Fan Monitor

The MAX6684 from Maxim (http://pdfserv.maxim-ic.com/arpdf/MAX6684.pdf) is a device for monitoring fans. Available in an 8-pin small outline SMD package the IC is capable of detecting if the fan is jammed or if it is turning too slowly. Although the device itself requires a supply voltage in the range +3.3 V to +5 V, it can be connected to fans operating on up to +24 V and drawing currents of up to 250 mA. An internal sense resistor to ground (PGND) is used to detect the current pulses of the fan motor and the pulses are processed according to their shape and frequency.