Building a Fan controller

I want to build a single fan controller so i can control the speed of my 120mm ( jet engine) fan that I want to put into my case.

It is a Y.S. Tech 120mm 8watt with a current draw of .80 max.

I tried a 7 volt mod but it was too slow. I was going to make a controller but I do not know what value potientometer (sp?) I should use. I was thinking of a baybus but I am too cheap

I want something that will not get too hot.

Something in the 25 - 30 ohm range gives me a nice range of controll. Just make sure the one you get is rated for at least 8 watts. You might be able to get away with 5, but it will probabally get warm if not hot.

Suggestions on where I can look on how to wire this thing up?

www.fanbus.com he has lots of info. Basically you're going to run a line from the 12V line on the PSU to one side of the pot, then a line from the center lead on the pot to the fan, then from the fan's ground back to the ground wire to the PSU. or something similar to that setup. You want to use one side and the middle of the pot.

A 20 ohm 5 watt rheostat wired into the 12 volt feed line as a variable resistor should give you good control of your fan. Basically, cut the twelve volt wire going to the fan and hook one cut end to one of the side terminals of the rheostat and the other cut end to the center terminal of the rheostat. With this setup you should be able to control the fan voltage from about 5 volts to the full twelve ovlts.

What about heat and a ground? Is this going to pose a problem for the power supply to see an ungrounded piece? Also what happens if it comes into comtact with metal in the case? A.k.a. the frame to monut the rhoestat?

Five watts or less shouldn't pose a heat problem. Yes, you need to hook the ground wire to the fan. No, do not hook the ground wire to the rheostat. Yes, use some kind of insulation or ensure that the rheostat connections ( you know, any one of the three terminals on the rheostat ) do not come in contact with the case. If you were able to do a seven volt mod you should be able to handle this with no problems.

Another possible means for connecting that rheo would be to leave the 12V line as is, connect the center lead of the rheo to the fan's ground/negative connection, connect one side of the rheo's coil to +5V, and the other side to GND. This way, you could vary between 12V and 7V with the rheo control. Let me know if you need a picture.

No matter which rheo you buy, make sure it can handle 8W. 10W would be a good figure to go on.

Good luck, and have fun! -- Paul

Could you please send in a picture? I am kinda electronically challenged. Thanks

Do not buy a 20 Ohm rheostat for your fan. Your fan is rated at 12v/.08A=15ohm. Try to find one rated at 10 Ohms.The rheostat that you get must be equal to or lower than this, or else it will smoke. You connect the ground wire on the fan to both the center pole and one of the outer ones. This should explain installation pretty well.

Gautam, please explain your reasoning for why the rheostat will smoke.

I have seen far too many people use rheostats that are not rated high enough for the fan they have, and have them burn out the fan and rheostat for sure, sometimes even more hardware. It is imperative to understand your fan's resistance and make sure that the rheostat you consider be rated for lower than this resistance.

Resistance = Voltage / Current,

For his fan = 12V / .8A = 15 ohm

A 20 ohm rheostat is not rated for such low resistance; its rated for 20ohm, not 15 ohm. He would need along the lines of a 10 Ohm rheostat. 15 Ohm would work fine, but is pushing the envelope. 20 ohm is asking for a fire.

Yes, except ohms are the resistence to flow, and the wattage is the power rating. (And the max wattage rating of the resistor is what's relevant to whether or not it's overloaded.)

This is why you'll see (for example), 100 kOhm resistors in both 1/4 Watt and 1/2 Watt varieties.

Regardless of the Ohm value he chooses, it needs to be rated for the total wattage that will pass through it. If he's planning on running 5V through the rheo so that the fan will get 7V, and if he's planning on a full .8A, that works out to .8A * 5V = 4W. He should probably count on a 10W rheo to be safe, though.

-- Paul

The wattage and resistance go hand in hand; power being Voltage x Current, and resistance Voltage/current. Thus, if you find a rheostat with the proper resistance rating, the wattage should be adequate just about all of the time. Both are imperative considerations, though. The same current will always be passing through the rheostat, and it needs to be able to handle it.

Clearly, they're related, but it's more like this:

The Ohm rating gives the voltage drop across the resistor (and hence the Wattage) as a function of current load.

V(i) = iR
W(i) = i V(i) = i^2 R.

So long as W(i) <= W_max of the resistor, you're fine. The resistor has two ratings: R and W_max. R allows you to determine the wattage of the particular application, and W_max tells you whether you are within the operating range of the resistor.

In this case, you're suggesting a 15 Ohm resistor, which would also work well. Okay, well, let's look at this.

You calculated an "internal resistence" of the fan of 15 Ohms. You have another 15 Ohms in series. Total resistence is 30 Ohms.

12V / 30 Ohms = .4 A total current in the fan-resistor circuit.

Okay, so

W(.4A) = (.4 A)^2 * 15 Ohms = 2.4 W.

So, the resistor in this case needs to be rated for 2.4W or better. This example would work, and I searched a bit for a 15 Ohm one that wouldn't work, but there aren't exactly a lot of 15 Ohm rheo's out there.

But here's an example to show that the two numbers aren't always the same:

Scroll down here, and you'll see two rheo's that are both rated for a maximum of 25 Watts, but one is only 10 Ohms, and the other 50 Ohms.

150330250 RHS10R 10 OHM 25 WATT RHEOSTAT $12.84 OHMITE

150331000 RHS50R 50 OHM 25 WATT RHEOSTAT $8.00 OHMITE

The two numbers can vary independently from one another, which is why I take the approach of choosing the Ohms value of the resistor for your design first, and then choose a resistor with that value that can handle the particular load for the particular application.

If we were talking about a 1.25 A current across the rheostat, then we'd have 23.4 W load on the resistor, and the 15W one linked above would no longer be appropriate, but it a 25W one would be.

At the end of the day, I think we're approaching the same numbers, but with different outlooks. -- Paul

Arrghh!
I am so confused!
Someone please just send me to a web site to buy one and give me the easiest way to wire it. I am REAL stoopid in wiring and electronics. I would buy a bus but I feel $30+ is too much for ONE fan.

Volts/(Amps * Ohms) = 1
-or-
Volts = Amps * Ohms

Amps = Volts/Ohms

Ohms = Volts/Amps

Watts = Volts * Amps

squirtle632 said:

Arrghh!
I am so confused!
Someone please just send me to a web site to buy one and give me the easiest way to wire it. I am REAL stoopid in wiring and electronics. I would buy a bus but I feel $30+ is too much for ONE fan.

Click to expand...

So buy more fans!!!

madman_sam81 said:

Volts/(Amps * Ohms) = 1
-or-
Volts = Amps * Ohms

Amps = Volts/Ohms

Ohms = Volts/Amps

Watts = Volts * Amps

Click to expand...

This is probably the nicest way to look at it. Just look at the link and info macklin01 gave. That rheostat would work great. And look here. To see how to install it. Its quite simple; I'm sure you won't have any problems.

Squirtle, follow the link in Gautams' first post for instructions on wiring the rheostat. Make sure you use a 15 or 20 ohm pot rated for at least 5 watts. Ignore everything else Gautam and Macklin01 say. They mean well, but they don't know what the f**k they are talking about when it comes to computing power or current or anything else to do with Ohms' Law.