Thermal switches provide circuit disconnect
Edited by Bill Travis
Mark Cherry, Maxim Integrated Products, Sunnyvale, CA -- EDN, September 4, 2003
A single temperature sensor can provide an interrupt to a microcontroller when the measured temperature goes out of range. You need multiple temperature sensors when you have to monitor more than one hot spot. A microcontroller implements the proper protective action when one of the temperature monitors detects an overtemperature condition. It is sometimes easier and more cost-effective to simply disconnect the offending circuit from the power supply without involving a microcontroller. A simple thermal-protection circuit (Figure 1) includes two temperature switches, IC1 and IC2, with active-high outputs. Temperature thresholds for these switches depend on resistors R1 and R2, and the switch outputs connect to the inputs of a dual-input OR gate, IC3. OR gates with more than two inputs are available if you need more than two temperature switches. When excessive temperature drives either input high, the OR gate's output switches high, causing an SCR (silicon-controlled rectifier) to crowbar the power supply and blow the fuse. You must take precautions to ensure that the SCR does not trigger on a false gate pulse. Power-supply transients can cause a false high signal at the output of the OR gate and cause the SCR to turn on. Once triggered, the SCR cannot turn off, and the fuse blows. A small RC filter, R3 and C3, suppresses any gate transients that would otherwise turn on the SCR.
