When a cooling fan fails, it can cause a cascade of system problems. Although you can monitor the temperature in a system, doing so adds cost and may be too late to tell you that a bearing is slowly wearing out. An alternative approach may be to sense the fan's revolutions per minute and set an alarm if it drops below what it should be at its operating voltage, but such sensing may require a special three-wire fan with tachometer or a similar output lead, or other extra circuitry to support it.

The TC670 IC fromMicrochip Technology offers a new approachto solving these problems. The TC670 senses the small voltage spikes a fan's ground lead generates; using a low-ohm resistor between the ground lead and actual dc ground senses these spikes. You establish the revolutions-per-minute alarm-level trip point using a resistor divider; typical trip-point settings are 40 to 80% of the fan's nominal speed, depending on model. The six-lead-SOT-23-packaged IC also includes a "clear" signal, so you can cancel the alert. Typical operating current is 90 µA for this 3 to 5.5V device, which costs 96 cents (1000).

Microchip Technology Inc, 1-480-792-7200, www.microchip.com.