Rheostat on Tornado

[Simpsonia]

Hi, first time poster long time lurker. I just tried the rheostat mod to adjust my 92mm tornado on my hsf. I used the 100ohm 5 watt rheostat from Jameco as was recomended in every rheostat tutorial I could find on the net. It works fine except that when I dial down the fanspeed the rheostat gets extremely hot, so hot I can't even touch it. Now Im kind of at a loss at what to do.

The rheostat also turns down the speed a little too much barely half a turn and the fan speed is cut by more than half. I was thinking of maybe trying out the little 25 ohm 1 watt radioshack one but that seems like it might be too small and I could fry it. Thanks

The Tornado is way too loud at full speed Im also thinking of just cutting my losses and going with a fairly normal 80mm fan. Btw Im using a swiftech MCX6400 on an 3500+
Any recomendations?

Simp,

 

[cgrant26]

I had that same problem initially with my DIY rheobus. I actually burned up one of my rheos becasue of it. I ended uo getting heatsinks for them. Pretty much anything that you can attach to the back of them will work, but If you look around, you can find some collar style heatsinks that will fit around your rheo.

Here's one example I found:

http://www.oselectronics.com/ose_p85.htm

Just do a google search on heatsinks & electronics

 

[Simpsonia]

So even if the wattage is way smaller it will be fine as long as I can dissipate the heat? I went to the local radioshack today and got the only rheostat they had there a 3 watt 25 ohm one the 25 ohms is perfect but it still gets hot as hell. The 92mm tornado uses 12.5 watts and Im going to order an 80mm but that's still 9.1 watts, less than what I have. Do I need a rheo that has a higher rated wattage than the fan I'm using?

 

[Interloper]

Why don't you put a waterblock on the rheo!>!>!>?!


(Sorry I just couldn't resist!!!)

I too was thinking the same thing...It seems to have great flow but damn loud. Too much wattage to run through a Rheo.. What about a Dimmer Switch for an overhead lighting system??? The dining room type dimmer switches are basically rheo's (NO?? IF I am wrong someone please correct me.) I know that the Overhead Ceiling Fans probably use a lot more power (but probably more volts and less amperage) - anyway, go to home depot and see if you could find a nice Overhead Fan Speed Controller switch.....

I am still contemplating the jump into Watercooling and the thought of a nice Tornado fan that was muted somewhat is highly appealing....

I haven't tried this personally, but I would think that a beefier Rheo would probably handle the wattage better than a heat sink on a smaller component....



Interloper

 

[Boki]

So even if the wattage is way smaller it will be fine as long as I can dissipate the heat? I went to the local radioshack today and got the only rheostat they had there a 3 watt 25 ohm one the 25 ohms is perfect but it still gets hot as hell. The 92mm tornado uses 12.5 watts and Im going to order an 80mm but that's still 9.1 watts, less than what I have. Do I need a rheo that has a higher rated wattage than the fan I'm using?

Ideally, you would like to have a rheostat with wattage rating as high as possible.
With a rpot set at 25ohm and a new 9W fan, you should be fine. Total power dissipated by rpot will be 2.1W.
Even if you use it with a 120mm, 12.5W fan, tpower dissipated by rpot is 2.6W.

 

[Interloper]

Ideally, you would like to have a rheostat with wattage rating as high as possible.
With a rpot set at 25ohm and a new 9W fan, you should be fine. Total power dissipated by rpot will be 2.1W.
Even if you use it with a 120mm, 12.5W fan, tpower dissipated by rpot is 2.6W.

Hey,

Its been a long time since my physics courses in school ... I forgot how the rheostat works..

Its basically a variable resistor but its not hooked in series to vary voltage??? Refresh my memory please, I seem to have forgotten.

And its just Ohm's law to calculate the Wattage on the rheo?? Hmmm. just tried doing the calcs but still not making sense - refresh my memory please how to do the calcs....


Thanks!

 

[Boki]

Hey,

Its been a long time since my physics courses in school ... I forgot how the rheostat works..

Its basically a variable resistor but its not hooked in series to vary voltage??? Refresh my memory please, I seem to have forgotten.

And its just Ohm's law to calculate the Wattage on the rheo?? Hmmm. just tried doing the calcs but still not making sense - refresh my memory please how to do the calcs....


Thanks!

A rheostat is just a wire wound resistor with a third node. Just made more robust to
handle higher current. See ->> Pic
Anyway, you must wire rheostat in series with a fan, not parallel, using only
2 nodes of the rheostat. Either left node+middle or right+midle
Same old ohm's law applies here if u wish to calculate power.
You know that fan draws 12.5W@12V according to specs.

P=V*I, so if you divide 12.5W/12V=1.04A.

Cal. fan resistance 12V/1.04A=11.5ohm

T(resistance)=Rf+Rpot->11.5+25ohm=36.5ohm

T(current)=Vin/Rt=12V/36.5=328mA(current is always the same in series cct)

T(power)=12V*328mA=3.93W

Vpot=(Rpot/Rt)*Vt=8.21V@25ohm(voltage drop across the rheostat)

Ppot=V*I-> 8.21V*328mA = 2.69W

Pfan=Pt-Ppot= 3.93W-2.69W=1.24W

Lowest voltage drop across the fan is 3.79V with rheostat set at 25ohm.
This may be too low for the fan to spin, so ideally you would like to set rheostat to equal fan resistance(11.5ohm). This would ensure that fan gets half the voltage

I hope this helps now.
cheers