Question How to control fan speed on 5V, via molex?

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Champion9

Champion9

Feb 20, 2024
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Hello everyone, sorry for dumb question, recently i made a bit of a mistake, I accidently purchased two 5V noctua NF-A14 1500RPM, instead of normal 12V, i found out I can run them through molex splitter 2x5V black+red, but sadly i dont want them to make too much noise and be stuck at 1500RPM is there any way to change their RPM?

I found this molex + 1x PWM controller, but sadly its black + yellow, which is 12V, will it work if i change the position of the yellow from right position to the left one, which should be red?
https://ibb.co/k66gFCr

Also found this, which SEEMS its molex black + red if im not blind, also has separate 4pin PWM, will that work on 5V fans to control RPM and not start a fire?
https://ibb.co/PG3L13K
 
I would have to see inside the cable on that second one, or a wiring diagram. Looks like there might be components under the heatshrink.

What I would do, probably just add a series resistor. Something like a 1/2 Watt .5 Ohm would make a pair of fans on a single 5V run down to 3.7v which should slow them down a lot. Double check the minimum starting voltage of the fans though. Might need to adjust accordingly, or the end RPM might be too low. So perhaps buy a range of resistor values and experiment. .5 ohms at 5V should dissipate .338 Watts, well under the .5 watts a 1/2 resistor can take. Also the common size you see on other noise suppression resistors. Which I thought was pretty commonly included with Noctua fans anyway (maybe not since most motherboards are PWM or at least DC controlled these days)
 
If its connected original like this 12V https://ibb.co/J21RXWj
if i swap the yellow to the red slot for 5V, that should power the fans at 5V 1500RPM right?
and the first photo molex, also has 4pin PWM control to mobo,
so teoreticaly, if i swap the cable, insert the 4pin pwm into mobo, they will work fine at 5V and i will be able to control rpm?
 
Neither of those adapters you link to can do what you need. The major reason is that PWM control ONLY works if the FAN is designed for that and includes a special chip to use the PWM signal from a mobo header. The fans you have are the older design in which the ONLY way to control fan speed is to reduce the VOLTAGE supplied to the fan on its Pin #2.


Eximo (above) has the right direction - insert a series resistor in the 5 VDC power supply line from the simpler adapter. That is the one with ONLY two wires from the Molex - Black Ground and Yellow +12 VDC. As you suggest, you MUST switch the yellow wire on that to the opposite end of the Molex so that it makes contact with the 5VDC supply RED wire from the PSU female Molex output connector. This will connect to the RED wire in the cable from your fan to its 3-hole female connector. Then you have a 5 V supply to use.

I think the numbers need re-assessment. The specs for that fan say it operates at full speed when supplied with 5 VDC, and will consume typically 0.209 A that way. Now, take a rough GUESS that, once the resistor is installed, it drops the supply Voltage reaching the motors and thus the current flowing. For a start, suppose the current drops to 0.150 A. Then the VOLTAGE drop across the resistor of resistance R Ohms is V = IxR = 0.150 x R, and the Voltage reaching the motor is (5 - V). Let R be 10 Ohms, and resistor drop is 1.50 V, and the motor supply is reduced to 3.5 V, about 70% of full voltage. That certainly will reduce its speed a lot. A 20 Ohm resistor would reduce the motor supply by more and hence the current by more - perhaps to about 0.12 A, and resistor Voltage Drop is 2.4 V, so motor Voltage reduced to 2.6 V. I don't know the minimum Voltage to avoid stalling of that fan. For a common fan fed by 12 VDC for full speed, it may stall under 5 VDC supply, or 42% of 12. IF that same guide applies to the 5 VDC fan, then 2 VDC (40% full) is the lowest you would dare to use, and you would want to try Resistors in the range 10 to 25 Ohms.

Regarding the power rating of the resistor, the heat power generated in the resistor is P = V x I, and V = I x R, so P = V^2/R. So a 10 Ohm resistor with an assumed 0.15 A current running through it to drop 1.5 V will dissipate 0.225 W of heat. A 20 Ohm resistor with 0.12 A flowing would produce 0.29 W heat. So any ½ W resistor in that resistance range should be sufficient.
 
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Thank you guys for your input, but im afraid im too retarded to do stuff like add resistors to fan to lower voltage
 
I may have messed up the math, I was using the Windows calculator and a mouse.

I was assuming two fans on a single wire. You could also do each independently and half the dissipation, but the end result is the same.
 
I have no idea where to connect resistors, never done it on fan, is there maybe anything more simple? 😀
 
Champion9 said:
Out of return window
Click to expand...
Unfortunately you bought an inappropriate specialty fan.
Chalk it up to experience.
Sell them and buy what you really need.

1500 rpm is not overly noisy with noctua fans if you wanted to use them.
But low noise adapters and such work by lowering the voltage to the fans from 12v, to 7v, and 5v.
However, most fans will not start at voltages lower than 5v.
 
geofelt said:
Unfortunately you bought an inappropriate specialty fan.
Chalk it up to experience.
Sell them and buy what you really need.

1500 rpm is not overly noisy with noctua fans if you wanted to use them.
But low noise adapters and such work by lowering the voltage to the fans from 12v, to 7v, and 5v.
However, most fans will not start at voltages lower than 5v.
Click to expand...
Yea i tought they would not be too noisy, but they kinda are, recorded with phone so you need to turn volume up a bit, the rotor sound can be heard a lot even when closed case and thats only 1 fan, did I little test and fired it up on 5V molex
https://streamable.com/ld1m77
 
To actually install the resistor you decide on is easy. First step is to get an adapter that is easy to modify. Try this

https://www.amazon.com/PNGKNYOCN-4-...s=Molex+to+fan+adapter&qid=1708532092&sr=8-18

That is MORE than you need, but you will use only part of it. It is a 2-pack and you will use only one. Each has TWO PAIRS of standard fan connectors for output to fans. For a 5 VDC power supply you use ONLY one of those with the RED and BLACK wires - those will match the red and black of the source female connector on wires from your PSU. At your FAN's connector its RED wire will mate to a RED on this adapter. If using only one output, I suggest you tape up all the others to avoid confusion.

So, on the ONE output connector fed by RED and Black wires you will use, you simply cut the RED wire in the middle and bare the two ends. For temporary connections to test exactly which resistor to use, you can make those connections simply by twisting wires together and keeping them separated so they don't touch the wrong thing. In fact, the voltage here is so low you can handle the bare wires to make and undo twisted connections while the power is on. Once you have decided which resistor to use you can make the connections secure by soldering them if you can, or even by wrapping the twisted wire junction points around a small machine bolt and tightening onto them a nut. Then wrap tape around to insulate against possible contact with a Ground.

Any resistor you get will be a small cylinder with colour bands around it and a straight single wire sticking out of each end. You just twist the bared wired you cut into from the adapter onto this wire at each end. If you go to an electronic parts shop or hobby store and tell them you want a few resistors in the range 10 to 25 Ohms each, and ½ W power rating, they can sell you those. They MAY only sell in packets of 5 of the same in a packet, or you MIGHT find individuals.

By buying those adapters I linked above (or similar) you can start with items with all the wires you need to work with and NOT have to move wires and connector pins in the Molex connector bodies. As I said, just tape up what you do not use. Then it becomes simple: cut one Red wire, twist its ends around the wires of a resistor, and try it out. Change which resistor as you like to see what fan speed you get. Finish by making the connections permanent and taping them.
 
Noctua normally supplies a couple of low noise adapters with their 140mm fans.
None is supplied with the 5v. variant.
I think all methods to reduce the voltage below 5v. will result in a fan not starting reliably at all.
 
^ That's an interesting device found by CountMike, and easy to use. It may not be suitable for OP's particular case because of the value of the resistor it contains: 47 Ohms. Here's my reasoning for that.

You cannot consider a small DC fan motor to be a simple resistance in a circuit, but you CAN use that model for a rough estimate. A Noctua fan model NF-A14 FLX is VERY similar to OP's 5V model, but uses 12 VDC max and consumes a current of 0.044 A at typical loads, and max 0.080 A. If we calculate its apparent "resistance" that comes to 12 / 0.044 = 273 Ohms "typical", and 150 Ohms for max current case. (I suspect that is the start-up current that is required briefly.) So for that max current brief time situation if you add in series with the motor a resistor of 47 Ohms, the total load "resistance" is 150 + 47 = 197 Ohms. Of that, the voltage drop across the motor alone is 152 * 150 / 197 = 9.1 V, surely sufficient to start that motor. For the "typical" scenario of 0.044 A current, the calcs say motor-only "resistance" is 12/0.044 = 273 Ohms, total is 273 + 47 = 320 Ohms, and motor Voltage is 12 * 273 / 320 = 10.2 Volts. That is certain to run the motor at less than max speed, but not a LOT less. The Noctua specs for that fan say IF you use the "Low Noise Adapter" they supply, it will reduce the motor from 1200 RPM to 1050 RPM. IF you use their "Ultra Low Noise Adapter" (with a higher resistance) the speed will drop to 900 RPM. So the 47 Ohm device found by CountMike CAN do something very similar to that for a fan designed for 12 VDC max power supply.

Let's apply the same guestimation technique to OP's 5 VDC fan. Its specs say it will draw 0.209A typical, and 0.260 A max given a 5 VDC supply. So "typical" condition says motor "resistance" is 5 / 0.029 = 24 Ohms, max current state "resistance" is 19 Ohms. Now add a 47 Ohm series resistor to the circuit. "Typical" conditions total resistance is 24 + 47 = 71 Ohms and the Voltage drop across the motor only is 5 * 24 / 71 = 1.7 V. For max current scase the numbers are total resistance 19 + 47 = 66 Ohms, and motor voltage is 5 * 19 / 66 = 1.44 V. I REALLY suspect that in BOTH these cases the motor will NOT start and could not even keep running if it had been started!

The idea is sound. IF one could find a device just like what CountMike linked, but with a resistor in the 10 Ohm range inside, that would be ideal.
 
^ not even that! The point of motors designed for 5 VDC versus 12 VDC power supply is NOT differences in motor max POWER, or ability to do its job. It is what VOLTAGE is required to get the required performance from the motor. Let's compare the 5V Noctua model OP has, the NF-A14 5V, to its more common 12 V relative, the NF-A14 FLX, also a 3-pin Voltage Controlled motor.

Spec 5V 12V
Max Input Power 1.3 W 0.96 W
Typical Input Power 1.045 W 0.528 W
Max Air Flow 140 m³/hr 115 m³/hr
Max static Pressure 2.08 in H20 1.51 mm H20
Typical Input Current 0.209 A 0.044 A

In this particular case the 5 VDC motor version actually is designed to consume MORE power and deliver MORE air flow, even up to higher backpressures!

The minimum STARTING Voltage for a fan is unique to each motor design. A common ROUGH guideline for 12 VDC computer case fans to KEEP RUNNING AND NOT STALL ONCE IT HAS BEEN STARTED ALREADY is not less than 5 VDC (42% of the full 12 VDC), and STARTING Voltage requirement is often higher by a couple of Volts. IF those 5 and 7 V numbers are correct for 12 VDC fans and we apply the SAME ratio for 5 VDC fans, those become 2.1 V no stall, and 2.9 V to start a stopped fan. NO guarantee these numbers are exactly correct, though.
 
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