4 wire computer fans work fine on a 3 wire header. They just run at a fixed
rate. Some may support variable speed via PWM-ing the power.
On Tuesday, December 10, 2019, 2:40:59 PM MST, Chris Albertson
<[email protected]> wrote:
In the 3-wire fan, the tach is not PWM. It is a two-pulse per
revelation TTL level signal. I actually build a fan controller once
using a "dead bug" 50 cent, 8-pin AVR microcontroller, and an external
transistor switch. So I had to look into this.
I think the real reason for the 4th wire is that with BLDC motors you can
no longer control the fan speed simply by putting PWM on the power. The
fan motor speed is determined by firmware inside the fan, not by voltage
control. Ging to a BLDC motor forced us of a separate control signal.
What happens if you fail to but PWM on the control wire? Hard to say
because it is controlled by firmware inside the fan. Likely you get zero
RPM on the fan.
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