Analog Devices,inc.

Kohji Yoshida

John Blake

David Hanrahan

An operating circuit ( ) for controlling the speed of a cooling fan motor ( ) for computer cabinetry ( ), and for determining an under-speed condition in the fan motor ( ) comprises a control circuit ( ) which controls a variable output signal generator circuit ( ) for outputting a pulse width modulated signal corresponding to the desired fan speed for switching a switching circuit ( ) in a power supply circuit of the fan motor ( ) for pulse width modulating the power supply to the fan motor ( ). A monitoring resistor (R ), a capacitor (C ) and a circuit ( ) provides a tachometer signal to the operating circuit ( ). A gate circuit ( ) gates the second and fourth pulses of the tachometer signal to a counter ( ) which counts clock pulses from a clock signal generating circuit ( ) between the rising edges of the second and fourth pulses which are in turn stored in a register ( ) for comparison with a reference count by a comparator ( ). The comparator ( ) outputs an alert signal in the event of the count in the register ( ) exceeding the reference count indicating under-speed of the fan. The control circuit ( ) controls the signal generator circuit ( ) for holding the pulse width modulated signal outputted to the switching circuit ( ) 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.

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Oomiya, Japan

Kohji Yoshida