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

A fan speed monitoring system for interfacing a fan, including a fan drive and sensing circuit for maintaining the voltage of the fan's TFSC pin above a minimum voltage level, thus maintaining continuous power to the fan. The fan drive and sensing circuit includes a control loop having an isolation resistor for enabling superposition of fan pulses asserted by the fan on the TFSC pin. The fan speed monitoring system also includes a filter circuit for filtering and amplifying the fan pulses and providing corresponding filtered pulse signals. The fan drive and sensing circuit includes an active control loop for maintaining the voltage of the TFSC pin at an average level above the minimum voltage level, while also enabling inclusion of the fan pulses. The filter circuit preferably includes an AC amplifier for amplifying the fan pulses and a level transition comparator for converting the amplified fan pulses to the filtered pulse signals having the desired form. In the preferred embodiment, a control circuit activates the fan drive and sensing circuit and receives the filtered pulse signals for determining a time value indicative of the speed of the fan. The control circuit converts the time value into a revolution per minute (RPM) value for identifying fan speed.