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Rheostat on Case fan(s)

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Tim the Toolman

Registered
Joined
Apr 20, 2001
Can you damage a fan by putting it on a Rheostat...
Anyone have some specs on rheostats they have they have installed on their systems? I have a monster Rotron 235 cfm coming and some "texas twisters" from Nidec going to test out.

Noise hmmm... I have a GWFop38 which is a little noisy but most of the time cant hear it over my 6 surround sound speakers :)
 

AMDGuy

Member
Joined
Apr 10, 2001
I put a rheo on my Sunon KD1212PMB1-6A and I'm currently testing the system. I used a Radio Shack Rheo outlined by Joe's article here http://www.overclockers.com/tips242/ and it tends to get a little warm when I max it's resistance. I'm thinking of putting a small heat sink on the back to help it keep cool. It says it's rated for 3 Watts but the fan is around 6 Watts. The specs for the rheo are here http://www.radioshack.com/product.a..._name=CTLG_010_003_003_000&product_id=271-265 (I hope that worked).
 

coldmop

Registered
Joined
Mar 4, 2001
This is a super important topic that goes mostly undiscussed.
In general DC motors like speed controllers not resistors.
And very much so when the motor comes under load. Now our fans don't load up much but a speed controller is a much better way to go because it switches the voltage on and off (quickly) to deliver the full 12V's the fan was built to run on. leave the switch off for longer and the motor slows down.

The real concern is what you do to the power supply, when you add resistence to decrease voltage. Comp powersupplies are sophisticated and when a twelve volt line is made to run at less it realy tries to make it 12Volts and stresses it. Most users have only been using rheostats for a little while so the end result is tough to predict. I'm speculating and there is a lot I don't know but I did buy a DC motor speed controller to use for my fans. They are available as little electronics kits for around $15.00 US. 12v in, then to a bus then to the fans. There is a 4 channel RHEObus being sold now for about $60 that seems to incorporate the mosfets and ICS needed for PWM (Pulse Width Modulation) but all the reviews that were done didn't even mention the difference between this and just plain rheostats. Heck the guys selling it don't even explain it.

Another good road is to use heat controlled fans, snip of the thermo sensor and install a rheostat there on the leads. Then the fans run as they were design to.
 

Hoot

Inactive Moderator
Joined
Feb 13, 2001
Location
Twin Cities
Reducing a fans speed by virtue of a rheostat does not damage the fan, nor does it imperil the PSU. It is the simplest method available and the price you pay is wasted energy in the form of heat. As long as the rheostat is capable of dissipating the amount of energy, in watts that you need it will work fine. Yes, there are more efficient methods, such as 3-terminal regulators, but even they dissipate heat and their output voltage is never equal to the input voltage, so full speed can not be achieved through them. Should you need to reach full speed at times, you can opt to bypass the regulator with a simple switch.

Hoot
 
M

MAV

Guest
I opened my 300w psu and found two sets of adjustments that could be made, the first one i noticed either decreased the fan speeds if i turned the setting screw anti-clockwise, when i turned the screw clockwise, i noticed that the fan speeds increased....the second adjustment screw , increased or decreased the voltage settings to the mobo, this i did not play with too much, but just to see what voltages were been increased or decreased,i noticed the following, the -12v i could either increase or decrease, the same with the +12v, the -5v & the +5v etc....interesting....anyone else come accross this on their psu's ?
 
OP
T

Tim the Toolman

Registered
Joined
Apr 20, 2001
VERY INTERESTING ESPECIALLY FOR OVERCLOCKING PURPOSES.... MY VOLTAGE MAXED IN MY BIOS ON MY KT7A READS IN MBM AND HMONITOR 1.94 AS OPPOSED TO HE 1.85 MY BIOS SAYS ... MAY LIMIT THE NEED FOR MOBO MODS FOR MORE VOLTS.... hmmmm ..... SOUNDS LIKE IT COULD BE A LITTLE DANGEROUS THOUGH PROBABLY WANT TO TAKE BABY STEPS AND TURN IT UP REAL SLOW....
 

coldmop

Registered
Joined
Mar 4, 2001
Hey Hoot, why does it not imperil the power supply. Or at least draw relatively a lot more current from it.
 

e_storm

Member
Joined
Mar 16, 2001
Location
Chicago, IL
And what's the difference between a rheostat and a 3 terminal voltage regulator? I saw that Hoot used the 3 terminal instead of rheostat for his dual purpose cooling mod.
 

Overload

Member
Joined
Dec 18, 2000
cold mop (Jun 06, 2001 05:30 a.m.):
Hey Hoot, why does it not imperil the power supply. Or at least draw relatively a lot more current from it.

Actually it will draw less current. when you add a resistor or reostat to a fan you add a voltage drop before reaching the fan and increase resistance to current. with reostat in the off position it acts like a sort (0 V drop and max Amps. in the fully on position it add voltage drop and decreases current.(theoretically think of it as going toward open circuit where all the voltage drop is across resistor and no current flow.) does this make any sence?
 

AMDGuy

Member
Joined
Apr 10, 2001
cold mop (Jun 07, 2001 05:04 a.m.):
I must not understand electricity very well

Don't sweat that. If you really get into overclocking you'll learn more than you ever watned to know with time.