Here is a simple circuit mains box heat monitor for sensing high temperature due to overheating. This circuit helps to prevent disasters caused by only sparking in the mains box due to any fault like short circuit over loaded. This circuit is so designed that it also automatically switches on a bright while LED (LED3) in case of power fails. This light is sufficient to check the mains box wring or fuses in darkness.
Circuit Description Mains Box Heat Monitor
The circuit is build around timer IC, a sensor and two transistors. The input AC mains is stepped down by using a 0-9v 500mA transformer and rectified by using bridge rectifier build using four diodes D1 through D4. The rectified voltage is further filtered by using 1000 µF of capacitor (C1) to smoothen the voltage and bypass the ripple. The output is given to LED1 through resistor R1 to indicate the power-on condition. Resistor R1 is used here as correct limiter.
Diode D5 is a germanium diode which is here act as temperature sensor and is used in reverse bias mode. The resistance of diode D5 is high at normal temperature areal as a result transistor T1 conducts to hold pin 4 of IC1 in low state. Potentiometer VR1 is used hear for adjust sensing capability of circuit.
Timer IC (IC1) is configured here as an astable multivibrator and generate frequency when the temperature around diode D5 rise due to overheating of the fuse. The resistance of diode D5 is decrease when temperature rise and transistor T5 stop conducting and this enable IC1. The output of IC3 is given to LED2 through resistor R6 for visual indication. A piezo buzzer (PZ) is connected parallel to LED1 as shown in circuit diagram for audio indication.