Rheostat switch

[John0904]

Hi all.

I have a Delta high speed CPU/Heatsink fan that runs around 7000+ Rpm’s.
I am going to make a control switch with a rheostat and a 4way Molex extender cable that will control the speed and noise of this beast.

What I need to know is what Ohm/Watt rating should I get?
I was thinking of getting a 100 Ohm 5 watt rheostat.

I have done some research on this, but was not able to find anything useful.
I am hoping that someone here with some experince can help me.

Btw, I don’t have this fan installed because it is too loud.
But when it was, it cooled the CPU under heavy load to about 35ºC. http://www.PCGuide.com/ubb/biggrin.gif

Thanks in advance for any assistance.

[mjc]

There are already similar products to what you want on the market, but they are not exactly cheap...

RheoBUS (http://www.pcmods.com/details.asp?ProdID=4) is one that comes to mind. They are rated at 17W per channel.....that should give you an idea, so I would say something in between would work. I think I would settle on about a 10w.

But you need to be careful when trying to mod a CPU fan....most motherboard monitor the speed, so if you get it too low you could end up shutting down due to "fan failure".

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mjc
Computer Links (http://www.dreamwater.org/tech/mjc/index.htm)

Celts are the men that heaven made mad, For all their battles are merry and their songs are all sad.


Please Post Questions in the forums, not my email. Thanks

[YODA74]

yup nice one mjc my lites are Blue, john what you have there is adiquit,A good rule of thumb is to use a rheostat equal in value to the fan resistance - this will reduce the voltage across the fan to half the supply, 6 volts.the one mjc shows is one I use but is a little over kill if your only useing one fan.I use 7 I need the controls/ A Radio Shack 25 Ohm Rheostat is enough to be an effective solution with 120 mm 5-6 watt fan.



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PUSH TO TEST. RELEASE TO DETONATE.
[Closed captioned for the thinking impaired.]

[John0904]

Thanks guys. I'll look for around for a 10W for starters.
I would rather be 100% sure that I'm not short. http://www.PCGuide.com/ubb/wink.gif

This motherboard just monitors speeds, but doesn't react on numbers.
It could be zero and the motherboard would keep on trucking.

Besides, this fan is heavy duty. If I plugged it in the motherboard fan header, it would burn that out since it draws too much power. http://www.PCGuide.com/ubb/tongue.gif

Also, once I have made this speed controller, I will be testing it out before I place it on the CPU to see how long it will last at full power. (Burn-in). http://www.PCGuide.com/ubb/biggrin.gif

Thanks again.

[mjc]

Ok, sounds like you got things coevered pretty well....

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mjc
Computer Links (http://www.dreamwater.org/tech/mjc/index.htm)

Celts are the men that heaven made mad, For all their battles are merry and their songs are all sad.


Please Post Questions in the forums, not my email. Thanks

[rond36]

ThermalTake makes a switch with H M L (http://2cooltek.safeshopper.com/62/358.htm?479) for HSFs

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[Reid]

You could also try connecting the fan across the +12V and +5V, which will run the fan at 7V. This link (http://www.procooling.com/articles/html/quiet_fans_-_power_control_met.shtml) gives more detail and information about rheostats and other options.

[John0904]

**Update**

I couldn't find the rheostat that I wanted and ended up going to Radio Shack and getting a 25 ohm/3 watt.

I made all connections and added the fan that is going to run on it.
No problem on the high settings.

But on the lowest setting, the rheostat gets rather hot!

Is this common? And is is possible that the rheostat can burn out causing the CPU to fry and causing me to cry? (Not in the literal sense.) :p

Btw, this setup is not connected to my CPU at the moment.

Thanks.

[Fruss Tray Ted]

Is rheostat and potentiometer the same thing?

Power absorbing resistors (rheostats incl.) tend to get hot because they are bleeding off or absorbing current otherwise intended for your CPU fan.

The fact you picked a low wattage one compounds the problem and may get too hot eventually and fail. I once made a set of switches for 12 volt halogen lamps with an array of ceramic, high watt resistors from Radio Shack. And they still got hot.

The lowest setting has the most current bleed so it will be the hottest setting. Is this something you can put in an airflow's path to keep it somewhat cool? If you use several resistors in parrallel, they will share the current bleed (or absorbtion) and not get as hot. But you will need to decide how many, use Ohm's law to come up with an ohmage that you will need and place the array near the exhaust side (to the rear) of your case.

Or get a higher rated pot.

[John0904]

Huh? :p

I am not an electrician by any means. Heck, I even had a hella time just soldering the wires on the rheostat. :D
Here is the setup of what I did basically. http://www.bit-tech.net/article/56/
And adding resistors to the equation? Woohoo! I just wanted a simple way to control the speed of the fan to lower the noise level of it. ;)

Unfortunately, I am unable to put the rheostat in any airflow's path.

This setup has been running for 3 hours at the lowest setting now. Still hot, but not hot enough to burn the skin. :)

My only real concern now is that I don't want the rheostat to burn out and fry the CPU.

Thanks for the reply.

[YODA74]

john where did you put these resistors? and for what purpose can you take maybe this pic here put it into paint and redraw where you put the resistors? this should get warm but not hot.

http://www.bit-tech.net/images/article/56/speedmod4.jpg

[John0904]

What resistors? I have none.
Those red things are just heat-shrink sleeves.

All I have is the rheostat between the yellow wire just like in the picture. :)

Should I get a higher ohm/watt rheostat rather than a 25 ohm/3 watt?

[YODA74]

And adding resistors to the equation? Ok I misunderstood you.I thought you added resisters for a lite or?? As long as that fan is not pushing more 3w your ok and be sure where ever you mount this especially on metal that you put a rubber washer or neopreme so the rehostate doesn't short out if you mount it in a Beisel it doesn't matter.

mine gets pretty warm ( where you really don't want to hold your fingers very long) but i run a bunch thru it

[Fruss Tray Ted]

Yoda,
That would be me, the do-it yourselfer with the light dimmer. :)

John,

As it appears to be a factory made and recommended rheostat, you should be fine. The rear of the case where it is installed will act as a heat sink so it shouldn't get too hot. Your trying it for an hour or so and the fact you still could touch it should be fine and you should be good to go.

In the picture the coils are exposed. This is good yet it can be bad. Good for cooling, bad for dust getting under the wiper arm if you change the settings a lot especially when you haven't dusted out your pc in a while.

If it does 'open' (circuit) and the fan stops or the safeguard reads too low a fan speed your pc should shut down as if you unplugged it. So you shouldn't need to worry. I overclocked mine too far once and that's exactly what mine did, shut right down when Windows began to load.

If you are still concerned about reliability, measure the ohmage you like to set it at and buy a resistor of the same measurement but a higher wattage/current rating. Solder that in line and you would have the same effect but no wiper arm to get dirty. Honestly I don't think you need to.

Just wear your headphones and turn the music up. ":eek: " There, can't hear the fan now, can you?!? :D

[John0904]

Well...I've done the deed.

I replaced the old fan/heatsink with the delta fan/heatsink controlled by the rheostat.
Running about 45ºC average at medium speed now. Whereas my average temp was close to 60ºC.

Think that 35ºC degrees that I mention before was during the winter with the fan running high and the case open.

Nevertheless, I'm happy. :D (Till the rheostat burns out.)