SMSC EMC2102 - Manual (Page 35)

Fan Speed Control with the EMC2102 Device
SMSC EMC2102
USER MANUAL
Revision 0.2 (09-17-07)
35
6 Appendix
6.1 DC Fan Basics - Poles, Tach Meter Pulses and Edges
An n-pole fan has n pairs of North-South magnetic poles which are generated by electromagnet coils.
At anytime, only one pair of coils are driven and which coil pair gets driven is determined by a
component called Hall Sensor. The architecture of a typical 2-pole DC fan is shown in
Figure 6.1, "A
Typical 2-pole DC Fan" .
The output of the hall sensor is also the TACH (or Tachometer) signal. When the magnetic field around
the Hall Sensor changes its direction, the sensor’s output level will follow the change to create a square
wave signal as shown in
Figure 6.2, "Output Signal of a 2-pole Fan" .
Assuming 2-pole fan is running at a speed of 6000 RPM, it will rotate 100 revolutions per second. With
2 pulses per revolution, the TACH pulse signal frequency will be 200Hz (
Figure 6.2 ). Since a higher
RPM will yield a higher TACH frequency, or a shorter period between pulses, the TACH signal can be
used by the EMC devices to determine the speed of the fan. Generally speaking, we have:
TACH Pulse Frequency (in HZ) = (RPM / 60) x (# of Pole)
EMC2102 uses a clock (32.768KHz for example) to fill in a window between a programmable number
of Tachometer edges. A counter starts on a specific rising edge and keeps counting until it sees the
‘set’ number of edges, and then saves the counted pulse numbers into register 58h, the TACH Reading
register.
Figure 6.1 A Typical 2-pole DC Fan
Figure 6.2 Output Signal of a 2-pole Fan
Hall
Sensor
Motor
Winding
Driver
Motor
Winding
1
Motor
Winding
2
Protection
Diode
VDD
GND
TACH
1
N +
S -
+
N +
S - +
-
-
Hall
Sensor
Fan Tach Signal
1 complete fan revolution
= 2 pulses (5 edges)