SMSC EMC2102 User Manual download pdf (Page 35)

Fan Speed Control with the EMC2102 Device

SMSC EMC2102

USER MANUAL Revision 0.2 (09-17-07)

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

Figure 6.1 A Typical 2-pole DC Fan

Figure 6.2 Output Signal of a 2-pole Fan

Hall

Sensor

Mo to r

Windin g

Dr i ve r

Mo to r

Winding

Mo to r

Winding

Protectio

Diode

VDD

GND

TACH

N +

S -

N +

S -

Hall

Sensor

Fan Tach Signal

1 complete fan revolution

= 2 pulses (5 edges)

1 2 ... 30 31 32 33 34 35 36 37 38

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SMSC EMC2102 User Manual Page 35

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