# Thread: CPU fan, PWM, Capacitors. Help.

1. 1kW User
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If you go to a store, you generally only need to say the ohm value because by default, they assume 1/4 watts or 0.25 watts.

I specified the wattage because for this particular case, it would be safer to user resistors rated for higher wattage as those won't heat up that much.

In one of my previous posts, I gave you links to articles and I also wrote the calculations I used to come up with the values 10 to 100 ohm. I thought I was very clear that you should pick several within that range and try and see which one gives you the best result when placed in circuit.

Not my fault you can't read and you're not willing to do further research (by clicking on the links that explain)

The voltage received by the fan will depend on the value the resistor has, so 10 ohm will mean very little drop in the fan's voltage, 100 ohm would mean a lot. Depending on how much the value is.
The 330ohm value may be too much, causing the fan to receive too little voltage, therefore not spin at all (fans and motors in general need a higher voltage to start and after the initial resistance they work with less), making you think I gave you crap advice.

And in the end... it's free help, you didn't pay anything for this so I don't really appreciate your attitude.

2. Originally Posted by mariush
If you go to a store, you generally only need to say the ohm value because by default, they assume 1/4 watts or 0.25 watts.

I specified the wattage because for this particular case, it would be safer to user resistors rated for higher wattage as those won't heat up that much.

In one of my previous posts, I gave you links to articles and I also wrote the calculations I used to come up with the values 10 to 100 ohm. I thought I was very clear that you should pick several within that range and try and see which one gives you the best result when placed in circuit.

Not my fault you can't read and you're not willing to do further research (by clicking on the links that explain)

The voltage received by the fan will depend on the value the resistor has, so 10 ohm will mean very little drop in the fan's voltage, 100 ohm would mean a lot. Depending on how much the value is.
The 330ohm value may be too much, causing the fan to receive too little voltage, therefore not spin at all (fans and motors in general need a higher voltage to start and after the initial resistance they work with less), making you think I gave you crap advice.

And in the end... it's free help, you didn't pay anything for this so I don't really appreciate your attitude.
Lets not jump onto some wrong conclusion. The thing is, as you said 0.5 to 2w. The resistors dont have any specs printed on them just colored bands. So for an amateur like me, i am caught in confusion. I did visit your links, several times, but all went over my head. Its easy for you because you KNOW about them. Its not easy for me. again i dont understand how i will know what wattage it has? Just tell me... should i forget about wattage & just look at ohm value?

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The higher the wattage, the thicker they are. If the seller doesn't know, it's probably 0.125w or 0.25 w. These will probably also work but as I said, they may heat up a lot so you'll have to be careful because they may melt plastic or whatever is close to them

If you're in US, buy them from digikey.com : http://search.digikey.com/scripts/Dk....dll?Cat=66690

Select the Ohm value, apply filters, then select the 0.25w, 0.5w, 1w and 2w (use Ctrl + Click), apply filters, then click on view page button to see results.

Sort by minimum quantity you can order so that you can buy only one if you want (you don't want to buy 4000 pieces) ... pick one or two of different power ratings. They cost anything between 4 cents and a dollar each.

For example:
8 cents - http://search.digikey.com/scripts/Dk...F14JT100RCT-ND 100 ohm 1/4w (0.25w)
33 cents - http://search.digikey.com/scripts/Dk...name=100W-2-ND same 100 ohm but rated for 2w

If you're in Europe, you have farnell.com : http://uk.farnell.com/fixed-value

Check on the resistance column the values you want and from the power rating 250mW, 500mw, 1w and 2w. Apply and pick whatever you want.

4. Thanks for that.

Now... what do think this one is rated?

2nd: Sorry... but why did you suggest me to cut the PWM wire? I did that and nothing happened. Nothing AT ALL! and as i said, it was red. When i installed the 3.3Kohms resistor on the red wire, the fan just moved an inch. So that was a good start, shows that i definitely need to try lower than 200'o'hm.

Please can you tell me why did you suggest me to cut PWM wire?

Edit: Most importantly, Does polarity matter while installing a resistor? I tired both sides and same results.

Just got bunch of different resistors from my friend.
Last edited by Serious Sam; 09-17-2011 at 09:18 AM. Reason: well... i pressed go advanced and it posted :/.

5. Flux Capacitor User
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PWM uses intermittent power to contol the fan speed. The fourth wire is necessary for a fan to be PWM.
A three wire fan is controlled by the voltage, the third wire reports the fan rpm.
I assumed with the PWM control removed the fan would default to a three wire fan.
Normally the first wire is ground, second is +12 volt, third is rpm, fourth PWM.
I will have to plug some PWM fans into three pin headers. It makes sense to me it would work.
The resistor has to go on the +12 volt wire. I believe the second one.
Wait for someone to tell me I'm wrong.

6. mariush, i dont see you!

So went to the 'central market for all things tech' for 2w resistors between 100 and 200ohm. Got 100, 120, 150, 220ohm.

Didn't work 220ohm, 150ohm.

Worked 120 and 100ohm. Though were very slow. So mariush, where did your calculations went wrong?

Now time to go again and try looking for below 100ohm.

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It's possible that my calculations were wrong but I doubt it.

More likely, it's because those are calculations for IDEAL conditions.

1. The connector on the motherboard doesn't always give you exactly 12.00 volts. It can be anything between 11 and 13 volts.

2. When any motor starts, it needs a bit more power to overcome FRICTION and INERTIA. Once the blades start spinning even just a bit, the voltage can usually be lowered further. If you think about it, it's the same with cars.. try to push a car and you'll notice it's much harder to start than to keep moving it forward once it started to move.

3. You mentioned the fan was 12 volts, 0.6 Amps - this value could represent the PEAK power draw, when the motor starts and has to overcome that friction and inertia. Once the blades start spinning, the fan may never use more than 0.3-0.5 Amps. You can't be sure exactly what that 0.6A represents unless you ask the manufacturer for the fan datasheet.

So I don't know what more to tell you - I suggested from my first answers to go with a potentiometer, which would allow you to adjust from 0 to about 100 ohms and save you all this trouble.
Not my fault you're cheap.

8. Originally Posted by mariush
It's possible that my calculations were wrong but I doubt it.

More likely, it's because those are calculations for IDEAL conditions.

1. The connector on the motherboard doesn't always give you exactly 12.00 volts. It can be anything between 11 and 13 volts.

2. When any motor starts, it needs a bit more power to overcome FRICTION and INERTIA. Once the blades start spinning even just a bit, the voltage can usually be lowered further. If you think about it, it's the same with cars.. try to push a car and you'll notice it's much harder to start than to keep moving it forward once it started to move.

3. You mentioned the fan was 12 volts, 0.6 Amps - this value could represent the PEAK power draw, when the motor starts and has to overcome that friction and inertia. Once the blades start spinning, the fan may never use more than 0.3-0.5 Amps. You can't be sure exactly what that 0.6A represents unless you ask the manufacturer for the fan datasheet.

So I don't know what more to tell you - I suggested from my first answers to go with a potentiometer, which would allow you to adjust from 0 to about 100 ohms and save you all this trouble.
Not my fault you're cheap.
Hey hey cool down miss angry pants. No need to act like that .

Now you didn't answer my previous questions...

and as for potentiometer, i told you its not ideal. First thing, i use a fan controller. So yeah. I am making it for someone, someone who wont understand 2 shits about when to control the fan etc etc.

9. But with a potentiometer you don't have to mess around with different value resistors, you just set it to where the fan is spinning at a good ratio of noise/air flow and forget about it.

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I assume you mean the questions in this post?

Originally Posted by Serious Sam
Thanks for that.

Now... what do think this one is rated?

2nd: Sorry... but why did you suggest me to cut the PWM wire? I did that and nothing happened. Nothing AT ALL! and as i said, it was red. When i installed the 3.3Kohms resistor on the red wire, the fan just moved an inch. So that was a good start, shows that i definitely need to try lower than 200'o'hm.

Please can you tell me why did you suggest me to cut PWM wire?

Edit: Most importantly, Does polarity matter while installing a resistor? I tired both sides and same results.

Just got bunch of different resistors from my friend.
1. about the rating of that resistor, it's probably 0.5-1w

Can't tell what it's rated for without a reference point. It could be 5mm wide or it could be 3 cm wide, I can't tell. People usually picture something along with a coin or something for which dimensions are well known.

2. 2nd: Sorry... but why did you suggest me to cut the PWM wire? I did that and nothing happened.

The more recent motherboards have a connector designed for the CPU fan with 4 wires. However, some older motherboards only have 3 pin connectors so most coolers that come with CPUs are designed to revert to the "dumb" design when the PWM wire is not connected to anything.

See the pinout of the connector here:

The colors can change but the wires are arranged in order of importance: ground, power, sense (tells the fan rotations per minute), pwm mode.
This way you can connect the fan even in two pin connectors - the first two pins are the power.

Control is the PWM wire. On motherboards with 3 pin connector, it just stays in the air unconnected.

If the circuitry in the fan receives no signal on the PWM wire, it falls back to "dumb" mode (always 100% speed) so by adjusting the voltage, you'll change the speed.

If the PWM wire remains connected, the fan may not work properly unless the voltage is at least 11v or something like that. So you're interesting in disabling the PWM part and you do that by cutting that wire.

So nothing happened... of course. The fan was working before in PWM mode at 100% speed and now without PWM, it still works at 100% speed. You probably deactivated in BIOS some settings like "Cool And Quiet" which adjust the speed of the fan based on the temperature of the CPU so your fan was staying at 100% all the time.

3. There's no polarity with resistors.

4. A potentiometer would have been a good idea still.

Just use it, adjust the speed to what you need, pull out the potentiometer and use a multimeter to determine the set value, then mount a resistor with a value close to that in its place.