A similar article appeared in the November, 2003 issue of EET.
The brushless DC fans found in many types of equipment can be crucial to the performance and longevity of that equipment. A quick indication of fan failure, moreover, can be essential in preventing major damage. Among the many approaches for identifying and indicating stalled fans, the circuit of Figure 1
is very simple and reliable.
Figure 1. The MAX6316 µP supervisor monitors a fan's tachometer output.
The fan's tachometer output connects to the watchdog input of a µP supervisor (U1). The LED remains off during normal operation. If the tachometer does not change state within a watchdog timeout period, U1 lights the LED by asserting its reset output. As a result, the LED pulses on and off as the supervisor goes through its watchdog/reset cycle. The LED in this example has a 200ms on-time and flashes with a period of 1.6s, which is suitable for most purposes.