0.48A through two 5.5-ohm resistors, so the power in each resistor is:

(5.5)*(0.48)^2 = 1.27 watts each

For the medium speed case, we have:

(5.5)*(0.64)^2 = 2.25 watts

So our worst case for heat in the resistors is the medium speed case of 2.25W. We also know that this is far below the 20W rating of the resistors.

For the thermal analysis I ran the fan for about an hour at medium speed and used a thermal camera to record the temperature of the hottest resistor. This was done at an ambient temperature of 60F (black color) and the hottest temperature recorded was 113F (white color). This is about 53 degrees above the ambient temperature.




This was repeated for the low-speed case where two resistors are used in series. Here the hottest temperature was 86F.



For completeness, the test was run on medium speed in the reverse direction to measure the temperature of the third resistor. It got to 111F, similar to the forward medium-speed case.



The maximum temperature for one-part silicone adhesive (room temperature cure) is 572F. This is the only thing actually touching the resistors. Ceramic resistors have a max operating temperature of around 250C (480F). Solder melts at about 190C (375F). The Standard glass-epoxy (FR-4) circuit boards have a service temperature up to about 110C (230F). The fan housing seems to be made of ABS plastic, which melts at around 105C (221F).

If we adjust the ambient temperature from 60F to 110F and add the increase in temperature of the resistor, we end up at 163F. This is far below the damage temperature of anything in the vicinity. In fact, it's not much hotter than domestic hot water.

So we can conclude that there is not a thermal issue with under-filling the resistors with silicone.