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Heatsink/ Heat Pipe / ThermoSiphon Cooling The cat will only make the mistake of putting its paw by your HSF once. :) Also the place to discuss the new high end heat pipe goodness.

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Unread 10-10-2003, 10:04 PM   #1
SparkedFire
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Default rheostat question.

what kind of (watt rating) rheostat will i need to slow these fans down pretty good? the fans pull 12watts. dows that mean i need a 12watt rheostat?

Link here to see the fans
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Unread 10-10-2003, 11:42 PM   #2
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I ran the numbers once...

You need a minimum of 1/4 of the power than you're trying to control. In your case, that would be 3 Watts. Of course you want a little bit more, vbecause you don't want to run a rheostat at maximum power, so I'd go with a common 5 watt unit.

Now where's that thread...
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Unread 10-11-2003, 07:30 AM   #3
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thanks, but i actually want the fan to run at full speed. eventually i will be using pelts, to cool my northbridge, and CPU.
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Unread 10-11-2003, 09:51 AM   #4
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Quote:
Originally posted by bigben2k
I ran the numbers once...

You need a minimum of 1/4 of the power than you're trying to control. In your case, that would be 3 Watts. Of course you want a little bit more, vbecause you don't want to run a rheostat at maximum power, so I'd go with a common 5 watt unit.

Now where's that thread...
Ummm, could be wrong here but if he has a 12watt fan he will want about 1/4 MORE of a rated rheostat. Say 14-16 watts. A rheostat is basically a variable resistor. If you turn it down the extra load is dumped by heat output. So if he used a 5watt rehostat and turned it down to say 6watts then the rheostat would have to dump 6 watts of load. If the thing is only rated at 5 watts then expect a fire. Seriously, been there done that.

You have to think backwards with rheostats. The more you turn it down the more load it is handling. The higher you run it the less load it is handling. Or maybe I just missed something...:shrug:
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Unread 10-11-2003, 12:01 PM   #5
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Quote:
Originally posted by jaydee116
You have to think backwards with rheostats. The more you turn it down the more load it is handling. The higher you run it the less load it is handling. Or maybe I just missed something...:shrug:
If the fan was simply a resistor (electrically), the most power the rheostat could dissipate would be 1/4 of the fan's 12V power dissipation. (Google "Maximum Power Theorem")

The maximum dissipation would occur when the voltage to the fan was at 1/2 of the total voltage. At this point, the current through the fan would be 1/2 the current at full voltage and the fan power dissipation would be:

1/2 * 1/2 = 1/4

Same for the rheostat.

Fans do not behave exactly like resistors though, so some margin is in order. I agree with Ben's suggestion.
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Unread 10-11-2003, 12:49 PM   #6
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i dont necessarily want to spend this kind of money, but will this rheostat do the job of controlling two high speed (120mm) fans? do you have any links to places that i could pick up a good rheostat?
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Unread 10-11-2003, 01:26 PM   #7
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Quote:
Originally posted by Since87
If the fan was simply a resistor (electrically), the most power the rheostat could dissipate would be 1/4 of the fan's 12V power dissipation. (Google "Maximum Power Theorem")

The maximum dissipation would occur when the voltage to the fan was at 1/2 of the total voltage. At this point, the current through the fan would be 1/2 the current at full voltage and the fan power dissipation would be:

1/2 * 1/2 = 1/4

Same for the rheostat.

Fans do not behave exactly like resistors though, so some margin is in order. I agree with Ben's suggestion.
I spent several months messing with pots and any fan rated over the pot makes the pot fry when the pot is turned down. The more you turn the pot down the hotter it gets. I fried several 3watt pots on 5watt fans when turning them down. I I run the pot wide open then the pot works fine at ever tripple the rated the fan. The pot is doing all the work not the fan.

Give it a shot and you will see what I am talking about. I know your an electronics guy but this is just my experience. I got 15 dead pots to prove it.

Sparked: here is the identical pot as that one from frozen for $1.50. http://www.allelectronics.com/cgi-bi...-15&type=store
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Unread 10-11-2003, 02:42 PM   #8
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thanks for the linkage

how do you know i know some electronics stuff? you remember me from the pelt cooling your northbridge thread?
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Unread 10-11-2003, 03:59 PM   #9
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Quote:
Originally posted by jaydee116
I spent several months messing with pots and any fan rated over the pot makes the pot fry when the pot is turned down. The more you turn the pot down the hotter it gets. I fried several 3watt pots on 5watt fans when turning them down. I I run the pot wide open then the pot works fine at ever tripple the rated the fan. The pot is doing all the work not the fan.
Yes, I wrote too simplistically. What resistance pot were you using?

Pots/rheostats should be sold with a current rating as well as a wattage rating. (Quality vendors do give both these specs as well as an end to end voltage spec.)

You are right, it is possible to burn up a pot while dissipating less than the pot's rated power. When the total resistance of the pot is higher than necessary for the job, you can get too much power dissipation in a small portion of the pot's resistance.

Ideally the resistance of the pot should be chosen so that when it is at maximum resistance, the voltage across it is 6V. (With the fan in series.) This will prevent the problem of excessive power dissipation in a small portion of the total resistance.
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Unread 10-11-2003, 04:06 PM   #10
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would cooling the rheostats also help in preventing any problims?
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Unread 10-11-2003, 04:08 PM   #11
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Happy birthday jaydee! just noticed it was your birthday, or the forum said so at least.
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Unread 10-11-2003, 05:32 PM   #12
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Quote:
Originally posted by SparkedFire
Happy birthday jaydee! just noticed it was your birthday, or the forum said so at least.
Yup, 27 today. Thanks!

I was refering to Since about being an electronics guy as he has been doing it forever professionally.

And Since I think we are on the same page just different paragraphs. I think what was buring my pots up (the cheap Radio Crap one's) was the amperage maybe? At about 6-7 volts the pots would get hot and start smoking. But they would run fine if they were turned all the way to 12V. Oh yeah and they were 25ohm I belive. I liked the 20-25 ohm because they didn't go below 6ish volts. So if I turned the computer on the fans would still turn on. Anything under 6 volts and the fans wouldn't start turning.
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Unread 10-11-2003, 05:58 PM   #13
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Sounds to me like those pots shouldn't have been rated anywhere near 3 Watts.

If the pot was wide open (max resistance) and the voltage across it was ~6V, then the power dissipation would have been:

P = V^2 / R = 6^2 / 28 = ~1.3 Watts

Nowhere near the 3 Watt spec. I'd just chalk that up to being sold junk by Radio Shack. Not too surprising.
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Unread 10-11-2003, 10:49 PM   #14
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just to point out a link that might help with your discussion here, check This out, specifically The Lab section. you guys tend to be more advanced than i am electronically (i can solder pretty well, but prefer not to design too much), but this site does a pretty good job of explaining things on a step by step basis, which was perfect for me when trying to understand the basics of fan controlling. for what its worth anyway.
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Unread 11-21-2003, 01:09 PM   #15
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OK, I got 2x 15w rheostats, and 2x high output 120mm fans from AllElecrtonics and so far so good. I am still working on my modding project, which involves getting these babies installed. thanks for all of your help.
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