Question slowing down a fan with a resistor

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loadyo

loadyo

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May 13, 2019
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I have just finished rearranging a server and put 5 of the hdd into a how swappable enclosure that fits into 3 of the 5.25 bay, it has a fan on the back to draw air over the drives, however, one of the fins has broken and it is off balance and rattling like crazy and i imagine will cause a lot of uneeded vibration, i have just raped an old fan from a computer CPU and plugged it in, its not rattling but it is running so fast the noise of the air rip is unacceptable, i read the i could add in a resistor to it to slow it down, i just tried two random resistors on the red wire but the fan does not spin, the fan is 12v 0.25a and has four wires red, black, blue and yellow, not sure where i should be adding the resistor or whether i have done something wrong, i dont understand the values and the fan is not for cooling a cpu, it just needs to draw a bit of air over the drives, can someone advise me how to do it ? i have lots of different resistors available.
 
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Very much doubt that the/a CPU fan will be very effective even if slowed down.

Plus using a resistor will just add heat to the overall system.

At least try to find or salvage a regular case fan.

Look at the original broken fan and match the specs as closely as possible.

I am all for DIY fixes and so forth. However, in the current situation you need to reconsider the options.
 
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Ralston18 said:
Very much doubt that the/a CPU fan will be very effective even if slowed down.

Plus using a resistor will just add heat to the overall system.

At least try to find or salvage a regular case fan.

Look at the original broken fan and match the specs as closely as possible.

I am all for DIY fixes and so forth. However, in the current situation you need to reconsider the options.
Click to expand...

Im not to worried about heat from the drives as the drives do not spin up unless they are being read/written to and then it only spins up the drive that contains the data, so a gentle flow of air will be ok. The fan has to be the size of a CPU fan as it sits in housing and is bolted to the back of the enclosure
 
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The formula you are looking for is applied fan voltage = 12 - R*I.
So a .25A fan will drop 0.25 volts across each 1 Ohm of resistance. 12 volt fans usually run okay from around 7 to 12 volts. A 20 Ohm 1/2 watt or one watt potentiometer or rheostat would be ideal for figuring out your fan speed. Then you can measure the resistance and substitute a fixed resistance near the value read from your multimeter.

https://www.mouser.com/ProductDetai...=sGAEpiMZZMvygUB3GLcD7jZSyzxDqivkfyRFaAmp2a4=
would work.
 
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Cant seem to log in here from my phone, possibly because i left it logged in on my laptop, anyway....

TJ Hooker said:
Heat from the resistor would be negligible.

Red wire should be the right one. Are your resistors color coded ones like these? http://www.resistorguide.com/resistor-color-code/

Something like 10-20 ohms should work. The higher the value the more it will slow down, until eventually it wouldn't spin at all.
Click to expand...

Yes they do look like that, i used a 1W one initially but the fan wouldnt spin, so i just grabbed a smaller looking one by eye and soldered that on, alas no spinning, guessing they are just to bigger value.

mark1776 said:
The formula you are looking for is applied fan voltage = 12 - R*I.
So a .25A fan will drop 0.25 volts across each 1 Ohm of resistance. 12 volt fans usually run okay from around 7 to 12 volts. A 20 Ohm 1/2 watt or one watt potentiometer or rheostat would be ideal for figuring out your fan speed. Then you can measure the resistance and substitute a fixed resistance near the value read from your multimeter.

https://www.mouser.com/ProductDetail/BI-Technologies-TT-Electronics/93PR20LF?qs=sGAEpiMZZMvygUB3GLcD7jZSyzxDqivkfyRFaAmp2a4=
would work.
Click to expand...

thats another project going forward, ill monitor temps once i have this going in a basic fashion, dont think i have a pot laying around and i want to get the server back in site by wednesday

USAFRet said:
REPLACE THE FAN.
$5 for a replacement case fan, rather than bodging up a DIY thing.
Click to expand...

Tried to find a replacement of the same value and couldnt find anything rated the same that wasnt from china, i dont have time to ship from there, although i could add it in later

USAFRet said:
Additionally, a CPU fan is just another fan.

Either you have it connected improperly, or it lacks the proper connectors to get speed adjustments form the motherboard/CPU.
Click to expand...

It doesnt make contact directly to the motherboard, the drive chasis has to molex inputs to power the 5 hdd's and the fan.
geofelt said:
A 12v fan will run slower at 7 or 5v.
Here is how:
https://www.silentpcreview.com/article6-page1.html
Click to expand...

The fan works with just one molex plugged in, i dont have the drives sitting in the chasis in case something goes wrong.

This how to you sent is very interesting, well written and something that an amateur could achieve, my soldering skills are very good as i do a lot of close up SMD work on consoles. I am a bit concerned about damaging anything with regard to feeding power back to the psu, i have a bench power supply that i can set voltage and ampage. so going forward for testing purposes, icould i actually use one of the molex plugs feeding the chasis, this way i am guaranteed that more is being drawn off than i am putting in ?
 
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loadyo said:
I tried to get a fan with same voltage and ampage but no joy there other than shipping them from china, the fan makes no contact with the motherboard, the drive chasis itself takes in two molex connectors which powers the 5 hdd and the fan, the fan works with just one plugged in.
Click to expand...
Just about every PC fan I've seen is 12V, and it doesn't necessarily need to have the same current of 0.25A (although if you want to be extra cautious you could make sure not to get one with higher current).

So are you not able to read your fan speed anywhere? E.g. in the BIOS, monitoring software, etc.

I wouldn't mess with the voltage going into the chassis, as then you're also affecting the supply to the HDDs.
 
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TJ Hooker said:
Just about every PC fan I've seen is 12V, and it doesn't necessarily need to have the same current of 0.25A (although if you want to be extra cautious you could make sure not to get one with higher current).

So are you not able to read your fan speed anywhere? E.g. in the BIOS, monitoring software, etc.

I wouldn't mess with the voltage going into the chassis, as then you're also affecting the supply to the HDDs.
Click to expand...

I have spare SYS_fan header on the mobo, and power fan header, i suppose i could plug it in there and get a reading from bios

i think running it at 7v would be ok 5 seemed a little to slow and 12v like a rocket
 
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didnt think of that, so i could just extend the wires to the fan and route it from the back of the unit straight into the motherboard header and set the speed through the bios ?, is it common place to be able to set the speed of the fan through the bios, the fan is a PWM but if i utilise that feature on the motherboard it will adjust speed according to case temp ?
 
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loadyo said:
didnt think of that, so i could just extend the wires to the fan and route it from the back of the unit straight into the motherboard header and set the speed through the bios ?, is it common place to be able to set the speed of the fan through the bios, the fan is a PWM but if i utilise that feature on the motherboard it will adjust speed according to case temp ?
Click to expand...
Well I have no idea what the physical layout looks like, but yes if you could get the fan connector attached to the motherboard fan header you should be able to control it. Being able to control the fan speed is pretty much universal for consumer boards. I'm not sure about server boards, but I don't know why it would even have a header if it couldn't be used to control speed.

Yes, the fan speed curve would typically control speed based on some temp sensor on the motherboard. But if you wanted you could just set a flat curve so that the speed is constant.
 
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Well, its not a server board, just a consumer one, i run an UNraid server. Going to do that now and will also play with the old fan and changing speed as its something i would like to learn. Thanks for your help
 
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For this you do not need fan speed control provided by a mobo fan header. You really want simple continuous operation of that fan from a fixed power source, such as is available from the PSU through existing wiring into the drive case; however, you want the speed reduced for noise reduction, so 12 VDC is too high a voltage. Regarding colour codes, from what you describe, the wires from the new fan motor should be: Black = Ground, Red = + DCV supply, Yellow = Speed signal, Blue = PWM Signal. For your purpose with no speed control or read-out, ignore and tape up the Yellow and Blue lines.

You have two ways to get reduced voltage to this fan. One simple way is to take slightly non-standard supply connections from a standard female 4-pin Molex output connector from the PSU. On them the two central lines are Black and grounds, Yellow on one end is +12 VDC, Red on the other end is +5 VDC. Whereas a common use is to connect between Yellow and Black for 12 VDC, using Yellow and Red gets you a difference of 7 VDC, sufficient to start and run a 12 VDC fan at reduced speed. To do this, connect Molex Yellow to Fan Red, and Molex Red to Fan Black.

The other way is to select the proper resistor and use the normal 12 VDC (Yellow and Black) connection from the Molex output. The voltage you supply to the fan should never be less than 5 VDC, and to be sure of start-up it really should be at least 7 VDC. Your "randomly-selected" resistor probably was just not the right value. So here's how to do the calcs, and there are two. One is the correct resistance value, the other is the proper wattage rating so the resistor can handle the heat it generates. Your fan draws at max 0.25 A when already running if fed a full 12 VDC, but more very briefly at start-up. So treat the motor as having a resistance of 12/0.25 = 48 Ohms. The added series resistor will have value R, yet to be found. We can set up two equations for your requirements.
Total resistance is R + 48 Ohms, total voltage across that is 12 V, current is unknown with value I.
Now the voltage is split with 7 volts drop across the motor at a current of I with a resistance of 48 Ohms, and a drop of 5 V across the resistor of R Ohms and a current of I.
So, I= 7/48 = 0.1458 amps from the motor,
and thus R=5/0.1458= 34.29 Ohms.

Just to check, if we put 48 Ohms (motor) and 34 Ohms (resistor) in series with 12 VDC across them the current will be 0.1469 amps, and the two voltage crops will be 7.02 V across the motor and 4.99 V across the resistor. The power dissipated in the resistor will be 4.99 x 0.1469 = 0.733 Watts.

So in rough terms you could use a resistor of 35 Ohms, with a 1 W rating.

Re-doing this for a 9 VDC supply to the motor (rather than 7 VDC), we get I= 9/48 = 0.1897 Amps, R = 3/.1875 = 16 Ohms, and resistor power rating = 3 * .1875 = 0.56W.

So use those two calcs as a "bracket". The Resistor value needs to be in the range 16 to 35 Ohms, with a power rating of 1 W. Use the lower end if you want the fan speed to be higher, and MAYBE use a 2 W resistor to be sure it does not get overheated. The real truth is that the effective "resistance" of the motor will not be as calculated above, because its real impedance is less that those values when it is running slower. But this range of Resistor value still will work.
 
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TJ Hooker said:
If you have fan headers available then using a resistor or your own DC power supply is probably unnecessary. You will likely be able to set the fan speed through the motherboard.
Click to expand...
TJ Hooker said:
Well I have no idea what the physical layout looks like, but yes if you could get the fan connector attached to the motherboard fan header you should be able to control it. Being able to control the fan speed is pretty much universal for consumer boards. I'm not sure about server boards, but I don't know why it would even have a header if it couldn't be used to control speed.

Yes, the fan speed curve would typically control speed based on some temp sensor on the motherboard. But if you wanted you could just set a flat curve so that the speed is constant.
Click to expand...

Alas no joy there, it only gives option for CPU fan to set to PWM, voltage or auto, the sys fan will only tell me the RPM and no other options

Paperdoc said:
For this you do not need fan speed control provided by a mobo fan header. You really want simple continuous operation of that fan from a fixed power source, such as is available from the PSU through existing wiring into the drive case; however, you want the speed reduced for noise reduction, so 12 VDC is too high a voltage. Regarding colour codes, from what you describe, the wires from the new fan motor should be: Black = Ground, Red = + DCV supply, Yellow = Speed signal, Blue = PWM Signal. For your purpose with no speed control or read-out, ignore and tape up the Yellow and Blue lines.

You have two ways to get reduced voltage to this fan. One simple way is to take slightly non-standard supply connections from a standard female 4-pin Molex output connector from the PSU. On them the two central lines are Black and grounds, Yellow on one end is +12 VDC, Red on the other end is +5 VDC. Whereas a common use is to connect between Yellow and Black for 12 VDC, using Yellow and Red gets you a difference of 7 VDC, sufficient to start and run a 12 VDC fan at reduced speed. To do this, connect Molex Yellow to Fan Red, and Molex Red to Fan Black.

The other way is to select the proper resistor and use the normal 12 VDC (Yellow and Black) connection from the Molex output. The voltage you supply to the fan should never be less than 5 VDC, and to be sure of start-up it really should be at least 7 VDC. Your "randomly-selected" resistor probably was just not the right value. So here's how to do the calcs, and there are two. One is the correct resistance value, the other is the proper wattage rating so the resistor can handle the heat it generates. Your fan draws at max 0.25 A when already running if fed a full 12 VDC, but more very briefly at start-up. So treat the motor as having a resistance of 12/0.25 = 48 Ohms. The added series resistor will have value R, yet to be found. We can set up two equations for your requirements.
Total resistance is R + 48 Ohms, total voltage across that is 12 V, current is unknown with value I.
Now the voltage is split with 7 volts drop across the motor at a current of I with a resistance of 48 Ohms, and a drop of 5 V across the resistor of R Ohms and a current of I.
So, I= 7/48 = 0.1458 amps from the motor,
and thus R=5/0.1458= 34.29 Ohms.

Just to check, if we put 48 Ohms (motor) and 34 Ohms (resistor) in series with 12 VDC across them the current will be 0.1469 amps, and the two voltage crops will be 7.02 V across the motor and 4.99 V across the resistor. The power dissipated in the resistor will be 4.99 x 0.1469 = 0.733 Watts.

So in rough terms you could use a resistor of 35 Ohms, with a 1 W rating.

Re-doing this for a 9 VDC supply to the motor (rather than 7 VDC), we get I= 9/48 = 0.1897 Amps, R = 3/.1875 = 16 Ohms, and resistor power rating = 3 * .1875 = 0.56W.

So use those two calcs as a "bracket". The Resistor value needs to be in the range 16 to 35 Ohms, with a power rating of 1 W. Use the lower end if you want the fan speed to be higher, and MAYBE use a 2 W resistor to be sure it does not get overheated. The real truth is that the effective "resistance" of the motor will not be as calculated above, because its real impedance is less that those values when it is running slower. But this range of Resistor value still will work.
Click to expand...

Thanks, appreciate that, unfortunately math is not my strong point and my understanding of what you just wrote is out of my remit just now, theres a lot more i need to understand about wattage and ampage before i could digest what you wrote, for now, i just want to bang in a resistor safely and if it is quieter i am happy for now...i can play with the damaged fan over the coming weeks to learn this as i am genuinely interested...with regard to spurring from one of the psu connectors, the chassis is using two molex, if i trace back down the lead toward power supply there is a sata connector free, i noticed that this has an additional orange wire which doesnt appear back toward the molex, i would like to use the spare sata connection if its safe to do so.

https://www.dropbox.com/s/cyxoynrmitwwm44/WIN_20190514_19_27_33_Pro_LI.jpg?dl=0
 
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Paperdoc said:
Your fan draws at max 0.25 A when already running if fed a full 12 VDC, but more very briefly at start-up. So treat the motor as having a resistance of 12/0.25 = 48 Ohms. The added series resistor will have value R, yet to be found. We can set up two equations for your requirements.
Total resistance is R + 48 Ohms, total voltage across that is 12 V, current is unknown with value I.
Now the voltage is split with 7 volts drop across the motor at a current of I with a resistance of 48 Ohms, and a drop of 5 V across the resistor of R Ohms and a current of I.
So, I= 7/48 = 0.1458 amps from the motor,
and thus R=5/0.1458= 34.29 Ohms.
Click to expand...
The effective resistance of the motor isn't a constant. It will also vary with the speed of the fan.
 
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loadyo said:
Alas no joy there, it only gives option for CPU fan to set to PWM, voltage or auto, the sys fan will only tell me the RPM and no other options
Click to expand...
This is what you want, these are different fan control methods you can choose from. Given it's a 4 pin fan, it should be PWM. Somewhere else in the BIOS there should be a fan profile you can adjust. If not, you can set it in Windows using something like Speedfan. http://www.almico.com/speedfan.php

Edit: Oops, I didn't see that you specifically said for CPU fan only.
 
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No, there is no adjustment available for the power header or sysfan header, on the same menu is the CPU fan, this does have options but only preset ones, I have been through every menu. The CPU cooler is a sealed water cooled unit, the cpu fan header is connected to the fan facing the radiator and another sysfan header is connected to the pump. This is being used as a server to run unraid and that resides on a usb drive and is basically the os. Windows does not come into play.
 
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just had a google for my board P55a UD5 and threw up this forum post (hope its ok to link to it)

https://forums.tweaktown.com/gigabyte/39494-p55a-ud5-fan-management.html

I looked for a bios update but it doesnt give any info about whats been added or fixed, i dont think this board will ever support fan control to any headers other than the cpu header. Maybe a work around would be to swap the fan that is facing the radiator to the power fan header, yes it will run at full tilt but the fan is much bigger, thats also assuming it is a four pin header fan !, if all is well i could then put the drive case fan to the cpu fan header
 
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