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What line to put rheostat on ?

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May 24, 2001
NY where the cows out number the people
I have some case fans with thermal sensors on them. And have found that they are quite useless because there temp range is so drastic ( 80C max) They only work at full speed if the probe is reading 80C. No way will I let anything in my case get close to that so Would I be able to cut off the thermal sensor and put a rheostat there instead ? If I do this mod will the fan still work at it's full potential when the rheostat is turned up all the way? How would I wire it ? The thermal probe has two wires coming into it and they are both white so how can I do this mod ? Thanks for your input any and all is greatly appreciated.
Not knowing exactly how your fans work in regard to temperature, could you enlighten me? Is there a simple mechanical thermalswitch inside the housing, that closes at a certain temp threshold? Do you provide an external sensor to the fan, thermistor, TC, ?
Do the fans have a make and model number that can be researched?
Mo info...

The fans are enermax fans they have an external thermal sensor with two leads that run back to the fan. The fan is powered by a connector that goes to one of the main board fan headers. The fan speed will vary depending on the temp. It starts at the lowest setting arround 30C and then speeds up as it gets hotter. The fastest speed is at 80C. That's all I know about them.
That's better.
I'd bet they are using a thermistor in the sensor. Thermistor resistance goes down as the temperature goes up. You don't need a rheostat, just a simple potentiometer (pot) as the current through the thermistor is small. You are going to simulate the resistance in the thermistor with a pot. Cut the sensor off and put a pot in it's place. You need to measure the resistance of the removed thermistor and choose a pot with a total resistance equal to it. IE lets say the thermistor reads 10k ohm at 25C. At 80C it will read about 1.6k ohm. Replace it with a 10k ohm pot and as you turn the resistance of the pot down, the fan will speed up. Actually, that's a pretty neat fan. Too bad the threshold temperature is so high. What was Enermax thinking?

Why not put a trimmer pot in between the thermistor and the circuit? Add a touch of resistance so the fan hits full speed at the temp you want it to.
The trimmer is a great idea. If you hook it up in parallel with the thermistor you should be able to lower the resitance to make ithe fan run full speed at any temp you want.
I'v never heard of a Trimmer Pot. Could you expand on what it does and where I could get one. The local radio shack guys are worthless here where I live. There all 16 year old high school kids who don't know anything about electricity. Oh well. Does radio shack carry these or do I have to go elsewhere to get one ? And please help me on what size to get. the little probe on the end has 103 printed on it. Don't know what that means. maybe it's resistance in ohms ?? Help me out here I'm in new ground with pots never heard of them. Thanks guys
works great use 10K or volume from radio let me get a link. HERE

check out sitka's line near the end.

Oh yea use the middle lead and one outside lead on rheostat, or on POT it is kind of obvious, use one of the pairs.
My favorite is the 47K one from the Shack I use for motherboard voltage mods. I doubt you need one with that much resistance. They are 49 cents and on the upper right hand corner of page 276 in the current catalog. You may want to go for the 15 turn trimmer just below these.
I looked at the 10K Ohm 15-Turn Trimmer on the shack website.

I see it has three connections on it. So If I can pick one up before work tomorrow how would I hook it up ? As I've described before the temp probe has two lines coming into it. Both white. What is correct way to hook this up ?
Connect the center lug to one of the end lugs. Connect a wire to that junction. Then caoonect another wire to the other end lug.
Well It's all been done. And I was sucesfull. Well sort of. I cut off the thermal probe and left my slef some slack in the wires just incase my whole experement didnt' work I would have enough wire to solder it back on and put it back to the way it was before.

First thing I tried was to connect the two leads togethere and found out that made the fan run at full speed. Cool, so I put a rheostat on the leads, one on the end and one in the middle. The fan wasn't affected by the rheostat. It didn't speed up or slow down. So I whiped out the wire cutters. Looked at the orange, yellow and black wires.....mmmm....Choped the black line and put the rheostat on that. I connected the two leads form the thermal probe together and tried it out that way. It works good this way. I can change the speed of the fan by changing the rheostat. I assume that the black line is the ground. Not sure if it was a good idea to put the rheostat on the ground line but it works so take that enermax.
Sure. Just mail me a self addressed stamped envelope and I'll put it in there for ya and mail it right back. I'll email my home address to you, don't need all the girls here sending me love letters. wink wink Hi ladies but sorry, my Porsche 911 GT2 didn't come free. I can't afford girls LOL, just nice cars for SPL competitions and car shows.
Thanks for the picture hoot. tried out some rheostats jumpered that way and they behave a little different. Wish I knew why exactly, but nice, more options. I guess the diagram shows a true Rheostat tap as opposed to a voltage divider? Found this little diagram Here
I love that link. My intention was not to use a high current resistor to drop the voltage, just to lower the resistance normally seen from a thermistor, to fool the fan control circuit into speeding up. In theory, all you should have had to do was substitute the variable resistor in place of the thermistor. Think of the thermistor as a variable resistor, whose resistance varies as a function of temperature, instead of turning a shaft. Regardless, I'm glad you have a working solution.