Intresting results with using both PWM and low noise adapter.

[Skanky Sylveon]

First off, I would like to thank @manikyath and @WoodenMarker for their information with my previous questions on the matter.

 

Since I had some low noise adapters lying around, I decided to try them out anyway.  The results were somewhat unexpected, not bad, just not how I expected them to work.

At minimum RPM (20%) the fans do spin a little slower, but not by much, around 50 RPM slower.  This is a huge difference from the 500 RPM slower at max speed.

Since my motherboard (ASUS) makes the fans spin at 100% when the temperature reaches 75C I still might have some use for them.  (The computer is quite a bit quieter when rendering, with minimal impact on temperature, and no, the CPU only gets to around 67C) 

I find it interesting that the low noise adapters make more of a difference, the faster the fan spins.

 

The fans that I used were the nf-p14s redux-1500 pwm.

I hope that this information proves useful.

[xc3ll]

It's pretty obvious why they do it.

The low noise adapter is nothing more than a resistor, which (simplified) reduces the supply-voltage of the fan. So it does not reduces RPM by let's say 500Rpm, but instead reduces the RPM by lets say 25%.

 

If you have a 2000Rpm fan, that means it will run at 1500Rpm max (100% PWM signal). If your fan would run at 200Rpm (10% PWM signal) without the low noise adapter, it would then run at around 150Rpm (25% less) with the low noise adapter. So the absolute difference in RPM is, of course, higher when running the fan at full speed.

 

I know that I neglected a lot of factors, such as that the voltage and current change with varying rpm, that motor-control is mostly non-linear etc. But I think OP wants a practial understanding of his results and the relationship between voltage, rpm and pwm-signal instead of a physics lecture.

But I'm happy to discuss this in more detail, if somebody really wants to

[Skanky Sylveon]

11 hours ago, xc3ll said:

It's pretty obvious why they do it.

The low noise adapter is nothing more than a resistor, which (simplified) reduces the supply-voltage of the fan. So it does not reduces RPM by let's say 500Rpm, but instead reduces the RPM by lets say 25%.

 

If you have a 2000Rpm fan, that means it will run at 1500Rpm max (100% PWM signal). If your fan would run at 200Rpm (10% PWM signal) without the low noise adapter, it would then run at around 150Rpm (25% less) with the low noise adapter. So the absolute difference in RPM is, of course, higher when running the fan at full speed.

 

I know that I neglected a lot of factors, such as that the voltage and current change with varying rpm, that motor-control is mostly non-linear etc. But I think OP wants a practial understanding of his results and the relationship between voltage, rpm and pwm-signal instead of a physics lecture.

But I'm happy to discuss this in more detail, if somebody really wants to

So the resistor doesn't resist a set amount, and is instead based off of the percentage the current that the fan is getting?

[xc3ll]

Well the resistance of the resistor is constant, but the amount of the voltage at the resistor depends on the current: U = R * I
 

If the fan runs faster, it needs more power and pulls a higher current through the resistor. This leads to more voltage at the resistor and less voltage at the fan (resistor and fan share the 12V from the mobo. if one gets more, the other gets less).

 

So the resistance is constant, but the "voltage-drop" depends on the current. The current depends on the Rpm, leading to: More Rpm = less voltage.

 

9 hours ago, Cinnabar Sonar said:

So the resistor doesn't resist a set amount, and is instead based off of the percentage the current that the fan is getting?

So if you mean "resists" as in "how much voltage gets dropped", this statement is correct as an approximation. But the resistance measured in "Ohm" is constant.

[Skanky Sylveon]

2 hours ago, xc3ll said:

Well the resistance of the resistor is constant, but the amount of the voltage at the resistor depends on the current: U = R * I
 

If the fan runs faster, it needs more power and pulls a higher current through the resistor. This leads to more voltage at the resistor and less voltage at the fan (resistor and fan share the 12V from the mobo. if one gets more, the other gets less).

 

So the resistance is constant, but the "voltage-drop" depends on the current. The current depends on the Rpm, leading to: More Rpm = less voltage.

 

So if you mean "resists" as in "how much voltage gets dropped", this statement is correct as an approximation. But the resistance measured in "Ohm" is constant.

Ah, then my understanding on how resistors work wasn't entirely correct.

The results make a lot more sense now.  Thanks for letting me know.

[WoodenMarker]

The resistor lowering the minimum rpm is circumstantial and will depend on the fan. Provided that you have a full range of control on the header the fan is plugged into, pwm using 12v should allow for the fan's minimum rpm. Depending on the fan, using a resistor with pwm can result in a lower or higher minimum rpm.