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Fan speed monitoring system for determining the speed of a PWM fan

  • US 5,727,928 A
  • Filed: 12/14/1995
  • Issued: 03/17/1998
  • Est. Priority Date: 12/14/1995
  • Status: Expired due to Term
First Claim
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1. A fan speed monitoring system for interfacing a fan unit, the fan unit including a fan speed control terminal for controlling a pulse width modulated (PWM) driven fan and the fan unit asserting fan pulses on the fan speed control terminal indicative of fan poles during the power portion of each PWM cycle, the fan speed monitoring system comprising:

  • a fan drive and sensing circuit for maintaining the voltage of the fan speed control terminal above a minimum level to maintain continuous power to the fan while also superimposing fan pulses asserted by the fan unit; and

    a filter circuit coupled to said fan sensing circuit for amplifying and filtering said fan pulses and for providing corresponding filtered pulse signals.

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