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Fan speed control using voltage control on Intel DZ77RE-75K Mobo


I had to replaced my old Rear Fan in my PC case. The new Rear Fan has 3-pins and the old one had 4-pins (PWM). The Rear Fan is plugged into the 4-pin rear header of my Intel DZ77RE-75K Mobo (with latest Bios 0066). The new Rear Fan is running however the Fan Speed is not reported anymore in the Intel Desktop Utilities (version v3.2.8.089). The speed old 4-pin Rear Fan was reported. It seems to me that the new Rear Fan is running at a constant speed and is not anymore controlled by the Intel DZ77RE-75K Mobo.

In the Intel DZ77RE-75K Mobo manual (March 2014 version), on page 12, there is a reference to "fan speed control using voltage control on Intel DZ77RE-75K Mobo (4-pin fan headers front, rear, and auxiliary) with selectable support in BIOS for 3-wire fans". I am assuming this would be a Voltage Modulation control of the new 3-pin Rear Fan speed based on the temperature detected by the Intel DZ77RE-75K Mobo. This is an option available on my old ASUS Mobo.

However, I could not find in the BIOS the option for selectable support for 3-wire fans.

Any suggestions?

Thank you.

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2 Replies

I verified the speed of a 3-wire fan can be increased or decreased by modifying the manual duty cycle. You may try with a third party application and the fan control mode set to auto.



There is no capability provided in BIOS for selecting support for 3-wire fans - and indeed none is necessary. With the exception of the Processor (CPU) fan header -- which supports only a 4-pin, PWM-controlled fan -- all other fan headers on the 7 and 8 Series Intel® Desktop Boards utilize a hardware circuit that automatically detects, at power-on, whether a 4-pin PWM-controlled fan is present or not. If a 4-pin PWM-controlled fan is detected, a full 12V output is supplied to the fan and it implements its own fan speed control based upon the PWM control signal. If a 2- or 3-pin fan is detected (or a non-compliant 4-pin fan; alas there are some out there), on the other hand, a voltage-variance control circuit is automatically enabled and the fan speed is controlled by varying the voltage between 0 and 12V.

OK, so your next question is "Why isn't the fan speed being reported?" There are a number of possibilities:

  1. If you are seeing the fan speed being displayed properly within BIOS Setup (or Visual BIOS) but not within Intel® Desktop Utilities (IDU), then the monitoring of this fan has been disabled within IDU. To re-enable monitoring, go into IDU's Options Menu, select entry "Set Sensor Threshold", click on the "Fans Redetect" button, close all IDU windows and then reboot the system.
  2. If the fan's speed is not being displayed properly within BIOS Setup (Visual BIOS), there may be something wrong with the tachometer output being generated (or not) by the fan or with the detection of this tachometer signal by the board. There are three possibilities that come to mind:
  • The minimum duty cycle for the fan is set too low and the voltage is being taken below 5V. In this case, the fan may continue to spin but the fan speed will not be detected. The circuit requires that the tachometer pulses reach the 5V threshold to be detected. To see whether this is the problem, raise the minimum duty cycle to 30% (or higher) and see if the fan speed is then detected.
  • If raising the minimum duty cycle doesn't help, there may be a problem with the tachometer signal being output by the fan. Your best bet at this point is to try replacing the fan and see if the problem fixes itself.
  • If replacing the fan doesn't make a difference, the tachometer detection circuit on the board may have been damaged (you did follow ESD protocols while working on it, didn't you?). There is no recourse for this except board replacement.

Hope this helps you get things working properly...