Airflow Monitor a

Fans are usually monitored by measuring their operating currents. If the current lies within a certain range, it is assumed that the fan is spinning properly and providing a stream of cooling air. If it falls below a lower threshold or exceeds an upper threshold, something is wrong with the fan: it is either defective or blocked by some sort of object. The cooling airflow generated by a fan can be directly monitored using the Analog Devices TMP12 sensor IC (www.analog.com). This IC contains a temperature sensor and a heater resistor, as well as two comparators and a reference-voltage source. Figure 1 shows the complete circuit diagram of an airflow monitor. The voltage divider formed by R1, R2 and R3 defines the temperature thresholds and the hysteresis for the switching points (via the current IREF flowing through the resistor chain).