# Question on high Wattage fans and Rheostats

#### Since87

##### Member
That rheostat will work fairly well with one fan.

It will allow you to turn the fan voltage down to 4.8V. (Which is probably too low for it to run.) And, the maximum dissipation in the rheostat will be 3.6 Watts.

It will also work with two fans, although not as well. With two fans:

The minimum fan voltage will be 3 V.
The maximum rheostat power consumption will be 7.2 Watts.

I think someone sells a 4 Ohm 10 Watt Rheostat. That would be much better for two of these fans.

#### lebe0024

##### Member
Thanks man. If you have the time or the desire, could you tell me how you got those numbers? (I searched all over this forum and couldn't understand anything (I've taken 2 years of physics, but I don't remember any of this stuff)

#### Gnufsh

##### Senior Member
current is equal to voltage divided by resistance. That's my story and I'm stickin' to it...

#### frostmeister

##### Member
Ok, I'm sticking my oar in as an electronic engineer. Gnufsh and since87 are both right anyway. The formula you're looking for is ohms law. I've attached the classic picture here. To use it, all you have to do is cover up the unknown quantity; So, if you have a 100 ohm resistor (R) and you put it across a 12 volt supply (V) then to find the current that flows through it, it's V over R or voltage divided by current. 12 divided by 100 = 0.12 or 120 milliamps. Conversly, to get the voltage, if you know 120 milliamps is flowing through that 120 ohm resistor, then .12 amps (I) multiplied by 120 (R) equals 12; - 12 volts.
Isnt learning fun? #### frostmeister

##### Member
Ok, clever clogs here forgot the image here's my magic triangle #### lebe0024

##### Member
ARGGG. But I don't know why variable resisters have a wattage rating. I know ohms law, but I can't figure out how to use it to find out if rheostat R will work with fan F.

For instance, take the fan and rheostat from above. SO, via ohms law, if I take a 1.2Amp fan and a 15ohm resistor, that's 1.2 * 15 = -18 volts drop? But since87 got -7.2 volts drop

#### lebe0024

##### Member
Theres's four numbers to deal with
FAN: 12 Volts, 1.2 Amps
Rheostat: 15 ohm 15 Watt.

These don't easily plug into the little triangle.

#### Since87

##### Member      Well I spent the better part of an hour writing a post deriving my calculations from three fundamental equations. I hit the preview button, and the forum software ate the post. At this point I'm going to just post the result of all this derivation.

Prheo-max = Pfan12 / 4

Where:
Prheo-max is the maximum power consumed in the rheostat.
Pfan12 is the power consumption of the fan when running at 12V.

Vfan-min = 12V * ( 12V / Ifan12 ) / ( Rrheo-max + ( 12V / Ifan12 ) )

Where
Vfan-min is the minimum voltage the fan can be set to.
Ifan12 is the current draw of the fan at 12V.
Rrheo-max is the maximum resistance of the rheostat.

If you don't have a current draw spec for your fan but you do have a spec for your fan's power consumption at 12V (Pfan12) you can calculate the current draw using this equation:

Ifan12 = Pfan12 / 12V

If you are connecting multiple fans up to the rheostat, then you need to add the current draws of all the fans, to get Ifan12.

I would advise selecting a rheostat with a wattage rating that is at least 125% of Prheo-max.

[/Edit]

Hope this helps.

Somebody make this a sticky. These questions come up about once a week.

Last edited:

You are the MAN!

#### Lithan

##### Disabled
WOOOT Read since87's post REAL REAL slow this time, and I can understand the HOW. But I still want to know the WHY. Can you maybe drop me some hints as to what books/chapters in books I'd find this stuff in? I've got a high school physics and I can probably get a college physics year one book. Or anything I could grab at the library.

#### Über~PhLuBB

##### Jedi Knight Senior
Since87 said:
I think someone sells a 4 Ohm 10 Watt Rheostat. That would be much better for two of these fans.

I've read all the stuff above... but I don't understand how a smaller rheo would be better for TWO giant fans than one big rheo would be for ONE giant fan. Doesn't make sense. #### Lithan

##### Disabled
Uber... With two fans MORE voltage must get to the fans or else they will dip below startup voltage and NOT spin. This is bad. So you get a weaker Rheo which at its highest resistance still does not resist current enough to stall the fans.

Remember more fans need more power.
AND
Resisters do not supply power, they Remove it.

Therefore, two fans needs more power
Weaker resistor amounts to more power.
You want a weaker resistor.

#### Über~PhLuBB

##### Jedi Knight Senior
Lithan said:
Uber... With two fans MORE voltage must get to the fans or else they will dip below startup voltage and NOT spin. This is bad. So you get a weaker Rheo which at its highest resistance still does not resist current enough to stall the fans.

Remember more fans need more power.
AND
Resisters do not supply power, they Remove it.

Therefore, two fans needs more power
Weaker resistor amounts to more power.
You want a weaker resistor.

Oh ok, I get it. Thanks. =)

#### Since87

##### Member
Lithan said:
WOOOT Read since87's post REAL REAL slow this time, and I can understand the HOW. But I still want to know the WHY. Can you maybe drop me some hints as to what books/chapters in books I'd find this stuff in? I've got a high school physics and I can probably get a college physics year one book. Or anything I could grab at the library.

My textbook for this stuff was Engineering Circuit Analysis.

I'm not sure how easy it'll be to get at a public library. Any book on basic electronics is likely to cover this stuff though.

What we're discussing here is a simple voltage divider composed of a voltage source (+12V to Ground) and two resistors. (A fan and a rheostat) All three are connected in a series loop.

The equation to calculate the voltage where the two resistors connect together is:

Vr1 = Vin * ( R1 / (R1 + R2) )

Where:
Vin is the voltage of our voltage source. (12V)
R1 is the resistor connected to ground.
R2 is the resistor connected to +12V.
Vr1 is the voltage drop across R1.

A brushless DC fan doesn't actually behave like a resistor, but it's a close enough approximation to get decent numbers. To get the "resistance" of the fan just use Ohms law:

V = I * R or R = V/I

So:

The fan's "resistance" = 12V / (The fan's current draw at 12V)

We'll call the fan resistance R1. At this point we know Vin and R1 of the voltage divider equation. We can either select a value for R2 and calculate Vr1 with the basic voltage divider equation:

Vr1 = Vin * ( R1 / (R1 + R2) )

Or we can select the voltage we want across the fan ( Vr1 ) and use the following version of the same equation to calculate a rheostat setting that gives us the desired voltage.

R2 = ( Vin * R1 ) / Vr1 - R1

To get the equation:

Prheo-max = Pfan12 / 4

I used a shortcut called The Maximum Power Theorem. Simply stated this theorem says that the rheostat will be dissipating the most power when its resistance is the same as the fan resistance.

Knowing that the maximum power draw of the rheostat would be when both resistances were equal, I used the following equation relating voltage and resistance to power:

P = V^2 / R

Suppose the fan resistance is 10 Ohms. With the fan running at 12V:

P = 12^2 / 10 = 14.4 Watts.

Now suppose we put another 10 Ohms in series representing the rheostat:

P = 12^2 / 20 = 7.2 Watts.

But of those 7.2 Watts, half are being dissipated in the fan, and half are being dissipated in the rheostat. The maximum dissipation in the rheostat is 3.6 Watts.

So the maximum power dissipation in the rheostat is 1/4 of the dissipation of the fan when it is running at 12V.

Lithan said:
With two fans MORE voltage must get to the fans or else they will dip below startup voltage and NOT spin. This is bad. So you get a weaker Rheo which at its highest resistance still does not resist current enough to stall the fans.

It would be more accurate to say that, "Two fans in parallel will draw twice the current of a single fan for a given voltage. To get the same voltage across two fans requires the resistance to be half that of the single fan case."

Aside from that nit picking, yep you've got it.

#### Intrepid

##### Member
people here is something very basic i dont see...
voltage is how much is given to the device and any device on an X voltage wire gets X voltage. if what he is saying about voltage getting split in half by 2 devices on one line. what about having 2 molexes on oe line pluged into 2 devices and both get 12 volts!!!???

ok that said now amperage is how much the device draws, voltage is how much the drivce is givem. also one is quantity the other is flow rate of the energy and all. and fruther more if you really want to know what ohm resistor is needed, just buy a bunch of 10 ohm resistors, like 15 of em, and than put em in series first onle one, than 2 than 3 in a chain adding more and more untill you his a resistance that causes the fan to coem close to a hault but not actauly stop. there is your max resistance!

Therei s much more in this but I don't have time to call up my physics prof on a sunday and get killed the next day i have his class.

#### Since87

##### Member
Intrepid6546 said:
people here is something very basic i dont see...
voltage is how much is given to the device and any device on an X voltage wire gets X voltage. if what he is saying about voltage getting split in half by 2 devices on one line. what about having 2 molexes on oe line pluged into 2 devices and both get 12 volts!!!???

I'm not sure where your confusion lies Intrepid. Have you seen Molex Y adapters that let you power two devices from a single Molex plug?

Intrepid6546 said:
if you really want to know what ohm resistor is needed, just buy a bunch of 10 ohm resistors, like 15 of em, and than put em in series first onle one, than 2 than 3 in a chain adding more and more untill you his a resistance that causes the fan to coem close to a hault but not actauly stop. there is your max resistance!

Experimentation is worthwhile. However, you need to be aware of the power rating of the resistors you use. The typical resistors you can get at radioshack are 1/4 Watt resistors. A 10 Ohm resistor in series with the four fans we are discussing in this thread will dissipate 2.975 Watts. Roughly 12 times the Watt rating of the resistor. It will get very hot, and it is likely to burn up.

Intrepid6546 said:
Therei s much more in this but I don't have time to call up my physics prof on a sunday and get killed the next day i have his class.

Hopefully you'll have a chance to discuss this stuff with your professor today.

#### Intrepid

##### Member
it is my understanding that they dissapate volts and yues i will ask becasue the results of my experimentation which i did with the help of a pr4efessional electrition show diferent results than your calculations, I want to know why there is such a diference and also, from other people on the ofrum, i have heared that about 100-150 ohm is good, this was from 3 months ago when i posted asking about the ammount of resistance needed for LEDs and they supplied me with a formula. and 2nd calm down.

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