this is a old post but maybe someone will answer, I have this issue the 4th pin is vcc... should I use a 4pin or a 3 pin if I want the mobo to change the fan speed ?
You can use either.
Gigabyte have rewired the header (sensibly imo) on my mobo at least, Z170X.
This specification for PWM control comes from Intel. What is written as 12V on some and Vcc on Gigabyte can be interchangeable.
Pin 1 Gnd,
Pin2 Control, Voltage(PWM)
pin 3 Sensor,
pin 4 VCC
The normal PWM header configuration is as follows :
pin 1 Gnd
pin 2 +12V
pin 3 Sensor
pin 4 Control Voltage(PWM)
An adaptor is easy enough to make.
On PWM there is usually a pull-up resistor attached to the 12V line to stop the control line from getting too much Voltage droop. It is also used to help supply current to larger fans, which is why you must be careful in fan choice when you connect to motherboard or other peripheral cards like graphics cards.
I have not seen any true analogue Voltage control done on a computer as a D/A would be prohibitively expensive and not needed.
You have to understand what PWM is, to understand what is happening, and therefore realise why a D/A is not required for Voltage control.
A PWM is a series of clock pulses that excite the coils of the motor to a greater or lesser extent, the only difference physically in most of these DC motors is that a PWM motor has a pulsed output twice every single revolution (Intel Spec). These pulses are read and the exact speed of the motor is then known.
Can you control a motor without the PWM Sensor via a PWM signal ? Yes you can. The PWM signal is just a series of on/off pulses which speeds up a motor or slows it down. If it is a 3 wire device then it can still be a PWM fan, it just does not have a wire to pull the Voltage up as this is done on the motherboard, however these are usually low power fans. You need to ask the manufacturer of the mobo, what the maximum safe current is.
Can a 2 wire fan be controlled ? Yes it can via proportional control. The 2 wire motor still receives the PWM signal just like any other fan and the amount of PWM it gets depends on the control of the fan, usuall set by you from a characteristic curve, although I suspect SIV uses current sensing. Usually by a third party control software like Argus Monitor works well, I try Speedfan, but it does not support the SMbus on the ITE IT8628E and MSI afterburner whilst being good for system and CPU fans, but does not let you control a fan from any temperature sensor that you want to select. A shame really.
The SM bus is just a low speed control bus 100KHz, that will read a device or write to a device. ie read a temperature sensor and write a control signal to a PWM controller.
I have a Gellid cooler on my graphics card, This is 2 two wire fans connected to the motherboard header and temperature controlled from the graphic card temperature sensor.
