Microchip Technology DM164123 Data Sheet
PICDEM™ System Management User’s Guide
DS41304B-page 16
©
2007 Microchip Technology Inc.
2.4.2
Three-wire Fan
FIGURE 2-1:
THREE-WIRE
Three-wire fans provide a power, ground and tachometer terminals. The tachometer
signal is typically generated by an internal Hall sensor that monitors magnetic field
switching due to rotation in the permanent magnet of the fan. These fans can also be
controlled through low-speed (10-100 Hz) PWM control of power input.
signal is typically generated by an internal Hall sensor that monitors magnetic field
switching due to rotation in the permanent magnet of the fan. These fans can also be
controlled through low-speed (10-100 Hz) PWM control of power input.
As the duty cycle of the PWM signal increases, the fan speed will also increase
non-linearly. Speed feedback is measured in the same way as the two-wire fan, but the
tachometer output of the fan on the third wire provides a square wave through an open
drain output.
non-linearly. Speed feedback is measured in the same way as the two-wire fan, but the
tachometer output of the fan on the third wire provides a square wave through an open
drain output.
2.4.3
Measuring the Speed of a Two and Three-wire Fan
It is important to note that when measuring speed on two and three-wire fans, some
additional conditions must be taken into account. The tachometer measurements are
only valid when fan power is on. Commutation noise is only measurable when power
is on and the Hall sensor output in three-wire fan is only valid when power is on. When
the PWM signal switches low, the Hall sensor output is no longer valid. Therefore, a
technique called pulse stretching must be employed to ensure accurate measure-
ments. Whenever a measurement is made, power must be provided to the fan for the
duration of the measurement. (See figure below for more details.)
additional conditions must be taken into account. The tachometer measurements are
only valid when fan power is on. Commutation noise is only measurable when power
is on and the Hall sensor output in three-wire fan is only valid when power is on. When
the PWM signal switches low, the Hall sensor output is no longer valid. Therefore, a
technique called pulse stretching must be employed to ensure accurate measure-
ments. Whenever a measurement is made, power must be provided to the fan for the
duration of the measurement. (See figure below for more details.)
FIGURE 2-2:
PULSE STRETCHING
Note:
Be sure to verify with the fan manufacturer that the fan specifications allow
for PWM control of input voltage.
for PWM control of input voltage.