On my motherboard (ASRock B150M Pro4S/D3) i have an unused 3-pin case fan header.

Can i use it to drive a single LED with only a resistor in series?

The fan can be configured in the BIOS to change speed according to the CPU temperature, so would like to use the LED as a visual indicator for that.

(I expect the LED to blink faster as the CPU temperature increases.)

  • 1
    \$\begingroup\$ I don't think fan controls work that way. Look that up before you get too carried away. I think they use pulse width modulation to control fan speed rather than changing the frequency. The LED would probably get brighter rather than changing the flash rate - which would be too fast to see, anyway. \$\endgroup\$ – JRE May 25 '17 at 21:55
  • \$\begingroup\$ My previous comment was wrong. allpinouts.org/pinouts/connectors/motherboards/… Two pins is a voltage, while the third one is the feedback. \$\endgroup\$ – Eugene Sh. May 25 '17 at 21:58
  • \$\begingroup\$ The PWM runs at 25 kHz, using the width to determine % control, according to this: en.wikipedia.org/wiki/… \$\endgroup\$ – jonk May 25 '17 at 23:24

With a 3 pin PC fan header, it should be possible.

  • Pin 1 is GND.
  • Pin 2 is 12V power to the fan. If the motherboard supports fan speed control via a three pin header, this 12V pin is PWM controlled by the motherboard.
  • Pin 3 is tachometer feedback from the fan. The fan pulls this pin to GND, usually twice per revolution. This pin has a pull-up resistor on the motherboard.

So yes, if you connect a LED with a current limiting resistor between GND and the 12V pin, it should vary in brightness according to the fan speed set in the software for that header.

With three pin headers, there is no standard PWM frequency, but I expect the frequency to be much too high for you to be able to percieve any blinking. What you will see is the LED getting dimmer as you "slow down the fan" in software.

And for completeness:

  • With four pin headers, the 12V is constant. Pin 4 has a 5V 25 kHz PWM signal that tells the fan how fast to spin.
  • 4 pin headers can often be configured to operate in 3 pin mode.

I= (12V-Vf)/R

I don't expect it to blink but you can run it in parallel with your CPU fan. They use PWM to create a DC voltage for the fan.

So if RED 20mA, Vf=2.1V then R=9.9/20m =495 ohm or nearest

Also, Pd(R)=20mA*10V = 200mW so 1/4W will get hot. but then the resistor can be near the fan ( lol )

If you want better UX, use 3 RED LEDS in series then when the fan is running very slow at say 6V, the LEDs are very dim.

R=(12V-3*2.1V)/20mA = 5.7V/20m = 285 ohms or nearest R

then Fan @ 6V LEDs dim or off and Fan @ 12V full RPM, = full brightness.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ obviously tach is not used. The Case Fan voltage is fixed so dont use that and stuff 24 AWG folded wire into crimp plug then add sealant to plug after tested working. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 25 '17 at 22:12
  • \$\begingroup\$ I will be using breadboard female wires that are AWG22 or 24. Btw are you sure i can't wire them directly to the case fan header? \$\endgroup\$ – eadmaster May 25 '17 at 22:34
  • \$\begingroup\$ Of course you can but why? The CPU fan speed is what you want to monitor. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 25 '17 at 23:01
  • \$\begingroup\$ the BIOS has a setting to control the speed of the case fans according to the CPU temperature, so it will be the same. Also for me it will be easier to wire up the LED separately instead than in parallel to the CPU fan. \$\endgroup\$ – eadmaster May 25 '17 at 23:06
  • \$\begingroup\$ ok thats a good reason but with only 1 LED,you wont notice much change \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 25 '17 at 23:25

From: ASrock B150M User Manual

So CHA_FAN_SPEED is an output from the motor as a tachometer to tell the CPU what speed it is going. So using it as a blinky light for temperature feedback is not possible.

Fan pinout

You may have better luck with the 4-pin connector.enter image description here


simulate this circuit – Schematic created using CircuitLab

This should work off of the 4-pin header, using PWM.

  • \$\begingroup\$ indeed the idea was to connect the LED to the 12V and GND pins and leave the tachometer pin unconnected... \$\endgroup\$ – eadmaster May 25 '17 at 22:21
  • \$\begingroup\$ @eadmaster So since 12V is 12V and GND is GND, how will you get your LED to blink? \$\endgroup\$ – StainlessSteelRat May 25 '17 at 22:26
  • \$\begingroup\$ i think the 12V output is not constant, they use PWM to reduce the case fan speed. \$\endgroup\$ – eadmaster May 25 '17 at 22:30
  • \$\begingroup\$ On the 4-pin header. It says 12V in the manual. On the 4-pin header (which uses PWM), it says FAN_VOLTAGE. \$\endgroup\$ – StainlessSteelRat May 25 '17 at 22:44
  • \$\begingroup\$ ok, i see the difference... for some reason the BIOS let me adjust the fan speed on 3-pin header too. The screen is similar to this, so probably is just a fake setting. :) Side question: to simplify, can i remove the transistor and the leave pin 4 unconnected? \$\endgroup\$ – eadmaster May 25 '17 at 23:01

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