What kind of resistors to splice into fan power?

[Joony]

I'm trying to silence my G4 Digital Audio tower, the fans in the system are quite loud.

Quiet replacement fans are quite expensive!

What is the usual resistor value that would do the trick? I happened to have some 1k ohm resistors (that would be 1000 ohms right?). Would that be too much or too little?

If I remember correctly from my physics knowledge: V = IR

hmm?

 

[akira34]

Why not just pick up something like this to control the fan speed? Otherwise you'll risk doing more damage to the system than anything else. Unless you're itching to get a replacement system. In which case, ship that old one to me, I'll put it to at least some use.

 

[timecop67]

I would start off between 30 to 70 ohms. If you have access to a decade resistance box you can find the value precisely that you want to use inline with the fan.

 

[wisdomtooth]

I just finished a soldering spree to slow down my case fans.

The fans in my system are 12v and usually draws between 0.08 amps to 0.15 amps.

A 100ohm, 0.5-watt resistor usually does the trick-- Slows the rotation speed from 5000+rpm down to about 1900rpm for fans rated at 0.08amp, and down to about 2800rpm for fans rated at 0.15 amps.

I definitely like using the resistors better than a fan controller because 1) I want the fans to spin slower permanently-- I never need to dial them up, and 2) it's cheap! A 5-pack of 100ohm/0.5W resistors at Radio Shack costs me two bucks.

In particular, splicing in that resistor for my Northbridge cooling fan will increase my system life, if anything... The original fan spins at a whopping 6000rpm's, and requires VERY frequent cleaning and lubrication. Otherwise, it begans to buzz loudly from the spindle wear. Slowing it down will increase its spindle life.

The caveat to slowing down fans is of course that you still need to maintain adequate cooling. That means you need to pair slow fans with the BIGGEST honking heatsinks you can get to fit in your case.

HTH!

 

[BonzaiDuck]

I see nothing wrong with this if you know what you're doing, but it seems a bit inflexible.

I started building "MOJO-I" last summer, and am still "tweaking" it. I chose an Aerogate-II CoolerMaster controller, and although there are some features I don't like about it as a manual fan-controller, it has been great for experimentally monitoring fan speeds and temperatures from strategically placed sensors. It has helped me with my ducting design to cool down my chipset (865pe) significantly.

But I'm starting to sour on manual control. First, I am still grappling with reducing the number of case fans for simplicity. I have conquered the noise problem. I've decided to live with twin intake 120's and twin exhaust 92's. The CPU fan is a 120x38mm on an XP120 cooler.

I've favored buying fans that have a wider speed range, gambling (with some success) that the high-decibel noise ratings apply mostly to the top-end, and that noise level drops dramatically by selecting fans for a lack of motor noise (despite decibel ratings), and moderating the speeds. I'm controlling my CPU fan automatically through the CPU_FAN header on the mobo with SpeedFan 4.20. I think I like pulse-width-modulated fan control better than use of a manual controller.

Two software-driven controllers have come to my attention: VLSystem Zephyrus and mCubed T-Balancer. They both communicate with the CPU through USB 2.0. Reviews say the first go-around on the software interface is buggy. They cost about $60 for either model.

If only mobo manufacturers would implement the full capabilities of the Winbond or other super-IO controller chips on the mobos, these other solutions would find only a marginal market. They are, after all, extra hardware with extra software overhead. I'd rather stand pat with SpeedFan and carefully chosen command-line parameters.

 

[KF]

Here is the math and theory.

The fan speed will drop roughly in proportion to voltage (I think). So will the current draw of the fan. A lot of fans are marked with the current and/or watts on the label. If it's watts, divide by the voltage, 12, to get current. Power = volts X amps. Or amps = Power / volts.

Resistance = volts / amps. That is, 12/ amps gives you the resistance of the fan motor.

If you want 1/4 speed, you need to drop 3/4 of the voltage (12) with the resistor and leave 1/4 (df 12)for the fan. That means the series resistor will need to be 3 times whatever the resistance of the fan motor calculates to.

Let F be the fractional speed you need. For instance, if you want 2000 RPM from a fan that spins at 3000 RPM at 12V, then F is 2000/3000 =0.66. The voltage across the fan will need to be F X 12. The voltage accoss the resistor will be whats left, (1 - F) X 12. That means the ratio of the dropping resistor to the fans resistance will need to be (1-F)/ F

Let R be the resiistance of the fan motor.

Then ( (1-F)/ F ) X R is the resistor you need.

Let G = (1-F) . (1-F ) X 12 = G X 12 is the voltage accoss the resistor.

When the fan is large and draws a lot of current, you may need to watch out that the resistor can handle it without burning up.

The wattage the resistor will disappate is
power = voltage squared / resistance =
(G X 12 X G X 12 )/ ( G/F X R) =
(G X F X 144)/R = ( (1-F) X F X 144 )/ R

The maximum power dissapated occurs when F = 1/2.

A rule of thumb is that you will need a resistor rated at twice the calculated wattage to avoid it getting so hot as to burn up anything it might touch. All the wattage rating for a resistor means is that the resistor will not go up in flames in a reasonably cool environment. IAC, you need a resistor that can handle 1/2 the wattage of the fan, worst case. If the resistor is larger than, or smalller than, the fans resistance, the resistor will disappate less power than at 1/2 voltage

I thought this was going to be more straightforward! I hope I didn't screw up at 3 AM.

 

[Bluefront]

Keep in mind that many fans simply will not start at low voltage, if you use a resistor. On the other hand, those same fans will start ok at the same voltage if you use a PWM fan controller.

Before doing alot of work, I suggest checking each fan for it's low-voltage starting ability. You can temporarily tap into the 5v line(red wire) on the molex connector....then see if the fan will start ok. If so, you can then use a resistor to lower the voltage to the fan, being certain it will start up.

 

[kornphlake]

well if you're going to tap into your 5v to test the fans, and they work why not hook up a switch to toggle between 5v and 12v, that way you can slow the fans down without a resistor and speed them up with the flick of a switch if you need more cooling. Or if you don't plan on ever needing the fans running at full speed just hardwire them to the 5v on the molex. I'm not sure if your fans will actually work at 5V though, you'll have to try it out.

 

[Navid]

Originally posted by: kornphlake
well if you're going to tap into your 5v to test the fans, and they work why not hook up a switch to toggle between 5v and 12v, that way you can slow the fans down without a resistor and speed them up with the flick of a switch if you need more cooling. Or if you don't plan on ever needing the fans running at full speed just hardwire them to the 5v on the molex. I'm not sure if your fans will actually work at 5V though, you'll have to try it out.

5V is good to test the fan to make sure it will start at lower voltages.
But, 5V is not enough for cooling. At 5V, the fan RPM will be too low. Usually, you can silence a fan by dropping its voltage to 7 to 8V.

As mentioned before, dropping the fan RPM has the negative side effect of degrading cooling. So, never do this unless you run tests and confirm that your system will still be cooled enough.

 

[wisdomtooth]

The behavior I've seen with many different fans is that if you try to drop their RPMs down to below 2000, a lot of them will indeed stall on start-up-- It will just shimmy back and forth and not turn. For that reason, proper resistor selection is important.

I find that 100ohm, 0.5-watt resistors work well with 12v fans that spin at a default speed of 5000rpm or more (which usually draws anywhere from 0.1 to 0.2 amps of current).

Slower fans don't necessarily mean poorer cooling... It just means you need to compensate with a BIGGER heatsink.

My Zalman CNPS7000AlCu heatsink with its 1300rpm 90mm fan cools better than Intel's stock 3000rpm fansink, and is far quieter.