I have a x370 gaming pro carbon motherboard. I just purchased 3 4pin pwm RGB fans and when I plugged two of them in and turn the computer on I only get the lights to turn on for roughly 5 seconds then they go out. Fans stay running. I'm aware that the two ports I plugged them into are by default set to DC and not PWM. Does that mean by them not getting a constant 12v that that's why they don't stay lit? The third fan I plugged in to the pump port stays lit and that one by default is set to pwm. I find it hard to believe that two fans are defective and am hoping that I just need to open BIOS and change the setting.
[SOLVED] 4 pin RGB pwm fan lights not staying on at start up
- Thread starter jmacman21
- Start date
Ah, some confusion here about types of lighted fans and their names. The first fans that included lighting effects are what I call LED fans. The LED devices built into them are normally of ONE colour only and are simply connected in parallel to the power being supplied to the fan motor. They have ONE power cable to the motor - either a 3-pin or 4-pin one, depending in the motor type. The LED's are always lit up when the fan gets power - well, MOST of the time. See below.
The newer designs are true RGB fans. In these the lighting devices are groups of LEDs of three different colours (Red, Green, Blue) and they actually have their own separate power supply cable in addition to the fan power cable. So in these, the fan motor works off one connection to a mobo fan header, and the RGB lights work off a different connection to a mobo RGB header. In these systems the lighting effects can be more complex and multi-coloured because there are three different LED colours to activate in varying combinations.
The fans you linked to are of the older single-colour, single-cable type. As I said, normally the LED's will always be lit when the fan runs. BUT since they are simply connected to the fan motor's supply, IF the signal from the mobo fan header to the motor is run at a very low Voltage to make the fan run slow, the LED's will get very dim or even just not light up. This can happen if you set the mobo header to use Voltage Control Mode (aka DC Mode). In your case, OP, here is what I suspect is happening. When a system first boots up, all the fan headers first set their outputs to max speed to be sure the fan starts up. In a few seconds it then turns control of fan speeds over to the normal automatic system that reduces the fan speed to what it required for the cool temperatures present at start-up. IF your fan header is using DC Mode, that is done by reducing the DCV supply on Pin #2. I think it's just going low enough to fail to keep the LED's lit on those two fans.
On the other hand, IF you set those two fans' headers to use PWM Mode (which actually is the right way to control these 4-pin PWM fans), the header will supply a constant 12 VDC to the fan on Pin #2, and the PWM signal on Pin #4 will be used by the fan to control the speed of the MOTOR part only. In the meantime, the LED parts will still get the full 12 VDC supply they need and work properly. That is what you are seeing for the third fan plugged into the mobo pump header which, I bet, is set to use PWM Mode. So, make those changes to the fan headers and it should work just fine.
The newer designs are true RGB fans. In these the lighting devices are groups of LEDs of three different colours (Red, Green, Blue) and they actually have their own separate power supply cable in addition to the fan power cable. So in these, the fan motor works off one connection to a mobo fan header, and the RGB lights work off a different connection to a mobo RGB header. In these systems the lighting effects can be more complex and multi-coloured because there are three different LED colours to activate in varying combinations.
The fans you linked to are of the older single-colour, single-cable type. As I said, normally the LED's will always be lit when the fan runs. BUT since they are simply connected to the fan motor's supply, IF the signal from the mobo fan header to the motor is run at a very low Voltage to make the fan run slow, the LED's will get very dim or even just not light up. This can happen if you set the mobo header to use Voltage Control Mode (aka DC Mode). In your case, OP, here is what I suspect is happening. When a system first boots up, all the fan headers first set their outputs to max speed to be sure the fan starts up. In a few seconds it then turns control of fan speeds over to the normal automatic system that reduces the fan speed to what it required for the cool temperatures present at start-up. IF your fan header is using DC Mode, that is done by reducing the DCV supply on Pin #2. I think it's just going low enough to fail to keep the LED's lit on those two fans.
On the other hand, IF you set those two fans' headers to use PWM Mode (which actually is the right way to control these 4-pin PWM fans), the header will supply a constant 12 VDC to the fan on Pin #2, and the PWM signal on Pin #4 will be used by the fan to control the speed of the MOTOR part only. In the meantime, the LED parts will still get the full 12 VDC supply they need and work properly. That is what you are seeing for the third fan plugged into the mobo pump header which, I bet, is set to use PWM Mode. So, make those changes to the fan headers and it should work just fine.
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