Update: I just got my hands on a new Vortex II fan, and this problem no longer exists. Here is a picture of the new circuit board with the resistors properly mounted against the board:
Unfortunately, they still use wire nuts with electrical tape (which is not holding well already), but those can be easily replaced with crimp connectors, which are far more reliable in a vibrational environment.
Unfortunately, they still use wire nuts with electrical tape (which is not holding well already), but those can be easily replaced with crimp connectors, which are far more reliable in a vibrational environment.
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.
I installed a Vortex II, but I had heard about reliability problems. By inspecting the circuit board, it was obvious that the big, heavy resistors could never hold-up in a high-vibration and shock environment of an RV. So I under-filled the resistors with silicon to support them better. This is actually a pretty common practice in electronics; I don't know why Vortex doesn't do it. It's been 5 seasons with no problems yet.
I tried a PWM speed controller on my old single-speed Vortex, but noticed that it had a lot of parasitic current draw, even when turned off. The one I used also made an objectionable high-pitch noise when it was on. I decided not to use it. Instead, I installed it in my stove vent fan circuit downstream of the switch, and I love it there.
Click For Full-Size Image.
I tried a PWM speed controller on my old single-speed Vortex, but noticed that it had a lot of parasitic current draw, even when turned off. The one I used also made an objectionable high-pitch noise when it was on. I decided not to use it. Instead, I installed it in my stove vent fan circuit downstream of the switch, and I love it there.
Click For Full-Size Image.
The multiple speed fans use resistors to select the speed. I bought the single speed fans and used a PWM and reversing switch so that there is variable speed forward and reverse. Doing it this way, it is quieter, more efficient and cost about the same or less.
