Electrical – PWM on negative pin to positive (the common) PWM for a fan’s control pin


I'm currently modding a 3D printer (Anet E10) by upgrading it's fan to a powerful 3.4A 12V fan. It's model is 9BMB12P2K01. The current layout on it's PCB does not allow such a powerful fan to be connected directly to it's designated MOSFET because it looks like it can't handle more than 1A safely. I can't take the motherboard apart to see the actual circuitry for the fan control. My assumption is that the CPU (Atmega2560) uses a PWM signal for the fan control.

The standard fan cable plugs into a 2 pin socket connector (this type). By what I was able to measure, there is 0 resistance between the positive pin of the connector and the positive side of the power supply. There is infinite (or at least > 200 ohm) resistance between the negative side of the connector and the negative side of the power supply (GND). I am not experienced with MOSFETs, so I cannot identify it's type or layout. Judging by this measurement, the actual "PWM" is happening on the negative pin.

The powerful fan in question requires a PWM signal on a special pin. Thus, my question becomes:

  1. What type of MOSFET is on the board? Is it an N-type, or a P-type? (it's covered by heatsink I cannot take off)
  2. How can I use this (possibly) existing PWM signal on the negative pin as an input to the control pin of the powerful fan?

Edit: The powerful fan will be connected directly to the power supply, leaving it's control on the control pin. It expects a PWM signal on the control pin.

Best Answer

Your current fan is likely connected with a low side N Channel MOSFET driver as shown below:

pwm fan

Fan PWM inputs are typically active low and pulled high internal to the fan. That way the fan runs at full speed when the PWM input is not connected. I could not tell from the data sheet if this is the case for your fan. If it runs at full speed with the PWM input floating and stops when you pull the PWM to GND, then it probably is. This is the general PWM info from the fan manufacturer.

In any case, the fan PWM input does not have high current draw, so it can be directly driven by the pin on the fan connector that is NOT connected to the power supply. That should pulse low to control the fan speed. The ground on the fan also needs to be connected to the ground on the power supply.