Question Does molex from the PSU have rpm controll?

May 21, 2020
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Can't seem to be able to find an answer to this so:

I have motherboard with only one fan header (2 if we include the CPU fan) and I have 2 case fans. One can be plugged in normally and have rpm controll but the other I've connect using Arduino cables to molex 12 v and ground. I see molex has another 5v cable and I was wondering if maybe that somehow gives rpm controll? Sounds stupid but I'm optimistic. I don't want my fan to run at full speed constantly and I don't want to put both fan on one header on the motherboard cuz I feel I'm gonna burn something from drawing too much current.
 

Vyrvelata

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May 19, 2020
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Nope there is no way to control your fan if you connect them to the PSU directly, unless you have fan controller on the fan itself has fan controller installed (or fan controller that is coming with the case itself).
The ways to control your fans cheap is:
  1. To buy fan with reduced max rpm (800rpm is relatively silent for 12cm fan).
  2. To buy header which reduces the current running to the fan.
 

Paperdoc

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Last post I find confusing. But its first part is right - there is NO way to control the fan speed when connected directly to the PSU Molex output.

You are too worried. Almost all mobo fan headers can output up to 1.0 A to the total load of fans connected to it using a Splitter. These days most fans draw at max 0.1 to 0.25 A at full speed, and even the high-draw LED fans are no more than 0.35 A. So you can easily connect two normal fans to a single header. Look on the fan label, or if necessary look it up on its maker's website, and look for the max amps it can consume.
 

Vyrvelata

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May 19, 2020
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Last post I find confusing. But its first part is right - there is NO way to control the fan speed when connected directly to the PSU Molex output.
Which part... fans with temperature sensors exist...if you put the sensor next to cpu cooler or gpu....when it starts to heat up.. the sensor would catch that and ramp up the rpm.

 

Paperdoc

Glorious
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You are correct, such items do exist. They are not ideal, and I'll say why. But they work and certainly are better than no control.

The largest problem with those systems is in identifying both the SITE for mounting the temperature sensor, and the TARGET TEMPERATURE they should use for control. You are concerned with a second case ventilation fan. Where is the right place to sense the temperature of the hot item to be cooled? And exactly what temperature is "right" for that spot? The lesser factor is how well your sensor mounting will stand up to disturbances that might disconnect it from the thing you want to measure.

When you use the system included on your mobo for control of a fan (or more than one) by a SYS_FAN header, the SITE, the TARGET, and the security of mounting all are taken care of by the people who designed that board. Your only concern is whether you can safely attach more than one fan to a single header. And that is answered entirely by the fan's max current consumption spec. For two fans, just add up those specs for each. As long as the total does not exceed 1.0 A, you have no problem at all.

A SPLITTER is a simple device that merely connect both fans in parallel to the output from the mobo header, so that's where all the fans' power comes from. Here's an example, but these are easy to find.

https://www.amazon.com/watersouprty-Splitter-Extension-Computer-Accessories/dp/B07YCC55KH/ref=sr_1_8?dchild=1&keywords=fan+splitter&qid=1590165414&rnid=2528832011&sr=8-8

That happens to be a 4-pin model, but it will work perfectly with 3-pin fans, too, of that is what you have. It has three outputs, but 2-output units are common.

Do NOT get a HUB. It is a different type of device and it can be identified because it has a third type of "arm" or cable that must plug into a power output from the PSU. You do not need that, and it is more expensive.

You should know one thing when using this device. The mobo header can only deal with the speed signal coming back to it from ONE fan. So it has all of its pins on only one of its output connectors. This has NO impact on speed control. It does mean you will never "see" the speed of the second fan.
 
Reactions: digitalgriffin
Can't seem to be able to find an answer to this so:

I have motherboard with only one fan header (2 if we include the CPU fan) and I have 2 case fans. One can be plugged in normally and have rpm controll but the other I've connect using Arduino cables to molex 12 v and ground. I see molex has another 5v cable and I was wondering if maybe that somehow gives rpm controll? Sounds stupid but I'm optimistic. I don't want my fan to run at full speed constantly and I don't want to put both fan on one header on the motherboard cuz I feel I'm gonna burn something from drawing too much current.
What wait?

Fans are typically 12V. A 5V pin on a molex is used for whatever device uses a 5V supply. Why would you think it supports a RPM signal?

Now your motherboard likely has at least 2 headers. Even my ITX has 3. One will be the CPU, one will be the chassis fan header. What you can do is buy a cheap fan splitter cable. If fan 1 is a 4 pin fan, make sure fan 2 is also 4 pin. Your motherboard will likely handle the current just fine. Attach it to the Chassis fan header.

https://www.amazon.com/Noctua-NA-SYC1-Y-Cables-Fans-Black/dp/B00KG8K5CY/ref=sr_1_1_sspa?crid=2YI3MHYFG6WMJ&dchild=1&keywords=pwm+fan+splitter+2+way&qid=1590167555&sprefix=pwm+fan+splitter,aps,157&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUE0WktFWVJURldDVjQmZW5jcnlwdGVkSWQ9QTA3MjU4MzhBR0RQQU9MTE9IQ0cmZW5jcnlwdGVkQWRJZD1BMDk4MzI2NjFYQkdWOUlQWEZTTkkmd2lkZ2V0TmFtZT1zcF9hdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl
 
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