Converted my fantastic vent fan to a PWM speed controller, cheap and highly recommended mod

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Vannautical engineer

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I was recently camping in some fairly hot weather, and I left my fantastic fan running on low almost all the time to keep the inside of the van at least at the ambient outside temp. Unfortunately even at the lowest of the three speeds, the fan draws around 1 or 1.5 amps, which equates (conservatively) to 24ah if you have it on for an entire day and night. That's about a quarter of my whole 100ah battery every day just to keep the fan going!

I converted the fan over to a PWM (pulse width modulated) speed controller. The factory speed controller on the fantastic fan is pretty much 1950s technology. It just runs the power through some coiled up resistive wires to lower the speed of the fan. The two big drawbacks of this are that these resistive wires use power all on their own, and they actually get hot, like uncomfortably hot to the touch. So as you are trying to cool your van, these little wires are wasting your battery and pumping out heat. Not ideal!

There are a few tutorials on this on youtube, but I like this one the best for one specific reason.



While this is not a roof vent fan, it appears to be a very similar stand alone fan made by Dometic. The reason I like this video is that he specifically calls out one important thing. Ideally you want to use a PWM motor controller with a frequency higher than 20khz. The reason for this is that any frequency lower than this is within the range of human hearing, meaning that your fan may emit a high pitched whine when using it. It will not harm the fan, but it could definitely be annoying. Here is a good PWM controller for only about $10 that runs at 25khz. It is rated to way, way more current than the fan runs at, but none of the smaller ones seem to run at over 20khz. Plus the higher rated controller will run much cooler when running at such a low load, probably making almost no heat at all.


After installing this controller, I can confirm that the fan seems to use somewhere around half the power that it did from the factory when I set it to a speed that's about equivalent to low speed on the factory fan.

Oh and that's the other big benefit, the speed is completely variable all the way from a crawl up to full speed. I can also confirm that this speed controller will fit inside the fan in the same spot where the factory speed control knob was. It's a tight fit, but it works. I still also have the power to the ran motor running through the thermostat, the switch that automatically shuts off the fan when the vent is closed, and the speed reversing switch, so it maintains all of the other factory functions.

One thing to note is that since you will probably be completely replacing the factory speed control knob, once you install this, power is effectively always going to this speed controller even if you have the speed set to the minimum, which is no fan rotation at all. Per my measurements, the speed controller and the rest of the fan's electronics uses about 0.05a of current when it is just sitting there not spinning the fan. (about 1.2ah per day.) So if you are a real stickler for parasitic drains on your battery, you may want to wire up an external on off switch that switches power to the fan so you can cut out that small drain when you are not using the fan.
 

tx2sturgis

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It works well, I installed a PWM controller on my 3 speed Fantastic fan many years ago.

Be aware, some PWM controllers will impart an audible 'whine' in the motor. This depends on the khz rate of the circuit. The OP listed one with a 25 khz rate...that should be high enough that it won't bother anyone with sensitive hearing.

One more caveat...the PWM controllers can cause RF interference in radio receivers....in my case specifically ham radio receivers, but even a portable AM/FM radio, near enough, might pick up the interference.

Your built-in AM/FM radio in the dashboard 'might' receive some interference, so if you are parked, and listening to the radio and running the fan, if your radio is not picking up the radio stations you expect, try adjusting the knob on the fan, or turning it off.

And of course, if you are sitting outside and listening to a portable radio on a nearby table, you can just move the radio a few more feet farther away and the reception should clear up noticeably.

But the main take-away is that a mod like this DOES reduce the overall power consumption of the fan. I did a variety of checks on current draw and the PWM controller is much more efficient, which will be important on solar and battery power.

Thanks to the OP for the report!
 

gone2day

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Nice mod. I bought a similar Drok controller to set speed on a motorcycle radiator fan that I will use for ventilation. I haven't set it up yet but looking at reviews from users it looks like the freq. is about 15khz. So it will be interesting to see if it will be noisy.

The fan motor has brushes so maybe that's why it has a lower operating frequency? The built-in switch turns completely off, so battery drain won't be a problem. It draws ~2 amps or so max.
 
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Vannautical engineer

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One more caveat...the PWM controllers can cause RF interference in radio receivers....in my case specifically ham radio receivers, but even a portable AM/FM radio, near enough, might pick up the interference.
Good point. If that becomes an issue, wrapping the controller in foil may help. However, make sure it's some kind of foil backed with something non-conductive since the controller is an exposed circuit board and will definitely short out if wrapped directly in foil.
 

Vannautical engineer

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The fan motor has brushes so maybe that's why it has a lower operating frequency? The built-in switch turns completely off, so battery drain won't be a problem. It draws ~2 amps or so max.
DC motors don't have any inherent operating frequency. Normally DC power is just one continuous feed of power, no frequency at all. What the PWM controller does is cycles that power on and off very fast. The more time the controller stays at the "off" state each time it flips the power on and off, the slower the motor will spin, and that's how it controls the speed.

The point is, ideally you want a controller that cycles it on and off at least 20,000 times per second (20khz.) Any lower than that, and you risk actually hearing that power being cycled on and off to the fan motor.

Like I said, I removed the fan's built in power/speed switch completely as it would not fit in the fan along with the new controller. The factory switch completely shuts off power to the whole fan when it is switched to 0, so there is no drain. But if you take out this switch and wire power right into the speed controller, even with the speed control set all the way down so the fan is not moving, power is still going to the controller along with the other electronics in the fan. So it creates a small continuous drain on the battery.

You can either wire in another switch somewhere else that shuts off power to the fan, which has the added benefit of now being able to turn the fan on and off from another location. Or, you could keep the factory switch and find somewhere else to put the PWM controller. If you go this route, you should put jumpers across those resistive wire coils on the switch. This will make it so that if you set the switch to any of the 3 speeds, it still supplies full power to the PWM controller, which is what you want, because the PWM controller is now controlling the speed.
 
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