Slowing down a FantasticVent

Anything electric, AC or DC

Postby shoeman » Sat Aug 23, 2008 8:21 am

My fan is just to loud to be a good "white noise" machine at night, and it also has the thermostat and when in cycles off and on it will wake you up. I run a fan in my bedroom all summer just for the noise effect but this vent fan is overkill. The fact that it is about three feet over your head probably makes it worse. I asked a good friend with an electronics background my original question and here is his reply:

"Your fan is 26w at 12v. Watt's law (you get to lean something new!) states I(current) * V(voltage) = P(power), so 26w / 12v = 2.16amps

Now, we know the current and the voltage, so we can calculate the resistance. Ohm's law is I * R = V. So we can take 12v / 2.16a = 5.5 ohms. The motor has 5.5ohms of resistance.

If you put another 5.5 ohm resistor in parallel, the current will be 1/2 of the 2.16 amps, and theoretically you will have 1/2 speed. The friction of the motor and some other factors will effect the actual speed, but it will give you a quieter motor. The power requirement of the resistor is calculated as 6v * 1.1amp = 6.6 watts - round up to 10watt resistor for safety. In case your wondering, the 6volts is because the motor and resistor are equal, so together they have 12v across them, with 6 volts across each. The 1.1 amps is because we doubled the resistance (motor + resistor) to 11 ohms. 12 volts / 11 ohms = 1.1 amps.

Personally, I think I would try installing two fans with a switch that would connect them in series, parallel or off. this was you don't need a power resistor, and you will still get similar airflow (to one fan) with less noise. If you need to, I can send you a schematic for that."


So there is a theory course for us all today. I spoke with him later and the thought came up that a good starter experiment would be to try two old automotive ballast resistors first! We used to race bikes together and those were the key to being able to use car coils in our bikes. They essentially drop the 12v from the battery down to about 6v. Dontcha love cross referencing hobbies like that!
But back on topic, I'll look at the Ohmite site too. Heat is always a concern with this stuff, so an engineer designed solution might be best. If you look at your fantastic fan, you'll notice there is a cut out for airflow around the speed switch. I bet that factory switch gets pretty warm and they thought it best to use some of the air movement to keep it cool. Don't want to melt anything!
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Postby Larwyn » Sat Aug 23, 2008 9:01 am

shoeman wrote:My fan is just to loud to be a good "white noise" machine at night, and it also has the thermostat and when in cycles off and on it will wake you up. I run a fan in my bedroom all summer just for the noise effect but this vent fan is overkill. The fact that it is about three feet over your head probably makes it worse. I asked a good friend with an electronics background my original question and here is his reply:

"Your fan is 26w at 12v. Watt's law (you get to lean something new!) states I(current) * V(voltage) = P(power), so 26w / 12v = 2.16amps

Now, we know the current and the voltage, so we can calculate the resistance. Ohm's law is I * R = V. So we can take 12v / 2.16a = 5.5 ohms. The motor has 5.5ohms of resistance.

If you put another 5.5 ohm resistor in parallel, the current will be 1/2 of the 2.16 amps, and theoretically you will have 1/2 speed. The friction of the motor and some other factors will effect the actual speed, but it will give you a quieter motor. The power requirement of the resistor is calculated as 6v * 1.1amp = 6.6 watts - round up to 10watt resistor for safety. In case your wondering, the 6volts is because the motor and resistor are equal, so together they have 12v across them, with 6 volts across each. The 1.1 amps is because we doubled the resistance (motor + resistor) to 11 ohms. 12 volts / 11 ohms = 1.1 amps.

Personally, I think I would try installing two fans with a switch that would connect them in series, parallel or off. this was you don't need a power resistor, and you will still get similar airflow (to one fan) with less noise. If you need to, I can send you a schematic for that."


So there is a theory course for us all today. I spoke with him later and the thought came up that a good starter experiment would be to try two old automotive ballast resistors first! We used to race bikes together and those were the key to being able to use car coils in our bikes. They essentially drop the 12v from the battery down to about 6v. Dontcha love cross referencing hobbies like that!
But back on topic, I'll look at the Ohmite site too. Heat is always a concern with this stuff, so an engineer designed solution might be best. If you look at your fantastic fan, you'll notice there is a cut out for airflow around the speed switch. I bet that factory switch gets pretty warm and they thought it best to use some of the air movement to keep it cool. Don't want to melt anything!


Shoeman,

Please do not take this wrong, but please do not act on the information you presented in this post without checking with others. Unless I am mistaken there are flaws in the electrical theory you have been given. I have great reservations about getting involved here, and I am sure your friend meant well with his "lesson".

There is little wrong with the math presented here other than the fact that in a parallel circuit, you have supply voltage across ALL loads. Therefore if you were to put the resistor in parallel with the fan, you would double the current required, half of which only serves to heat the resistor. In this case the 5.5 ohm resistor with 12 VDC applied across it will equal 26 watts. So if you were to use that 10 watt resistor you will at best burn up the resistor, and at the worse, the heat generated in the process could ignite any surrounding material.

I would advise you research the matter very carefully before proceeding with any knowledge gleaned form the "lesson" which was presented here. In my opinion that could be rather dangerous misinformation.

In addition, you would find that the parallel resistor would have no effect on the fan speed.
Last edited by Larwyn on Sat Aug 23, 2008 9:47 am, edited 1 time in total.
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Postby planovet » Sat Aug 23, 2008 9:08 am

Heck, I didn't understand a word of what his friend said anyway! :?

One thing I like about the Fantastic Fan is that it comes in a low profile option. Anyone know how far above the roof a Northern Breeze fan sits? I will have limited clearance to get the teardrop in the garage.
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Postby shoeman » Sat Aug 23, 2008 10:40 am

The resistor needs to be in series. He later corrected that for me. As an experiment I just put a 5w 30 ohm resistor in the power feed and it slowed the fan right down to a crawl. Too slow, but I expected that from 30 ohms. I also looked closer at the factory speed switch and it appears to have a resistor across two posts. I'm guessing they do the same as I'm thinking but use a lower resistance to keep speed up. I'll get a closer look at it and try and read the rating.
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Postby Larwyn » Sat Aug 23, 2008 11:06 am

I believe that the only way you will control fan speed by adding parallel loads, is to add enough of them to increase current flow to the point that it trips a breaker of opens a fuse. Even though I have found that a Fantastic Fan is very quiet at zero rpms, I have also found it to be less than useful at that speed..... :lol:

To further describe resistance in a parallel circuit I should include the fact that in the above example the current demand doubled because the resistance of the two parallel paths were equal. Had they not been of equal resistance, the current demand would still increase but it would be by a factor proportional to the resistance of the added path.

Parallel resistance is computed by taking the reciprocal of the sum of the reciprocals of all the resistance values of the parallel paths.

or, a simpler method if there are only two paths is;

the product of the resistance values of the parallel paths divided by the sum of the resistance of the values of the parallel paths.

In this case, since there were two parallel paths of 5.5 ohms, my statement that current doubles is valid (5.5 / 2 = 2.75). If you apply either or both of the other formulas you get the same result; 2.75 ohms.
At 5.5 ohms current = 12 volts / 5.5 ohms = 2.18 amps.
At 2.75 ohms current = 12 volts / 2.75 ohms = 4.36 amps. Or double.

Even though each added parallel load is resistance, it provides an additional current path therefore decreasing total circuit resistance, resulting in a higher current demand on the supply source. The voltage drop across each parallel circuit will remain at supply voltage, only the current will vary depending on the resistance of that particular current path. The flow in each path is determined by supply voltage and the resistance of each individual path, independently of the flow in any other parallel path.

This information was provided for the benefit of those who would read my previous post and dive in on me for oversimplifying the computation of resistance/current flow in parallel circuits.
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Postby S. Heisley » Sat Aug 23, 2008 11:13 am

Hi, Mr. Shoeman:

I don't know if you've tried calling the Fantastic Fan company yet; but, I'm betting they've had similar complaints to yours. While your answer is absolutely amazing, the factory just might have a product already made for your purpose (fingers crossed and lotsa hopin') or, at least, they may have some useful suggestions.

Either way, I think what you are about to do is very valuable to the TD & TTT community and I, for one, thank you for your research and testing. Please, take lots of pictures of what you ultimately do and post them along with your explanations and results. There will be many curious and grateful people.

Whatever you do, please be very careful. I think we all want the same for you: to be on this earth, with all your being functioning, and posting here for a very long time! If life is a ride, let's make it a long one!

Wishing you success,
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Postby Larwyn » Sat Aug 23, 2008 11:58 am

Okay, good deal, glad to get that out of the way...... :thumbsup: :thumbsup:

Keep in mind that it could be that the manufacturer already took all this into consideration. The low speed current draw is an advertising point for these types of products. The lower the speed, the lower the current draw. So it might be that the fan is delivered with the best compromise of speed (battery life)/life expectancy of the product. To run it at a slower speed could compromise operating life of the fan. I do not know this to be the case but I do consider it to be likely.

Yep, series resistance works much differently. I know your use of the 30 ohm resistor was only an experiment. But the following example should help you in determining the resistance and wattage requirements to accomplish the job. The minimum voltage at which the fan can be reliably operated will be the limiting factor on how slowly you should operate the fan.

In your experiment you had a 30 ohm resistor in series with a 5.5 ohm fan. For a total circuit resistance of 35.5 ohms (yes, the math is much more straight forward on series circuits).

12 volts / 35.5 ohms = .338 amps

The resistor

30 ohms * .338 amps = 10.14 volts droped across the resistor

.338 amps * 10.14 volts = 3.43 Watts so the 5 watt resistor is okay.

The fan

5.5 ohms * .338 amps = 1.859 volts droped across the fan

I would check with the manufacturer to get their recommendation of the minimum voltage at which the fan should be operated. From there it should be easy to compute the resistor size needed to get the lowest rpm from your fan. Operation at too low a voltage for too much time could result in the fan overheating and failure of the motor. I do not know what voltage value would be required to keep the fan going for a reasonable period of time. The 5.5 ohms suggested by your friend would provide 6 volts for the fan, I have no idea if it will operate reliably at 6 volts.

In this case "applied to the fan" includes the fan switch. The voltage across the motor itself would depend on the switch/motor circuitry.

Let us know what you find out, could be an interesting exercise. :thumbsup:
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Postby aggie79 » Sat Aug 23, 2008 4:02 pm

There is another route to take besides a resistor. You might be able to use a PWM (pulse width modulation) controller.

Before going further I need to state that electricty is not my cup of tea. I learned about these helping my stepson "overclock" his computer - making it run faster than spec'ed - for his computer gaming hobby. A PWM controller does not alter the current - it only just provides it in "pulses" rather than continuously. For some fan motors, though, it can introduce "clicking" noises.

Here is a link to one for computers, but if you google it, you can find schematics to build your own:

http://www.frozencpu.com/products/992/bus-34/FrozenCPU_Black_PWM_6x_Fan_Controller_525_Bay.html?tl=g36c17s240&id=GGRjsLIy
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Postby starleen2 » Sat Aug 23, 2008 4:39 pm

Larwyn wrote:Okay, good deal, glad to get that out of the way...... :thumbsup: :thumbsup:

Keep in mind that it could be that the manufacturer already took all this into consideration. The low speed current draw is an advertising point for these types of products. The lower the speed, the lower the current draw. So it might be that the fan is delivered with the best compromise of speed (battery life)/life expectancy of the product. To run it at a slower speed could compromise operating life of the fan. I do not know this to be the case but I do consider it to be likely.

Yep, series resistance works much differently. I know your use of the 30 ohm resistor was only an experiment. But the following example should help you in determining the resistance and wattage requirements to accomplish the job. The minimum voltage at which the fan can be reliably operated will be the limiting factor on how slowly you should operate the fan.

In your experiment you had a 30 ohm resistor in series with a 5.5 ohm fan. For a total circuit resistance of 35.5 ohms (yes, the math is much more straight forward on series circuits).

12 volts / 35.5 ohms = .338 amps

The resistor

30 ohms * .338 amps = 10.14 volts droped across the resistor

.338 amps * 10.14 volts = 3.43 Watts so the 5 watt resistor is okay.

The fan

5.5 ohms * .338 amps = 1.859 volts droped across the fan

I would check with the manufacturer to get their recommendation of the minimum voltage at which the fan should be operated. From there it should be easy to compute the resistor size needed to get the lowest rpm from your fan. Operation at too low a voltage for too much time could result in the fan overheating and failure of the motor. I do not know what voltage value would be required to keep the fan going for a reasonable period of time. The 5.5 ohms suggested by your friend would provide 6 volts for the fan, I have no idea if it will operate reliably at 6 volts.

In this case "applied to the fan" includes the fan switch. The voltage across the motor itself would depend on the switch/motor circuitry.

Let us know what you find out, could be an interesting exercise. :thumbsup:


What the....???? :oops: :shock: Now it's starting to sink in
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Postby Toytaco2 » Sat Aug 23, 2008 10:12 pm

For some reason my Fantastic Fan doesn't seem so loud anymore! :lol:
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Postby Leon » Sat Aug 23, 2008 10:57 pm

aggie79 wrote:There is another route to take besides a resistor. You might be able to use a PWM (pulse width modulation) controller.

That's how some of the better transformers for model railroad use work. It can provide a slower rotating DC motor that still has sufficient torque to do the job.
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Postby shoeman » Sun Aug 24, 2008 7:41 am

I plan on calling the factory Monday. I may get lawyer-speak for an answer, but it also may be the shortest route to some good info. This has been a heck of a good discussion so far. I'll post what I hear ASAP.

Larwyn wrote:Okay, good deal, glad to get that out of the way...... :thumbsup: :thumbsup:

Keep in mind that it could be that the manufacturer already took all this into consideration. The low speed current draw is an advertising point for these types of products. The lower the speed, the lower the current draw. So it might be that the fan is delivered with the best compromise of speed (battery life)/life expectancy of the product. To run it at a slower speed could compromise operating life of the fan. I do not know this to be the case but I do consider it to be likely.

Yep, series resistance works much differently. I know your use of the 30 ohm resistor was only an experiment. But the following example should help you in determining the resistance and wattage requirements to accomplish the job. The minimum voltage at which the fan can be reliably operated will be the limiting factor on how slowly you should operate the fan.

In your experiment you had a 30 ohm resistor in series with a 5.5 ohm fan. For a total circuit resistance of 35.5 ohms (yes, the math is much more straight forward on series circuits).

12 volts / 35.5 ohms = .338 amps

The resistor

30 ohms * .338 amps = 10.14 volts droped across the resistor

.338 amps * 10.14 volts = 3.43 Watts so the 5 watt resistor is okay.

The fan

5.5 ohms * .338 amps = 1.859 volts droped across the fan

I would check with the manufacturer to get their recommendation of the minimum voltage at which the fan should be operated. From there it should be easy to compute the resistor size needed to get the lowest rpm from your fan. Operation at too low a voltage for too much time could result in the fan overheating and failure of the motor. I do not know what voltage value would be required to keep the fan going for a reasonable period of time. The 5.5 ohms suggested by your friend would provide 6 volts for the fan, I have no idea if it will operate reliably at 6 volts.

In this case "applied to the fan" includes the fan switch. The voltage across the motor itself would depend on the switch/motor circuitry.

Let us know what you find out, could be an interesting exercise. :thumbsup:
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Postby mechmagcn » Sun Aug 24, 2008 1:01 pm

While I am a believer in FV for RV use, I bought a Shurflo Comfort Air Silver vent for my TD. This fan utilizes a reostat speed control for infinate adjustment. On the lowest setting I can barely hear the fan running and believe it will be about perfect for air flow in the TD. This is the bare bones edition, they have the gold and platinum series that have many more features. It can also be used with the cover closed as a ceiling fan.
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Postby aggie79 » Sun Aug 24, 2008 8:58 pm

I just found this:
http://www.casitaclub.com/forums/lofiversion/index.php/t10037.html
Scroll down about half-way to the diagram.
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Postby starleen2 » Sun Aug 24, 2008 9:21 pm

Here ya go - from the above mentioned post

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