Patent Number: 6,528,987

Title: Method and apparatus for determining fan speed

Abstract: An operating circuit (1) for controlling the speed of a cooling fan motor (3) for computer cabinetry (7), and for determining an under-speed condition in the fan motor (3) comprises a control circuit (15) which controls a variable output signal generator circuit (18) for outputting a pulse width modulated signal corresponding to the desired fan speed for switching a switching circuit (8) in a power supply circuit of the fan motor (3) for pulse width modulating the power supply to the fan motor (3). A monitoring resistor (R1), a capacitor (C1) and a circuit (12) provides a tachometer signal to the operating circuit (1). A gate circuit (22) gates the second and fourth pulses of the tachometer signal to a counter (20) which counts clock pulses from a clock signal generating circuit (19) between the rising edges of the second and fourth pulses which are in turn stored in a register (23) for comparison with a reference count by a comparator (25). The comparator (25) outputs an alert signal in the event of the count in the register (23) exceeding the reference count indicating under-speed of the fan. The control circuit (15) controls the signal generator circuit (18) for holding the pulse width modulated signal outputted to the switching circuit (8) on during a duty cycle between the rising edges of the second and fourth pulses of the tachometer signal so that the tachometer signal remains available until the fan speed has been determined.

Inventors: Blake; John (Limerick, IE), Hanrahan; David (Waterford, IE), Yoshida; Kohji (Oomiya, JP)

Assignee: Analog Devices, Inc.

International Classification: G01P 3/489 (20060101); G01P 3/42 (20060101); G06F 1/20 (20060101); H05K 7/20 (20060101); G01P 003/489 ()

Expiration Date: 03/04/2020