PWM splitter + resistor?

XeonKyle · January 11, 2019

If I use a PWM splitter but want the slave fan to run slower than the main, could I run a resistor in series on that slave fan’s +12V main?

New user here! Currently doing a HP Z400 build, more posts on that later... I’m needing this solution because I would like to run a small fan on my motherboard’s VRM but I lack another PWM slot. I’m going to use a PWM splitter and run it with the CPU’s heatsink fan, but I don’t think the vrm fan needs to blow full tilt to be of use, and the less noise the better. Do you think this will work for my purposes?

WoodenMarker · January 11, 2019

Yes, you can. As long as the resistor is used between the splitter and the fan as opposed to between the splitter and power source, there should be no issue.

XeonKyle · January 11, 2019

1 minute ago, James Evens said:

Yes and no. Best solution would be a fan "hub" doing this for you but not many people have this.

Yes it will do something. No it might case instability or wired behavior if you plan on feeding in a pwm signal.

Check that the resistor can dissipate the power loss. If the fan draws 250mA and you want 8V you will dissipate 1W at the resistor.

I’m planning on using an Arctic F8 fan, which draws 160mA. I read somewhere that the most common resistors were rated at 1/4 watt so I think I’ll be going with a series of low resistance resistors so I can keep the dissipated heat per resistor down and fine tune the ratio of fan speed. On that note... who even sells resistors? I live in the US btw.

Cortmarshal · January 18, 2019

If your fan draws 160ma at 12v, that means it's a 75 ohm resistance. If you add a 50 ohm resistor in front of the fan, it'll give you 8v at the fan, and should only dissipate 1/2w, but keep in mind the resistor does get warm.

To be safe, I'd recommend you use two 1w resistors in parallel, meaning you need a rating of 100 ohms.