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 These temperature sensors provide a direct measure of overheating, yet may trip alarms or shutdown too late to prevent damage to electronic components. |
F or the designer of electronic equipment packing more power into less space, heat is a persistent adversary. The i ncreased density of modern electronic devices increases the amount of heat to be dissipated in a smaller box. A smaller enclosure containing less air also increases the rate at which heat builds up, should forced-air cooling fail.
The choice of overheat protection sensors and strategy depends on value, application, and sensitivity of equipment, installed cost of sensors, and the presence or absence of forced-air cooling. There are numerous options in forced-air cooled circuitry, including redundant overheat protection. One manufacturer of test systems for Very Large Scale Integrated (VLSI) circuits redesigned its high-performance product with this in mind. The tester actually has five miniature solid state air flow sensors and a network of temperature sensors on circuit cards throughout the enclosure. The two-tier strategy combines indirect and direct overheat detection to safeguard the overheat sensors. For example, the use of the compact Warren G-V Series VE and C-8 sensors for a response to surface temperature on circuit boards or ambient temperature in the electronic enclosure can help speed this approach.
Indirect overheat protection is tied to cooling airflow sensed through the motion of a fan or the velocity of cooling air itself. Warren G-V makes its Series LS, FS, and SAF airflow sensors with time delays to ignore transient airflow interruptions. Combination sensors, such as the company's Series DLS Thermulator, monitor both cooling air velocity to provide early warning and temperature for overheat shutdown. The comparison chart in this article shows the common technical approaches to overheat protection. It rates each sensor based on effectiveness, installed cost, and other considerations. (see chart).
Cooling fans rarely fail, and overheat sensors may never be activated. However, their cost can be justified by the expense of a single cooling failure. Direct-measurement overheat protection based on thermostats is common, and temperature sensors themselves can be accurate and inexpensive. Commercial-grade Series C8 and VE sensors made by Warren G-V, for example, can be set for trip points from -10 °C to +150 °C with setting tolerances of ±3 °C. For most commercial applications, 40 °C is a typical alarm setpoint.
Thermostats and other direct-measurement over-temperature sensors, however, often warn only after the fact. Temperatures within an electronic enclosure continue to rise even after power is cut. By the time individual sensors reach warning temperatures, even a shutdown of the equipment may not prevent overheat damage. Indirect-sensing overheat protection, which responds to lack of cooling air, shuts equipment down or triggers other remedies before the harm is done.
In case of overheating, temperature sensors can trigger an alarm, increase the flow of cooling air, activate a backup system, signal a clogged filter, or cut power altogether. Thermal momentum nevertheless continues to build up heat once power is disconnected. The most effective overheat sensor is consequently one which anticipates overheating to save dense, sophisticated circuitry from its effects.
With less space and higher circuit density, the placement of an overheat sensor demands careful study and testing. The designer needs a clear understanding of the environment within an electronic enclosure. While direct temperature measurements are easy, air flow measurements and their relation to cooling effectiveness typically require more advanced calculation. Obstructions can rob some components of cooling air. Turbulence can confuse air flow sensors and trigger false alarms. The use of an anemometer at the prototype stage can help establish cooling requirements and find areas of low turbulence in which to locate sensors. Whatever the approach to overheat protection, the engineer needs a detector sensitive enough to respond quickly but stable enough to avert nuisance alarms and shutdowns. A time delay of less than 10 seconds usually provides protection from transient events. Trip points should not be set too close to nominal operating thresholds. Warren G-V airflow sensors, for example, are usually calibrated for trip points around 50-60% average airflow. The engineer searching can best validate a trip point by testing three sample sensors in prototype enclosures.
tWarren G-V,
One Apollo Drive, Whippany, NJ 07981. 973-386-1200.
